Cisco CCNP 2.0
Building Cisco Multilayer Switched Networks
Knowledge measured according
Campus network models
Cisco Hierarchical Internetworking
LAN Switching and hardware
Spanning Tree Protocol
Protocol Independent Multicast
Hot Standby Routing Protocol
Do NOT use this study guide as your sole
study resource. Successful completion of the 504 exam requires both
practical experience as well as lots and lots of reading.
On the actual exam you will encounter
questions on IOS commands as well as many terms. For the commands,
Cisco’s original documentation has very detailed coverage. You may
follow the links we provide to read these original Cisco documents for
more information on the commands.
Readings from the Cisco Web Site
Configuring VLAN Trunks
IOS Switching commands
Configuring IP MLS
Configuring IP services
IP Multicast Commands
Configuring Multicast Services
In the past, the primary target of
local workstations would be workgroup servers in the same neighborhood
Nowadays, performance of Layer 3
device is critical
Nowadays, resources are shared mostly
in the core layer of the Cisco Hierarchical Model
80 % of the campus traffic will
traverse the core while 20 % will stay local with the LANs
The new trend for the networking world
when we have a high percentage of traffic destined for the enterprise
80 % of the network traffic should be
local, while 20 % of the network traffic should move across the
This is the “old school of
Not suitable for nowadays web based
environment where activities and processing are consolidated to
Cisco Hierarchical Internetworking Model
This refers to the network design
where we have 3 different layers:
This model facilitates scalability,
performance and troble-shooting
High-speed switching backbone
Designed to switch packets as fast as
Does not perform packet manipulation
Address or area aggregation
Departmental or workgroup access
Broadcast or multicast domain
Allows local end users into the
Uses access lists to filter the needs
of a particular set of users
Shared / switched bandwidth
MAC layer filtering
OSI Layers and Protocols
Cisco Switches in the different layers:
Switches Interfaces and Management
The 5000/6000/6500 series switches use
a set based CLI similar to the Unix csh style interface. The most
commonly used commands are Set (for configuration changes), Show ( for
showing config information) and Clear (for removing config settings).
You may manage the switches with
In-band management, meaning you do it through telnet or SNMP through
modem or line module.
You may also manage with Out-of-band
management, meaning you are doing it via the console port directly
connected to the Supervisor module.
You may seek help from Cisco by
approaching Cisco’s Technical Assistance Center TAC. TAC has
Customer Support Engineers CSEs to help you out
Support may also be obtained via Cisco
Connection Online in Cisco web site
Early LANs and Broadcasts
The first Local Area Networks (LANs) were fairly small and
required few network devices to provide basic connectivity. They were
built around broadcast domains, with all devices sharing the same network
access in a flat network, and all packets visible to every device on the
network. Early on it was seen that broadcast congestion was a major
impediment to growth because this would limit the number of devices that
could share the network.
Just think “broadcasts bad”. Broadcasts take up bandwidth,
and worse yet, every device that can hear it must interrupt its other work
to process it. Adding more devices to a flat network results in more
broadcasts, less bandwidth available to other purposes, and more
interruptions for end-devices to process.
When the network became saturated, a bridge, or what we
would today call a two-port store-and-forward switch, would be installed.
Bridges flood broadcasts, multicasts, and unknown unicasts to all
segments; all the bridged segments together form a single broadcast
domain. The primary advantage to bridges is that they do keep user traffic
isolated and allow more hosts to be added to the network by reducing the
size of the collision domain.
Layer-2 switches are micro-segmentation devices (a review of
the OSI model is available at the end of this document). In other words,
you can think of them as bridges with dozens of ports. I’ve heard it said
jokingly that the only reason they named them switches was because if they
called them a bridge, they’d never sell any. Because switches facilitated
the move away from shared media for end-devices, they have the affect of
increasing available bandwidth without adding additional complexity. The
packets do not need to be modified when data is passed between devices on
the same VLAN (Virtual Local Area Network). This allows data to travel at
wire-speed through the switched network fabric.
One of the problems in a bridged or switched environment is
containing broadcasts, which, since these devices work at layer-2, are
forwarded to all ports. VLANs are a technology created to address this
issue. Each port on a switch can be configured to be part of a specific
VLAN, which can be thought of as a subnet. These represent a broadcast
domain defined by a set of ports; and some form of layer-3 device, such as
a router, is required to move traffic between them.
The now out-of-date 80/20 rule refers to the goal of keeping
80% of the traffic on a network segment bound for a local destination
(peer-to-peer and workgroup servers), and that no more than 20% of the
network traffic should be directed across the backbone. Under this older
design principle, workgroup servers would be deployed as the primary
target of local workstations, allowing most of the traffic to be contained
within the local subnet. This was done to conserve valuable bandwidth when
media was shared and bandwidth was extremely limited.
The philosophy of network design has been reversed in the
last few years, and this is reflected in the new 20/80 rule, which has the
bulk of traffic directed at shared resources on the core layer of the
Cisco Hierarchical Model. These design principles have 80% of the traffic
routed off the local domain, usually to the core, and 20% (I would say,
probably significantly less than that) is kept within the same broadcast
domain. This has driven the improvement in layer-3 device performance.
Because bridges limit collision domains, but not broadcast
domains, routing was introduced on the LAN to provide control, and to
actually segment the network into separate entities limiting the effect of
broadcast traffic. This was an important step in the evolution of the LAN,
but it must be remembered that router ports are expensive, both in pure
dollars and in processing overhead.
Routing, which occurs at layer-3, is much more complex than
bridges and switches because packets must be ripped apart and reassembled
as they pass through the router. This activity is CPU intensive. Routers
do allow a great deal of control over data through the use of access
lists, static routes and dynamic routing protocols.
The diameter of a network is the number of router
hops from any one device to another. Cisco recommends having a
consistent diameter. Their way of achieving this is through the use of the
Hierarchical Design Model (if you are unfamiliar with the HDM, it is
defined at the end of this document).
Those who have worked on Cisco routers in the past will be
comfortable with the Cat1900/2820 and 2900XL series access switches. The
command nomenclature is familiar and, other then a few new commands, the
same rules apply.
The Cat5000/6000/6500 series of switches use a different
style of CLI, which is based on the Unix csh or c-shell prompt. This is
commonly called the Set-based CLI, since this is one of the three commands
used on these devices. They are:
Set – Implements configuration
Show – Verifies and provides
information on the configuration
Clear – Removes configuration
A VLAN is an extended logical network that is
configured independent of the physical network layout. Each port on a
switch can be defined to join whatever VLAN suits the Network Architect’s
plans. Since each VLAN is a separate broadcast domain, routing must be
enabled between them if data is to be passed.
