Prefix Suppression

Load isis.cfg

#IOS-XE
config replace flash:isis.cfg

#IOS-XR
configure
load bootflash:isis.cfg
commit replace
y

Configure ISIS so that only the loopbacks are advertised into the IGP. Only use interface-level commands on R2 and XR2.

Answer

#R1, R3-R6
router isis
 advertise passive-only

#R2
int gi2.12
 no isis advertise prefix
int gi2.23
 no isis advertise prefix
int gi2.24
 no isis advertise prefix

#XRv1
router isis 1
 add ipv4
  advertise passive-only

#XRv2
router isis 1
 int gi0/0/0/0.1920
  suppressed

Explanation

Previously, routers advertised their loopback (using the passive-interface Lo0 command) and their directly connected interface network on all interfaces that had ISIS activated:

In ISIS, the link address is not part of the underlying topology. It is simply advertised as an additional prefix in the IP reachability TLV. By configuring advertise passive-only we instruct the router to suppress all prefixes except for the passive interfaces, which in our case is only Lo0.

This can be done on a router-by-router basis. It does not need to match within the network.

The question might arise then: How does R2 have the correct nexthop for R1’s loopback? We don’t see R1 advertising its nexthop anywhere in the LSP. The answer is that R1 advertises its interface address in the Hello. This is how R2 knows what to use for the nexthop to 1.1.1.1.