Forwarding Adjacency

Load mpls.te.forwarding.adjacency.init.cfg

#IOS-XE
config replace flash:mpls.te.forwarding.adjacency.init.cfg

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

A bidirectional TE tunnel is setup between CSR8 and CSR10 via CSR9. Use forwarding adjacency and IGP metric (if necessary) to force CSR2 to use only CSR10 to get to 8.8.8.8/32.

Answer

Using forwarding adjacency announces the tunnel as an adjacency to the IGP. CSR8 lists an adjacency to CSR10 with the ISIS default metric (which is 10):

Likewise CSR10 announces the same adjacency to CSR8, and now all routers in the IGP include this link in the graph of the topology.

We will now see that CSR2 is doing ECMP to get to 8.8.8.8/32:

i L2     8.8.8.8 [115/20] via 132.2.10.10, 00:00:09, GigabitEthernet2.520
                 [115/20] via 132.2.9.9, 00:00:09, GigabitEthernet2.529

To force CSR2 to only use CSR10’s tunnel, we can change the IGP metric of the tunnel or change the CSR2-CSR10 link cost. I thought we could change the TE tunnel’s isis metric but that doesn’t seem to work on CSR1000v. Instead I changed the link cost. (Note that on XRv, changing the ISIS metric of the tunnel does work, by configuring it under ISIS just like any other interface).

#CSR2, CSR10
int gi2.520
 isis metric 9

CSR2 only uses CSR10 to get to 8.8.8.8/32:

i L2     8.8.8.8 [115/19] via 132.2.10.10, 00:00:01, GigabitEthernet2.520