Switches are used in VLANs to act as
entry points for end-station devices into the switched fabric, and to
provide flexibility in configuring group users, ports, or logical
addresses and to make filtering and forwarding decisions.
Most VLANs use frame filtering
(frame tagging) to examine particular information about each frame based
on user-defined offsets, and uniquely assign a user-defined ID to each
Each hub segment connected to a
switch port can be assigned to only one VLAN.
VLAN ports on a switch can be
assigned statically using a VLAN management application or by working
directly within the switch. A more convenient approach, Dynamic VLANs,
are ports on a switch that can automatically determine their VLAN
Assign Ports to a VLAN on a Set-Based Switch
Switch (enable) set vlan vlan-number module/port
Example: Port 6, on module 4 needs to be assigned to VLAN
3. Keep in mind that you can assign several ports at once by using
wildcards, such as “4/1-12” for the first twelve ports on module 4.
Switch (enable) set vlan 3 4/6
Assign Ports to a VLAN on an IOS-Based Switch
Switch(config-if)# switchport mode access
VLAN membership mode for the port
Switch(config-if)# switchport access vlan 6
port to VLAN 6
Verify Port VLAN Status on an IOS-Based Switch
show interface interface-id switchport
Verify Port VLAN Status on a Set-Based Switch
Switch (enable) show vlan
Switch (enable) show port
A point-to-point link configured on a single Fast-Ethernet,
Gigabit Ethernet, or Fast- or Gigabit EtherChannel bundle and another
network device, such as a router or second switch. Trunks transport the
packets of multiple VLANs over a single network link.
The available trunking encapsulation types for Ethernet are
Inter-Switch Link (ISL) - a
Cisco-proprietary trunking encapsulation that adds a 26-byte header and
4-byte trailer to the frame.
IEEE 802.1Q (dot1q)- an
industry-standard trunking encapsulation that does not change the size
of the frame. Because multiple vendors support dot1q, it is becoming
more common in newer switched networks.
Negotiate - The port negotiates with
its neighbor port to mirror its encapsulation configuration, either ISL
(preferred) or 802.1Q trunk. This configuration option is only available
in switch software release 4.2 and later.
There are five trunking modes
On - Forces the port to become a
trunk port, even if the neighboring port does not agree to the change.
Off – Forces the port to become non-trunking, even if the neighboring port does not agree to the change.
Desirable - Causes the port to
actively seek to convert the link to a trunk. The port becomes trunked
if the neighboring port is set to either “on”, “desirable”, or “auto”
Auto - Makes the port available to
serve as a trunk link. The port becomes a trunk port if the neighboring
port is set to either “on” or “desirable” modes. This is the default
mode for both Fast- and Gigabit Ethernet ports.
Nonegotiate - Puts the port into
permanent trunking mode but the neighboring port must be manually
configured as a trunk port in order to establish a trunk.
For trunking to be auto-negotiated
on Fast Ethernet and Gigabit Ethernet ports, the ports must be in the
same VTP domain.
Not all switches support all
encapsulation methods; for instance the Cat2948G and Cat4000 series
switches support only 802.1Q encapsulation. In order to determine
whether a switch supports trunking, and what trunking encapsulations are
supported, look to the hardware documentation or use the "show port
For trunking to be enabled on
EtherChannel bundles the speed and duplex settings must be configured
the same on all links. If part of an EtherChannel bundle fails, traffic
will still be passed, but at a slower rate.
The command to enable trunking on a SET based switch is:
Switch (enable) set trunk module/port
on|desirable|auto|off|nonegotiate [vlan-numbers] [isl|dot1q]
The command to disable trunking on a SET based switch is:
Switch (enable) clear trunk module/port vlan-numbers
The command to verify trunking status on a SET based switch
Switch (enable) show trunk [module/port]
The command to enable trunking on a 2900XL is:
Switch(config-if)# switchport mode
Switch(config-if)# switchport trunk encapsulation isl|dot1q
The command to verify trunking on a 2900XL is:
Switch# show interface
The command to enable trunking on a Cat1900/2820 is:
Switch(config-if)# trunk [on|off|desirable|auto|nonegotiate]
The command to verify trunking on a Cat1900/2820 is:
Switch# show trunk
Spanning Tree Protocol (STP)
When multiple bridges or switches are installed, the
possibility of loops forming and causing broadcast storms is a significant
concern. A layer-2 loop occurs when a frame is transmitted from an
end-device and detected by two different bridges or switches. These
switches populate their address tables with the MAC address from the
source address of the frame. Once their table is updated they forward the
frame to the second segment, and then pick up the same MAC address from
the other switch, feeding each other back the same information repeatedly.
In other words, multi-active paths between stations create loops in the
network, causing hosts to receive redundant messages and forcing switches
to learn duplicate host MAC addresses from multiple ports.
The Spanning Tree Protocol (STP) was developed
to prevent loops in the network and to route around failed elements.
It is a link management protocol that provides path redundancy and
prevents undesirable loops in the network. The Spanning Tree Algorithm (STA) calculates the best loop-free path throughout a switched network -
switches send and receive spanning-tree frames at regular intervals, using
them to construct a loop-free path, and forcing redundant data paths into
a standby (blocked) state. All this is done in a way that is transparent
operationally to the network hosts. The election of root bridges is
performed through an exchange of data messages called Bridge Protocol Data
Units (BPDUs). STP can be manually disabled on a per-VLAN or a global
The following are characteristics of the STP
Where redundant links exist, any but
the one with the least distance from the root switch are blocked.
STP convergence can take upwards of
Broadcast traffic within the layer-2
domain (VLAN) interrupts every host.
Broadcast storms within the layer-2
domain affect the whole domain.
Isolating problems can be time
Network security options within the
layer-2 domain (VLAN) is limited.
STP can be configured two ways
Per-VLAN Spanning Tree (PVST) – A
Cisco proprietary method of connecting through 802.1Q VLAN trunks, the
switches maintain one instance of the spanning tree for each VLAN
allowed on the trunk, versus non-Cisco 802.1Q switches which maintain
one instance for ALL VLANs. This is the default STP used on ISL trunks.
