IEEE 802.1Q Standard


The IEEE 802.1Q standard is an industry standard for trunking and can be used when a Cisco device needs to have a trunked link to a non-Cisco device. On the Cisco router, you can use the encapsulation dot1q command on Cisco IOS version 12.0.1(t) or higher.

You can configure the trunk link to use the IEEE 802.1q protocol on the Catalyst 5000 series software version 4.1 or later, using the set trunk command as shown below:

 DCSCatalyst5000>(enable)  set trunk 2/1 ?  <mode>                     Trunk mode                                 (on,off,desirable,auto,nonegotiate)   <type>                     Trunk type                                  (isl,dot1q,dot10,lane,negotiate)   <vlan>                     VLAN number DCSCatalyst5000>(enable)  set trunk 2/1 on dot1q  Port(s)  2/1 trunk mode set to on. Port(s)  2/1 trunk type set to dot1q. DCSCatalyst5000>(enable) 

The output above shows the module 2 port 1 set to use the IEEE 802.1q trunking protocol. The 802.1Q header is somewhat different than that of the ISL header. Only 4 bytes are added to the frame. ISL adds 30 bytes to an ISL header and trailer. Instead of encapsulating the frame, as ISL does, 802.1Q adds the VLAN number inside the frame.

Most trunking issues result from a misconfiguration of the VLANs or the interfaces on each side of a trunk link. ISL and 802.1Q cannot be used on the same trunk link.

Inter-VLAN Routing Troubleshooting

The Route Switch Module (RSM) is one of many external modules and cards called internal route processors that are available for the 5000 and 6000 families of switches. These cards and modules give the switch a local resource to resolve Layer 3 addresses and perform inter-VLAN routing.

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Although you need to know about the RSM role in troubleshooting and that it uses an IOS similar to that used on a router, you need to learn about other cards as well. These cards include the Multilayer Switch Module (MSM), Multilayer Switch Feature Card (MSFC), Route Switch Feature Card (RSFC), FlexWAN module, NetFlow Feature Card (NFFC), and NetFlow Feature Card II (NFFCII).


The internal route processor can be used to provide routing between VLANs. Using an external router to route between VLANs adds additional overhead and complexity that an internal route processor can relieve. An internal route processor on the switch uses an internal connection to the switch and does not need to use up an interface on the switch to create a trunk port from the switch to the internal route processor.

An external router should be used when you need a higher performance level, such as that provided by the 7000 series routers or the 12000 Gigabit Switch Router (GSR). You can also combine the RSM with a NetFlow Feature Card (NFFC or NFFC II), which is a daughter card that can provide multilayer switching (MLS).

The internal route processor uses an IOS that is similar to that used on a router. To access an internal route processor on the Cisco Catalyst 5000 family of switches, use the session command followed by the slot number:

 catalyst5000>(enable)  session 3  Building configuration... Current configuration: ! version 11.3 service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname RSM1 ! interface Vlan1  description VLAN1  ip address 38.187.128.10 255.255.255.0  no ip redirects  standby 1 timers 5 15  standby 1 priority 10  standby 1 preempt  standby 1 ip 38.187.128.11 

To configure an external router for each type of connection, each VLAN must be defined on a subinterface, and the main interface must be left without a configuration. To keep track of the VLANs on an interface, it's good practice to make the subinterface number the same as the VLAN number. The following code displays the commands involved in configuring FastEthernet port 2, subinterface 1 for VLAN 1, and setting the encapsulation method on the link for ISL trunking:

  interface fastethernet 0/2   no ip address   no shutdown   full-duplex   interface fastethernet 0/2.1   description vlan1   ip address 10.1.1.1 255.255.255.0  encapsulation isl 1 

show vtp domain Command

The VLAN Trunk Protocol (VTP) is used to maintain a consistent VLAN configuration throughout the switches in the network. In a VTP management domain, a configuration change is done only once on a VTP server “configured switch. The new configuration is propagated throughout the network. The show vtp domain command provides the status and configuration information for VTP.

An example of using the show vtp domain command follows :

 DCSCatalyst5000>(enable)  show vtp ?  domain                     Show VTP domain information   statistics                 Show VTP statistic information DCSCatalyst5000>(enable) show vtp domain Domain Name  Domain Index  VTP Version  Local Mode  Password -----------  ------------  -----------  ----------  -------- DCS     1             2            server      sean1 Vlan-count Max-vlan-storage Config Revision Notifications ---------- ---------------- --------------- ------------- 5          1023             355             enabled Last Updater    V2 Mode  Pruning  PruneEligible on Vlans ------------    -------  -------  ---------------------- 68.38.127.5     enabled  enabled  2-1000 

The show vtp command is used to gather similar information on the Cisco CLI-based switches. There are three syntaxes. We will look at the output from the counters and status syntaxes below:

 4506#  show vtp ?  counters  VTP statistics   password  VTP password   status    VTP domain status 4506#show vtp status VTP Version                     : 2 Configuration Revision          : 17 Maximum VLANs supported locally : 1005 Number of existing VLANs        : 5 VTP Operating Mode              : Server VTP Domain Name                 : DCS VTP Pruning Mode                : Disabled VTP V2 Mode                     : Disabled VTP Traps Generation            : Disabled MD5 digest                      : 0xC6 0x5B 0x77 0xDD 0xA1 0x84 0x14 0xC3 Configuration last modified by 0.0.0.0 at 3-24-02 10:08:05 Local updater ID is 10.1.2.55 on interface Vl1       (lowest numbered VLAN interface found) 4506#  show vtp counters  VTP statistics: Summary advertisements received    : 0 Subset advertisements received     : 0 Request advertisements received    : 0 Summary advertisements transmitted : 0 Subset advertisements transmitted  : 0 Request advertisements transmitted : 0 Number of config revision errors   : 0 Number of config digest errors     : 0 Number of V1 summary errors        : 0 VTP pruning statistics: Trunk         Join Transmitted Join Received    Summary advts received from                                                 non-pruning-capable device ------------- ---------------- ---------------- --------------------------- 4506# 

The above output shows the different items displayed by the show vtp counters command. The VTP counters are shown as zeros since the interface counters have just been reset.



CCNP CIT Exam Cram 2 (642-831)
CCNP CIT Exam Cram 2 (Exam Cram 642-831)
ISBN: 0789730219
EAN: 2147483647
Year: 2003
Pages: 213
Authors: Sean Odom

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