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TE Tunnel with Explicit-Null

Load mpls.te.base.config.with.ospf.cfg

#IOS-XE
config replace flash:mpls.te.base.config.with.ospf.cfg

#IOS-XR
configure
load bootflash:mpls.te.base.config.with.ospf.cfg
commit replace
y

Configure bi-directional TE tunnels between CSR8 and XR11 that takes the path:

  • CSR8

  • CSR9

  • XR12

  • XR11

Configuring the least number of routers, ensure that an explicit-null is used at the penultimate hop for both TE tunnels.

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Answer

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Explanation

By default, both IOS-XE and IOS-XR tailends advertise an implicit-null to their previous hop (phop). To change this, we must globally configure the tailends to advertise an explicit-null instead.

However, on IOS-XE, an explicit-null is interpretted as implicit-null, and a Pop action will still happen. This means that no matter what you advertise on the tailend router, the IOS-XE phop will always do a Pop action. We can see this on R9 before adding the “interpret” command:

R9 has a “Pop” action even though R8 is advertising exp-null for the tunnel for which is it acting as tailend:

To change this, we must use the following command on R9:

Note that this change is disruptive. R9 sends a PathError message up to XR11, prompting XR11 to re-signal the tunnel. R9 is now using explicit-null:

IOS-XR already interprets exp-null as exp-null. For this reason, no configuration on XR12 is necessary. It is already using exp-null as the outgoing label for the tunnel for which XR11 is the tailend:

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