Since each VLAN has its own instance of STP, there is more granular
control of the path selection process, and fewer sub-optimal paths may
be invoked. Because the size of the STP topology is reduced, this can
have the effect of reducing convergence time and increasing scalability
Common Spanning Tree (CST) - When
connecting a Cisco switch to a non-Cisco device through an 802.1Q trunk,
the Cisco switch combines the spanning-tree instance of the 802.1Q
native VLAN of the trunk with the spanning-tree instance of the
non-Cisco 802.1Q switch. The primary advantages of CST are that only one
set of BPDU’s are used; it is only necessary to track changes for a
single instance of STP, and non-Cisco switches can be added to the mesh.
However, with only one STP algorithm running, sub-optimal paths are more
likely to be selected than under other methods. With CST, less bandwidth
will be used to negotiate a root bridge, although with only one root
bridge for the entire network, it may take longer for STP to recalculate
when a change occurs.
Bridge Protocol Data Units (BPDUs) are multicast frames sent
out periodically to announce the presences, resources and recent changes
of a switch’s configuration. They
Propagate bridge IDs in order for
the selection of the root switch to take place.
Are used to determine loop locations
within a network.
Provide notification of network
Remove loops by placing redundant
switch ports in a backup state.
The Bridge ID defines which device will be the root bridge.
It is made up of two parts; the 2-byte priority, a default value that can
be changed by the Network Architect; and the 6-byte MAC address of the
switch or bridge.
There are two factors involved in the root port selection
As BPDU leaves a port, it applies the root port cost. Path
Cost is the total sum of all of the port costs, and is what STP uses to
determine which ports should forward and which ports should block. If the
path cost is the same for several ports, STP will use the lowest port ID.
Hello timer - How often the switch
broadcasts Hello messages to other switches.
Forward delay timer - Amount of time
a port will remain in the listening and learning states before going
into the forwarding state.
Maximum age timer – How long
protocol information received on a port is stored by the switch.
STP Port States
Ports on an STP domain will progress through the following states
Blocking – Listens for BPDUs from
other bridges, but does not forward them or any traffic.
Listening – An interim state while
moving from blocking to learning. Listens for frames and detects
available paths to the root bridge, but will not collect host MAC
addresses for its address table.
Learning – Examines the data frames
for source MAC addresses to populate its address table, but no user data
Forwarding – Once the learning state
is complete, the port will begin its normal function of gathering MAC
addresses and passing user data.
Disabled – Either there has been an
equipment failure, a security issue, or the port has been disabled by
the Network Administrator.
Notes about STP Port States
A port in blocking state does not
participate in frame forwarding - switch always goes into blocking state
immediately following switch initialization.
When a port changes from the
listening state to the learning state it is preparing to participate in
Port in the Forwarding state
actually forwards frames (User data, BPDUs, etc.).
Root Switch Selection
Rather than allowing STP to define the root bridge, a good
Network Architect will select the switch to be the root that minimizes
unnecessarily convoluted data migrations. They will change the
Spanning-tree bridge priority from the default value (32768) to a
significantly lower value, ensuring that the switch becomes the root for
the specified VLANs.
Spanning Tree Commands
Enable spanning tree on per-VLAN or global basis:
Switch (enable) set spantree enable [vlans]
(enable) set spantree enable all
Verify that spanning tree is enabled:
Switch (enable) show spantree [vlan]
Configure a switch as the root switch:
Switch (enable) set spantree root vlans [dia network_diameter] [hello
Change the global port priority for a port:
Switch (enable) set spantree portpri
Change the port-VLAN priority for a VLAN on a switch port
Switch (enable) set spantree portvlanpri
mod_num/port_num priority [vlans]
Change the global port cost for a switch port
Switch (enable) set spantree portcost
Change the port-VLAN cost for a VLAN on a switch port
Switch (enable) set spantree portvlancost
mod_num/port_num cost cost [vlans]
Set the bridge priority for a VLAN
Switch (enable) set spantree priority
Set the Hello time for a VLAN
Switch (enable) set spantree hello interval [vlan]
Set the forward delay time for a VLAN
Switch (enable) set spantree fwddelay delay [vlan]
Set the maximum aging time for a VLAN
Switch (enable) set spantree maxage agingtime
By default, all ports on a switch are assumed to have the
potential to have bridges or switches attached to them. Since each of
these ports must be included in the STP calculations, they must go through
the four different states whenever the STP algorithm runs (when a change
occurs to the network).
Enabling PortFast on the user access ports is basically a
commitment between the Network Architect and the switch agreeing that the
specific port does not have a switch or bridge connected, and therefore
this port can be placed directly into the Forwarding state; this allows
the port to avoid being unavailable for 50 seconds while it cycles through
the different bridge states, simplifies the STP recalculation and reduces
the time to convergence.
The command to enable PortFast on a SET based switch is:
Switch (enable) set spantree portfast
Switch (enable) set spantree
portfast 4/1-12 enable
The command to disable PortFast on a SET based switch is:
Switch (enable) set spantree portfast
Switch (enable) set spantree
portfast 4/1-12 disable
The command to verify PortFast status on a SET based switch
Switch (enable) show spantree
The command to enable PortFast on a 2900XL is:
Switch(config-if)# spanning-tree portfast
The command to disable PortFast on a 2900XL is:
Switch(config-if)# no spanning-tree portfast
The command to verify PortFast on a 2900XL is:
Switch# show spanning-tree
The command to enable PortFast on a Cat1900/2820 is:
Switch(config-if)# spantree start-forwarding
The command to disable PortFast on a Cat1900/2820 is:
Switch(config-if)# no spantree start-forwarding
The command to verify PortFast on a Cat1900/2820 is:
Switch# show spantree
Convergence time on STP is 50 seconds. Part of this is the
need to determine alternative paths when a link between switches is
broken. This is unacceptable on networks where realtime or
bandwidth-intensive applications are deployed (basically any network).
If the UplinkFast feature is enabled (it is not the default)
AND there is a least one alternative path whose port is in a blocking
state AND the failure occurs on the root port of the actual switch, not an
indirect link; then UplinkFast will allow switchover to the alternative
link without recalculating STP, usually within 2 to 4 seconds. This allows
STP to skip the listening and learning states before unblocking the
The command to enable UplinkFast on a SET based switch is:
Switch (enable) set spantree uplinkfast enable
The command to disable UplinkFast on a SET based switch is:
Switch (enable) set spantree uplinkfast disable
The command to verify UplinkFast status on a SET based
Switch (enable) show spantree
The command to enable UplinkFast on a 2900XL is:
Switch(config)# spanning-tree uplinkfast
The command to disable UplinkFast on a 2900XL is:
Switch(config)# no spanning-tree uplinkfast
The command to verify UplinkFast on a 2900XL is:
Switch# show spanning-tree
The command to enable UplinkFast on a Cat1900/2820 is:
The command to disable UplinkFast on a Cat1900/2820 is:
Switch(config)# no uplink-fast
The command to verify UplinkFast on a Cat1900/2820 is:
Switch# show uplink-fast
BackboneFast is used at the Distribution and Core layers,
where multiple switches connect together, and is only useful where
multiple paths to the root bridge are available.
