Troubleshooting and Verification 71

tag switching is being used in the service provider network, the command show tag-switching interfaces should be used. The output of the two commands is virtually identical in Cisco IOS. If MPLS or tag switching is not configured for an interface, you’ll need to add it. If MPLS or tag switching is configured on an interface that it does not need to be configured on, use the no form of the MPLS or tag switching command to disable it.

In the network in Figure 2.7 that we’re using as a troubleshooting example, the Atlanta POP, Core, and Raleigh POP routers have been configured with tag switching.

On the Atlanta POP router, only the Serial 0/0 interface has tag switching enabled. To verify this, execute the show tag-switching interfaces command on the Atlanta POP router as follows:

Atlanta#show tag-switching interfaces

On the Core router, the Serial 0/0 and Serial 0/1 interfaces have tag switching enabled. To verify this, execute the show tag-switching interfaces command on the Core router as follows:

On the Raleigh POP router, only the Serial 0/3 interface has tag switching enabled. To verify this, execute the show tag-switching interfaces command on the Raleigh POP router as follows:

Raleigh#show tag-switching interfaces

Label Distribution and Bindings

The IOS command you use to verify that labels are being exchanged between neighbors depends on whether you are using MPLS or tag switching. To verify that MPLS labels are being exchanged, use the show mpls ldp discovery command. If tag switching is being used in the service provider network, the command show tag-switching tdp discovery should be used. In the network in the troubleshooting example, tag switching is being used.

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The output of the show tag-switching tdp discovery command as executed on the Atlanta POP router is as follows:

Atlanta#show tag-switching tdp discovery

Local TDP Identifier: 204.134.83.1:0

TDP Discovery Sources: Interfaces:

Serial0/0: xmit/recv

TDP Id: 204.134.83.2:0

There are many similarities in output between the MPLS and tag switching versions of commands executed on a Cisco IOS router. For example, if the show mpls ldp discovery command is used, the output displays LDP.

The output of the show tag-switching tdp discovery command as executed on the Core router is as follows:

Core#show tag-switching tdp discovery

Local TDP Identifier: 204.134.83.2:0

TDP Discovery Sources: Interfaces:

Serial0/0: xmit/recv

TDP Id: 204.134.83.3:0 Serial0/1: xmit/recv

TDP Id: 204.134.83.1:0

The output of the show tag-switching tdp discovery command as executed on the Raleigh POP router is as follows:

Raleigh#show tag-switching tdp discovery

Local TDP Identifier: 204.134.83.3:0

TDP Discovery Sources: Interfaces:

Serial0/3: xmit/recv

TDP Id: 204.134.83.2:0

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If you don’t see a neighbor, and you are certain that either tag switching or MPLS has been enabled, you’ll need to verify the neighbor with the ping command. If you can’t ping the missing neighbor, you need to do basic troubleshooting such as verifying that the interface is properly attached, is up, or has the proper encapsulation to fix the connectivity problem.

Binding Verification

If you want more information on all the label bindings in the network, there are many IOS commands you can use. You have already learned about the show mpls forwarding-table and show tag-switching forwardingtable commands. If you really want to get the nitty-gritty on labels (or tags), use the show mpls ldp bindings or show tag-switching tdp bindings command.

The output of the show tag-switching tdp bindings command as executed on the Atlanta POP router is as follows:

Atlanta#show tag-switching tdp bindings

tib entry: 192.168.3.4 255.255.255.252, rev 16 local binding: tag: imp-null

tib entry: 204.134.83.1 255.255.255.255, rev 4 local binding: tag: imp-null

remote binding: tsr: 204.134.83.2:0, tag: 26 tib entry: 204.134.83.2 255.255.255.255, rev 8

local binding: tag: 28

remote binding: tsr: 204.134.83.2:0, tag: imp-null tib entry: 204.134.83.3 255.255.255.255, rev 6

local binding: tag: 27

remote binding: tsr: 204.134.83.2:0, tag: 27 tib entry: 204.134.83.4 255.255.255.252, rev 10

local binding: tag: imp-null

remote binding: tsr: 204.134.83.2:0, tag: imp-null tib entry: 204.134.83.8 255.255.255.252, rev 2

local binding: tag: 26

remote binding: tsr: 204.134.83.2:0, tag: imp-null

The output of the show tag-switching tdp bindings command as executed on the Core router is as follows:

Core#show tag-switching tdp bindings

tib entry: 192.168.3.4 255.255.255.252, rev 15

remote binding: tsr: 204.134.83.1:0, tag: imp-null

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tib entry: 192.168.3.8 255.255.255.252, rev 16

remote binding: tsr: 204.134.83.3:0, tag: imp-null tib entry: 204.134.83.1 255.255.255.255, rev 4

local binding: tag: 26

remote binding: tsr: 204.134.83.3:0, tag: 26 remote binding: tsr: 204.134.83.1:0, tag: imp-null

tib entry: 204.134.83.2 255.255.255.255, rev 8 local binding: tag: imp-null

remote binding: tsr: 204.134.83.3:0, tag: 27 remote binding: tsr: 204.134.83.1:0, tag: 28

tib entry: 204.134.83.3 255.255.255.255, rev 6 local binding: tag: 27

remote binding: tsr: 204.134.83.3:0, tag: imp-null remote binding: tsr: 204.134.83.1:0, tag: 27

tib entry: 204.134.83.4 255.255.255.252, rev 10 local binding: tag: imp-null

remote binding: tsr: 204.134.83.3:0, tag: 28 remote binding: tsr: 204.134.83.1:0, tag: imp-null

tib entry: 204.134.83.8 255.255.255.252, rev 2 local binding: tag: imp-null

remote binding: tsr: 204.134.83.3:0, tag: imp-null remote binding: tsr: 204.134.83.1:0, tag: 26

The output of the show tag-switching tdp bindings command as executed on the Raleigh POP router is as follows:

Raleigh#show tag-switching tdp bindings

tib entry: 192.168.3.8 255.255.255.252, rev 16 local binding: tag: imp-null

tib entry: 204.134.83.1 255.255.255.255, rev 4 local binding: tag: 26

remote binding: tsr: 204.134.83.2:0, tag: 26 tib entry: 204.134.83.2 255.255.255.255, rev 8

local binding: tag: 27

remote binding: tsr: 204.134.83.2:0, tag: imp-null tib entry: 204.134.83.3 255.255.255.255, rev 6

local binding: tag: imp-null

remote binding: tsr: 204.134.83.2:0, tag: 27