This is a Cisco proprietary feature that speeds recovery
when there is a failure with an active link in the STP. Usually when an
indirect link fails, the switch must wait until the maximum aging time
(max-age) has expired before looking for an alternative link. This delays
convergence in the event of a failure by 20 seconds (the max-age value).
When BackboneFast is enabled on all switches, and an inferior BPDU arrives
at the root port - indicating an indirect link failure - the switch rolls
over to a blocked port that has been previously calculated.
The Primary difference between
UplinkFast and BackboneFast is that BackboneFast can detect indirect
link failures and is used at the Distribution and Core layers; while
UplinkFast is aware of only directly connected links, and is used
primarily on Access layer switches. If UplinkFast is turned on for the
root switch, it will automatically disable it.
There is no BackboneFast command for
IOS based switches. Since this is an enhancement for Core and
Distribution layer devices only, and these are all Set-based
The command to enable BackboneFast on a SET based switch is:
Switch (enable) set spantree backbonefast enable
The command to disable BackboneFast on a SET based switch
Switch (enable) set spantree backbonefast disable
The command to verify BackboneFast status on a SET based
Switch (enable) show spantree backbonefast
(enable) show spantree summary
VLAN Trunk Protocol (VTP)
In a switched environment a subnet corresponds to a VLAN,
and a VLAN may map to a single Layer 2 switch, or it may span several
switches, especially at the access layer. Also, it is likely that one or
more VLANs may be present on any particular switch.
VLAN Trunk Protocol (VTP) is a layer-2 messaging protocol
that centralizes the management of VLAN additions, deletions and changes
on a network-wide basis. This simplifies the management of large switched
networks with many VLANs.
A VTP domain is specified by the Network Engineer and
consists of one or more interconnected switches that share the same VLAN
configuration. A switch can only be configured as a member of a single VTP
domain. Changes to the global VLAN configuration for the domain can be
implemented using either the CLI or an SNMP session.
Switches defined as part of VTP domains can be configured to
operate in any of three VTP modes:
– Advertise VLAN
configuration to other switches in the same VTP domain and synchronize
with other switches in the domain. Can create, modify, and delete VLANs
as well as modify VLAN configuration parameters such as VTP version and
VTP pruning for the entire domain. This is the default mode for a
configuration to other switches in the same VTP domain and synchronize
their VLAN configuration with other switches based on advertisements
received over trunk links; however, they are unable to create, change,
or delete VLAN configurations.
- Does not
advertise its VLAN configuration and does not synchronize its VLAN
configuration with other switches. In VTP version 2, transparent
switches do forward VTP advertisements.
Advertisement types include: requests from clients, summary
advertisements and subset advertisements. An advertisement contains the
VLAN IDs, the Emulated LAN names for ATM LANE, the 802.10 SAID values for
FDDI, the VTP domain name, the VTP configuration revision number, the MTU
size and the Frame format.
Early design specifications touted the ability of VTP to
create global VLAN groups that the Network Engineer could use to have
VLANs that would span vast networks; however, in recent years it has
become obvious that this has generated unnecessary and expensive wide area
traffic for not much gain. Most design specifications now suggest creating
VTP domains for each facility, and limiting the VTP advertisements sent
over limited and expensive wide area links.
VTP advertisements carry configuration revision numbers that
are incremented every time a VLAN is modified. This is used to identify
the most recent changes to the network topology. When a switch finds an
advertisement with a higher configuration revision number, it will save
the new VTP database over the old one. A VLAN that does not exist in the
new database is automatically deleted from the switch, and any ports that
were in the VLAN will be orphaned.
It is a common problem for a newly ordained Network Engineer
(also called a “newby” or “loser-boy”) to add a switch that has been used
on a separate or test network to a production network, not being aware of
the revision number. Since test networks change much more often than
production networks, the new switch likely has a higher configuration
revision number than the production VTP domain. The result is that the
entire domain’s VTP database gets overwritten and any ports assigned to
the lost VLANs lose their VLAN membership and become unavailable to users.
If you receive a call that all the switched ports on a network have
suddenly locked up and no traffic is being passed, one of the first places
to look is the red-faced newest member of the network team who thought he
was helping when he put a new switch in the network. Good documentation
and control over physical access to network devices are probably your best
defense against this problem. Also, to prevent this problem, the command
“clear config all” should be used on any switch before it is added to a
VTP pruning is a technique to limit VTP broadcast from
branches of the network that do not contain member ports of a specific
VLAN. By default, VTP pruning is disabled. VTP pruning must be enabled on
a VTP server and promulgates pruning eligibility through the entire
management domain. By default, VLAN 1 is always pruning-ineligible, and
VLANs 2 through 1000 are pruning-eligible.
Configure a VTP Domain
Enter the VTP configuration mode
Switch (enable) vlan database
Set the VTP domain name to “Primary”
Switch (vlan) vtp domain Primary
Set the VTP domain password to “scubbie”
Switch(vlan) vtp password scubbie
VTP version 2 is enabled (to return to ver.1 - “no vtp
Switch(vlan) vtp v2-mode
Set the switch to VTP server mode. The client or transparent
arguments could be used instead.
Switch(vlan) vtp server
Verify VTP Operation
Display the VTP switch configuration and statistics
Switch show vtp status
Display the VTP counters for messages sent and received
Switch show vtp counters
Adding VLANs to a VTP domain
Enter the VTP configuration mode
vlan vlan-id name
Example: Add VLAN 6 to the domain and name it “accounting”.
If a name is not specified, it defaults to the VLAN number designation, as
in this case would be “VLAN0006”:
vlan 6 name accounting
Display the VLAN configuration
show vlan name
vlan-name show vlan name
Displays a list of configured VLANs
show vlan brief
Deleting VLANs from a VTP domain
Enter the VTP configuration mode
no vlan vlan-id
Example: Remove vlan 6 from the VTP domain and orphan any
ports assigned to that VLAN:
no vlan 6
VLAN commands - Brief
Vlan database - enter into VLAN
Vtp domain domain-name - configure a
VTP administrative-domain’s name
Vtp password password-value - set
the password for the VTP domain
Vtp server - configure the switch as
Vtp client - put the switch in VTP
Vtp transparent - put the switch in
VTP transparent mode
Show vtp status - show VTP
No vtp v2-mode - disable VTP version
Global Information in a VTP
Advertisement includes VTP Domain Name, VTP Configuration Revision
Number, Update Identity, Update Timestamp, MD5 Digest.
VLAN Information in a VTP
Advertisement includes VLAN ID, VLAN Name, VLAN Type, VLAN State.
VTP Version 2 has features not
supported in VTP version 1, including Token Ring LAN Switching and VLANs, unrecognized Type Length Value, Version Dependent Transparent
Mode and Consistency Checks. Please note that all the switches in the
VTP domain must run the same VTP version.
In general, don’t enable VTP version
2 in the VTP domain unless all the switches are running version 2 as
well. However, if the network is Token Ring, you must enable VTP version
VTP Pruning increases bandwidth by
controlling traffic flow to the vital trunk links and to block flooded
traffic to VLANs in the pruning eligible list. Enabling VTP pruning on a
VTP server will enable it on the entire management domain.
Max 250 active VLANs supported by a
switch. Watch out though, as some switch models only support 6 VLANs.
When creating a VLAN, the switch
must be in VTP server or transparent mode.
Default VLAN Configurations -
Ethernet Parameters have an ID Range 1-1005. No limit on VLAN Name, and
the MTU Size is 1500.
Am emulated LAN is a group of ATM-attached devices treated
as an independent broadcast domain. Think of it as a single Ethernet
segment or independent Token Ring. ELANs are made up of two components:
the LAN Emulation Client (LEC) and LAN Emulation (LANE) services. The LEC
can be located in the same device(s) as the LANE Services. LANE services
are made up of a LAN Emulation Configuration Server (LECS), the LAN
Emulation Server (LES), and the Broadcast and Unknown Server (BUS), and
all of them can be located in the same device or distributed among one,
two, or three devices.
To join an emulated LAN, LEC needs to contact the LECS in
order to obtain its ATM address via reconfigured address for the LECS,
ILMI or the well-known address of the configuration service. When two
hosts are in the same emulated LAN, switches are enough for data
transmissions. When two systems reside in different emulated LANs, a
layer-3 router or switch must be used to interconnect them, regardless of
the physical connection.
All nodes on an Ethernet network can transmit at the same
time, so the more nodes you have, the greater the possibility of
collisions happening, which can slow the network down.
LAN segmentation means to break up collision domains by
decreasing the number of workstations per segment using bridges or
switches. Switches are sometimes called micro-segmentation devices,
because there may be as little as one host per collision domain.
Switching is a layer-2 data manipulation that forwards
through the network by destination MAC addresses.
These are the common Cisco switching techniques
Store-and-forward – receives the
complete frame before forwarding. Copies the entire frame into the
buffer and then checks for CRC errors. Higher latency then other
techniques. This technique is used on Cat5000s.
Cut-through – checks the destination
address as soon as the header is received, and immediately forwards it
out, lowering the latency level.
Fast switching - The default
switching type. It can be configured manually through use of the “ip
route-cache” command. The first packet is copied into packet memory,
while the destination network or host information is stored in the
Process Switching - This technique
doesn’t use route caching, so it runs slow; however, slow usually means
SAFE. To enable, use the command “no protocol route-cache”.
Optimum Switching – From its name
you can understand what it is – high performance! This is the default on
Multi-layer Switching is the ability to use a combination of
layer-2 switching technology, with layer-3 routing and layer-4 application
Layer-2 switching is hardware-based, using
Application-Specific Integrated Circuits (ASICs) to bridge a network. The
performance difference between a Layer-2 switch and a shared hub is
significant. A layer-2 switch can be thought of as a bridge on steroids.
It has all the same characteristics and limitations as bridging.
Problems with layer-2 switched networks
They provide scaling and performance
issues on large bridged networks.
The broadcast radiation increases
with the number of hosts; broadcasts are seen by all end stations.
STP can have slow convergence on
Traditional routers use CPUs that are general purpose
devices, while a layer-3 switch uses an ASIC, a piece of high-speed
hardware designed to perform a more limited set of tasks, in this case to
achieve efficient routing (in some cases and under certain circumstances,
wire-speed). For most purposes you can consider a layer-3 switch a device
that integrates layer-2 and layer-3 (and sometimes layer-4) functionality
in a single piece of equipment.
Depending on the network design, including what protocols,
interfaces, and features are required, layer-3 switches can be used in
place of routers and allow almost wire-speed routing. Standard routing
protocol can be used for route determination, including OSPF, EIGRP, RIP,
A router is used to determine conversations between
end-devices, and then switching techniques continue the conversations. It
has the following advantages: Hardware-based packet forwarding,
high-performance packet switching, scalability, low latency, lower
per-port cost, flow accounting, security and control over Quality of
Layer-4 switching refers to hardware-based routing, using ASICs, which takes application specificity into consideration.
TCP or UDP flows include port number in the packet heading,
which serves to identify the application under consideration.
Cisco routers have the ability to control traffic based on
Layer-4 information using extended access lists and
To support multi-layer switching, you will need to have the following
Engine (MLS-SE) - Catalyst 2926G series switch, or Catalyst 5000 series
switch with the NFFC (NetFlow Feature Card) or NFFC II. The NFFC is
a daughter-card upgrade to the Supervisor Engine that is an ASIC-based layer-3 switching engine.
Multilayer Switching-Route Processor
(MLS-RP) - A Route Switch Module (RSM) or an externally connected Cisco
router with software that supports MLS. The RSM is an IOS-based router
on a blade that uses the same Reduced Instruction Set Computing (RISC)
processor as the RSP2 engine in 7500 series routers. When MLS
is enabled, the RSM or externally attached router continues to handle
all non-IP protocols while offloading the switching of IP packets to the
Multilayer Switching Protocol (MLSP)
– A protocol running between the MLS-SE and MLS-RP.
You’ll hear the configuration with an external router
referred to as a “router-on-a-stick” or a “one-armed-router”.
When using an external router, the
ideal set up is one directly attached external router per switch to
ensure proper caching.
You can use Cisco high-end routers
for MLS when they are externally attached to the switch, make the
attachment with multiple Ethernet connections on an one per subnet basis
or by using Fast or Gigabit Ethernet with Inter-Switch Link
Router interfaces with input access
lists or reflexive access lists cannot participate in MLS. However, you
can translate input access lists to output access lists to provide the
When an output access list is
applied, the MLS cache entries for that interface are purged. However,
entries associated with other interfaces are not affected at all.
Flow mask mode is destination-ip
when there is no access list on any MLS-RP interface. When there is a
standard access list, the mode is source-destination-ip. When there is
an extended access list, the mode is ip-flow.
Setting up Multi-layer Switching
These are the commands necessary to configure an internal or
external Multi-layer Switch Route Processor:
Switch(enable) mls rp ip
Entering into the router interface
Assign VLAN ID to the route processor interface
Switch(enable-if)# mls rp vlan-id
Place the external route processor in the interface of the
VTP domain switch
Switch(enable-if)# mls rp vtp-domain
Enable the RSM interface
Switch(enable-if)# mls rp management-interface
Based only on layer-3 addresses.
NFFC (or NFFC II) maintains layer-3
switching table (MLS cache) for the layer-3-switched flows.
Whenever the Layer-3 switching entry
for a flow ages out, the flow statistics will be exported to a flow
Maximum MLS cache size is 128K.
Cache larger than 32K increases the
likelihood that a flow will get forwarded to the router.
When a layer-3 packet is switched
from source to destination, the switch performs a packet rewrite based
on information learned from the router and stored in the MLS cache.
If Host A and B are on different
virtual LANs, when Host A sends a packet to the MLS-RP to be routed to
Host B, the MLS-SE recognizes that the packet was sent to the MAC
address of the MLS-RP, and will check the MLS cache to find the matching
MLS-SE uses flow mask modes to
determine how MLS entries are created; the flow mask mode is based on
the access lists configured on the MLS router interfaces.
Unicast – A frame that will only be
processed by the destination host (one machine to one machine)
Broadcast – A frame that every host
on the broadcast domain must process (one machine to all machines)
Multicast – A frame that will only
be processed by multicast members on the broadcast domain (one machine
to a select list of machines)
Using Multicasts, an application can send a single stream of
packets to a defined group of computers, instead of sending it one by one
to each recipient, or flooding the network with broadcasts. Class-D
addresses are reserved for multicast traffic and are allocated
To manage multicast by allowing directed switching of
multicast traffic, and also to dynamically configure switch ports so that
IP multicast traffic is forwarded only to the appropriate ports, Cisco
Internet Group Management Protocol (IGMP) - Standard protocol to manage the multicast transmissions passed
to routed ports. One of the problems with this protocol is if a VLAN on
a switch is set to receive, all the workstations on that VLAN will get
the multicast stream.
Cisco Group Management Protocol (CGMP) - Cisco proprietary protocols to control the flow of multicast
streams to individual VLAN port members. Solves the problem sited above.
Requires IGMP to be running on the router.
CGMP and IGMP software components run on both the Cisco
routers and switches. Remember that CGMP is Cisco proprietary. When the
CGMP/IGMP-capable router receives an IGMP control packet, it creates a
CGMP or IGMP packet that contains the request type, the multicast group
address, and the MAC address of the host. These request types can either
be “join” or “leave” messages. The router sends the packet to a well-known
address to which all switches listen, so that the supervisor engine module
interprets the packet and modifies the forwarding table automatically. If
a spanning-tree VLAN topology changes, the CGMP/IGMP-learned multicast
groups on the VLAN are purged and the CGMP/IGMP-capable router generates
new multicast group information. If a CGMP/IGMP-learned port link is
disabled, the corresponding port is removed from any multicast group.
CGMP/IGMP-capable routers send periodic multicast group
queries, so if a host wants to remain in a multicast group, it must
respond to the query. If, after a number of queries, the router receives
no reports from any host in a multicast group, the router sends a CGMP/IGMP command to the switch to remove the group from the forwarding
tables. CGMP fast-leave-processing allows the switch to detect IGMP
version-2 leave messages sent to the all-routers multicast address by
hosts on any of the supervisor engine module ports.
Display information on dynamically learned and manually
configured multicast router ports
show multicast router mod_num/port_num vlan_id
Display total number of multicast address groups in each
show multicast group count vlan_id
Enable CGMP on the switch
Switch (enable) set cgmp enable
Verify that CGMP is enabled
Switch (enable) show cgmp statistics vlan_num
Enable CGMP fast-leave processing on the switch
Switch (enable) set cgmp leave enable
Verify that CGMP fast-leave processing is enabled
Switch (enable) show cgmp leave
Display information on those multicast router ports learned
dynamically using CGMP
Switch (enable) show multicast router cgmp
Display information about multicast groups learned
dynamically through CGMP
Switch (enable) show multicast group cgmp mac_addr
Display total number of multicast address groups in each
VLAN that were learned dynamically through CGMP
Switch (enable) show multicast group count cgmp
Display CGMP statistics
Switch (enable) show cgmp statistics vlan_id
Disable CGMP fast-leave processing on the switch
Switch (enable) set cgmp leave disable
Disable CGMP on switch
Switch (enable) set cgmp disable
Enable IGMP snooping on the switch
set igmp enable
Verify that IGMP snooping is enabled
show igmp statistics vlan_num
Enable IGMP fast-leave processing on the switch
set igmp fastleave enable
Verify that IGMP fast-leave processing is enabled
show igmp leave
Display information only on those multicast router ports
learned dynamically using IGMP snooping
show multicast router igmp mod_num/port_num vlan_id
Disable IGMP snooping on the switch
set igmp disable
Protocol Independent Multicast (PIM)
PIM is used to forward multicast packets through a network.
It must be enabled for a Cisco interface to perform IP multicast routing.
Enabling PIM on an Interface also enables IGMP operation on that
Interface can be configured to be in dense mode, sparse
mode, or sparse-dense mode - the modes determine how the router populates
its multicast routing table and how the router forwards multicast packets
it receives from its directly connected LANs. For PIM to work, it must be
in one mode, although there is no default mode setting as multicast
routing is disabled on an interface by default.
Dense-mode interfaces are always
added to the table. Dense mode is used when multicast group members are
densely distributed throughout the network and there is plenty of
bandwidth available. Dense mode PIM floods the multimedia packet to all
routers and prunes routers that do not support members of that
particular multicast group.
Sparse-mode interfaces are added to
the table only when periodic “join” messages are received from
downstream routers, or when there is a directly connected member on the
interface. Sparse mode is used when members are more spread out and
there is limited bandwidth available. Sparse mode PIM relies on
rendezvous points. For this purpose the PIM neighbor with the highest IP
address is elected to be the Designated Router (DR). If no PIM queries
are received from this DR after a certain period of time, the election
mechanism will run again.
Sparse-dense mode interfaces are
treated as dense mode if the group is in dense mode, or in sparse mode
if the group is in sparse mode.
A significant difference between Dense and Sparse modes is
that a dense mode router assumes all other routers are willing to forward
multicast packets for a group, while a sparse mode router requires an
explicit request for the traffic.
Enable dense-mode PIM on the interface
ip pim dense-mode
Enable sparse-mode PIM on the interface
ip pim sparse-mode
Quality of Service (QoS)
Quality of Service refers to the capability to provide
higher levels of access network resources based on the type of traffic. It
is defined as being over various network technologies, including Frame
Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks,
SONET, and IP-routed networks. QoS was created to provide
Cisco IOS QoS software allows control over complex networks
to provide the ability to predictably service a variety of applications
and traffic types. It provides
Enhanced control over, and more
efficient use of, network resources
Better network analysis management
and accounting tools
The ability to consistently service
the most important traffic, while still providing access for less
Enables ISPs to offer tailored
grades of service to their customers
There are three fundamental pieces for QoS implementation
Within a single network element -
queuing, scheduling, and traffic shaping tools
Signaling techniques for
coordinating QoS from end to end between network elements
QoS policy, management, and
accounting functions to control and administer end-to-end traffic across
There are three basic levels of end-to-end
Best-effort service - basic
connectivity with no guarantees
Differentiated service - soft QoS
- some traffic is treated better than the rest via statistical
Guaranteed service - hard QoS -
absolute reservation of network resources for specific traffic
There are two traffic-shaping tools
Generic traffic shaping (GTS) - GTS
reduces outbound traffic flow by constraining specified traffic to a
particular bit rate while queuing bursts of the specified traffic
Frame Relay traffic shaping (FRTS) -
FRTS provides parameters useful for managing network traffic congestion:
committed information rate (CIR), FECN and BECN, and DE bit
CDP (Cisco Discovery Protocol)
A proprietary Data Link layer protocol used between Cisco
devices to pass information about local conditions. CDP uses a data-link,
multicast address with no protocol ID or network layer field, and cannot
The only way to prevent their being passed is to configure
“no cdp enable” on those interfaces on which you do not want to run
You can configure a MAC-layer filter to deny a multicast address as an
alternative method to block these packets.
Asynchronous Transfer Mode (ATM)
Developed as a compromise between voice and data needs, ATM
is commonly found either on large telecom networks or built into networks
that have a strong need for QoS (Quality of Service) needs.
ATM uses Cells that are uniform in size - 53 bytes; 5
bytes for a header, and 48 bytes for payload. This allows a great deal of
control over traffic and allows for QoS, but is wasteful in that the
header is a greater percentage of the traffic than in other methods.
ATM is connection-oriented with traffic traveling from
end-to-end over either
There are two types of interfaces
There are four major layers in the ATM reference model
(equivalent to the OSI Model)
Higher layers – ATM signaling,
addressing and routing.
AAL (ATM Adoption Layer) – Converts
from higher level to ATM cells.
ATM – Defines ATM cell relaying and
Physical – Defines the physical
network media and framing.
DDR Dial-on-Demand Routing
DDR has two important applications
When there is a WAN link that needs
to be available, but rarely sees traffic, the Network Architect might
provision a pay-per-use wide area connection - such as BRI - and use DDR
on the routers to only activate the link when there is “interesting
traffic”, and rip it down when the conversation is over.
When there is a critical WAN link
and there must be a redundant connection. If there were a T1 between two
sites, and it was imperative that the link see very little downtime, DDR
might be enabled on a BRI ISDN port. If the T1 were to fail, the BRI
would establish connectivity over at least one of its data channels
(B-channel), and could be configured to enable the second channel if
traffic needs were to reach a defined threshold.
DDR spoofs routing tables to provide the image of full-time
connectivity using Dialer interfaces and filters out interesting packets
for establishing, maintaining, and releasing switched connections.
Interesting traffic is defined by an access list.
Encapsulation Methods for DDR
– recommended, as it
supports multiple protocols and is used for synchronous, asynchronous,
or ISDN connections. It is also non-proprietary.
- supported on
synchronous serial lines and ISDN connections only, and supports
multiple protocols, with NO authentication.
- works on asynchronous
interfaces and is IP only, with NO authentication.
- works on synchronous
serial lines and a single ISDN B channel.
The Institute of Electrical and Electronics
Engineers, a professional organization that, among other things,
developments communications and network standards.
IEEE 802.1 -
IEEE specification that describes an
algorithm that prevents bridging loops by creating a spanning tree. The
algorithm was invented by Digital Equipment Corporation. The Digital
algorithm and the IEEE 802.1 algorithm are not exactly the same, nor
are they compatible.
IEEE 802.2 -
IEEE LAN protocol that specifies an
implementation of the LLC sublayer of the data link layer. IEEE 802.2
handles errors, framing, flow control, and the network layer (Layer 3)
service interface. Used in IEEE 802.3 and IEEE 802.5 LANs.
IEEE 802.3 -
IEEE LAN protocol that specifies an
implementation of the physical layer and the MAC sublayer of the data link
layer. IEEE 802.3 uses CSMA/CD access at a variety of speeds over a
variety of physical media. Extensions to the IEEE 802.3 standard
specify implementations for Fast Ethernet. This is the specification that
IEEE 802.4 -
IEEE LAN protocol that specifies an
implementation of the physical layer and the MAC sublayer of the data link
layer. IEEE 802.4 uses token-passing access over a bus topology and is
based on the token bus LAN architecture. This is the specification that
describes Token Ring Bus.
IEEE LAN protocol that specifies an
implementation of the physical layer and MAC sublayer of the data link
layer. IEEE 802.5 uses token passing access at 4 or 16 Mbps over
STP cabling and is similar to IBM Token Ring.
IEEE 802.6 -
IEEE MAN specification based on DQDB
technology. IEEE 802.6 supports data rates of 1.5 to 155 Mbps.
This is the specification that describes Municipal Area Networks (MAN).
More 802.x standards
802.8 Fiber-optic LANs
Integrated Voice & Data
802.10 LAN/MAN Security
802.12 VGAnyLAN (HP’s answer to FastEthernet
Tag switches support multicast by utilizing data link layer
multicast capabilities: all tag switches that are part of a given
multicast tree on a common sub-network must agree on a common tag so that
forwarding of multicast packets to all downstream switches on that
sub-network is possible.
Tag switching can mark packets as belonging to a particular
class after they have been classified the first time, which is an
important aspect of QOS.
The tag-switching forwarding paradigm is based on label
swapping, which is the same as in ATM forwarding, tag-switching technology
can be applied to ATM switches.
Tag information can be carried in a packet in many ways,
such as a small "shim" tag header inserted between the Layer 2 and the
network-layer headers, as part of the Layer 2 header (e.g. ATM), as part
of the network-layer header (e.g. Ipv6). This is why tag switching can be
implemented over any media type.
When a packet with a tag is received, the switch uses the
tag as an index in its Tag Information Base (TIB). If the switch finds a
matching entry, then for each component in the entry the switch replaces
the tag in the packet with the outgoing tag. The switch also replaces the
link-level information in the packet with the outgoing link-level. This is
called Label Swapping.
There are two principal components
tag information carried by packets and the tag-forwarding information
maintained by a tag switch to perform packet forwarding.
maintaining correct tag-forwarding information among a group of
interconnected tag switches.
Remote Monitoring (RMON)
RMON has 4 groups
Statistics Group for port
utilization and error statistics
History Group for periodic
Alarm Group for sampling interval
Event Group for logging events to
network management station
Cisco Device Management
There are two ways of managing routers and switches:
General Troubleshooting Tips
SPAN is the Enhanced Switched Port Analyzer that monitors traffic
for analysis by other tools.
CWSI CiscoWorks Switched Internet Solutions is a management suite
that consists of CiscoView, VlanDirector, and TrafficDirector.
A cable tester device is used to look for cable breaks.
Time Domain Reflectometer measures cable length and impedance; loose
or incorrect device connection can also be detected.
Always isolate network segment problems by checking the devices on
the same segment to see if they can communicate. In an IP environment,
use the “ping” and “traceroute” commands.
Switch LEDs indicate problem based on color: red = failure, orange =
less severe problem. If the Output Fail LED = Red, check the power
To troubleshoot other problems, try using the show commands to find
out what is going on: Sh config, Sh int, Sh module, Sh spantree, Sh
trunk, Sh vlan, Sh port, Sh mac, Show test and Show log, etc.
A Protocol Analyzer can capture and display protocol information,
while Network monitors can continuously monitor network traffic.
A STP failure generally results in a bridging loop.
For point-to-point links, a duplex mismatch occurs when one side of
the link is hardcoded full duplex, while the other side is
auto-negotiation, and eventually the link ends up in half-duplex.
When a link is experiencing many physical errors, a number of
consecutive BPDUs could be lost, leading a blocking port to transition
STP is software based: if the CPU is over-utilized, it is possible
that it can lack the resources necessary to send out BPDUs. Also,
software bugs are possible.
When the age field of a BPDU goes beyond max age, it is discarded -
this occurs if the diameter of the STP network is too large, making the
root switch too far from some distant switches.
To limit the risk implied by the use of the STP, it is recommended
that you reduce (as much as possible) the number of blocked ports -
prune VLAN not needed off your trunks and use the PortFast command on
those user ports that will never have switches or bridges installed.
Keep traffic off the administrative VLAN and avoid having a single
VLAN spanning the entire network.
Avoid hand-tuning STP parameters - Catalyst software provides macros
that perform fine-tuning of most significant STP parameters.
IOS commands for troubleshooting
“debug spantree events” displays STP events to help determine
problems. Be careful that this doesn’t overwhelm the CPU of the switch.
“logging buffered” captures debug information in the device's
“show interface” verifies interface utilization, packet
corruption, speed and duplex status of the specified port.
“show processes cpu” checks CPU utilization.
Catalyst OS Commands for troubleshooting
“set logging level spantree 7 default”
increases the default level of STP related event to debugging.
“set logging buffer 500” sets a maximum number of
messages in the switch's buffers.
“show port <module#/port#>” give you details of the
“show system” give indication on the backplane utilization.
“show spantree statistics <module#/port#> <vlan#>
gives accurate information on suspected ports.
Hot Standby Routing Protocol (HSRP)
Provides a means of having two default gateways to protect
against an equipment failure locking out a group of users from the wider
The default priority for each router is 100, but can be
change to give one priority as the most likely default gateway (if say,
one unit were faster than another).
TAC and CCO
Cisco’s Technical Assistance Center (TAC) provides 7x24x365
technical support on all their products. The Center follows the sun, with
offices around the globe. It is staffed by Customer Support Engineers
Cases can be opened by phone, e-mail or through Cisco
Connection Online (Cisco’s exceptionally well designed website at http://www.cisco.com/). When a case is
logged, a call number is generated and assigned to a CSE who will work
with client to answer questions, provide advice on system use, help with
system configuration, or correct a system malfunction.
There are four priority levels
Priority 1 - existing network is "down", which is critical
Priority 2 - network is severely degraded, which has a significant
Priority 3 - operational performance of the network is impaired,
although business operations remain functional
Priority 4 - little or no impact to the business operation at this
The OSI Model
The OSI is a common tool for conceptualizing how network
traffic is handled. In this document we will be interested primarily in
the lower four levels. Just a reminder, that you can use the old mnemonic
“All People Seem To Need Data
Processing” as a way to help remember the sequence.
7. Application – User interface tools (such as
Telnet, SMTP, FTP, etc.)
6. Presentation – Encoding/Decoding
(such as ASCII, MPEG, GIF, JPEG, etc.)
5. Session – Creating,
managing and terminating Presentation layer
4. Transport –
Error checking and recovery, flow control and multiplexing (TCP, SPX,
3. Network – Routing (IP, IPX, etc.)
LLC – Manages communications
MAC – Manages
addressing and access to the physical layer
1. Physical –
Establish and maintain physical connectivity
Cisco Hierarchical Internetworking Model
– The point at which
users join the network. VLANs, WAN connections, RAS services are all at
this layer. Cat1900 or 3500 series switches with 10BaseTx and 100BaseTx
ports might be appropriate for the Access layer, where high port density
and low per-port costs are major concerns.
– Control layer;
Aggregation of traffic, access lists, compression, encryption and other
services that provide the glue between Access and Core layers. Cat6000
series with 16-port Gigabit Ethernet modules, or Cat5500 series switches
with internal RSMs might be appropriate at the Distribution layer.
Performance becomes more of an issue at this layer.
– Concentrates all
traffic traversing the network. The focus is on speed. Fast switching,
Gigabit Ethernet, and ATM are commonly deployed at this layer. Cat8500
or 6500 series switches - high speed and expensive - are probably most
appropriate at the core, where passing traffic is the primary