Prefix suppression

• Topology  

• Task

Configure ISIS so that only the loopbacks are advertised into the IGP. Only use interface-level commands on XR2 and XE7.

• Answer

XR1,XR3-XR4
router isis
 address-family ipv4 unicast
  advertise passive-only

XR2
router isis 1
 interface GigabitEthernet0/0/0/0
  suppressed
 interface GigabitEthernet0/0/0/1
  suppressed
 interface GigabitEthernet0/0/0/2
  suppressed

XE5-XE6,XE8
router isis
 advertise passive-only

XE7
interface GigabitEthernet1
 no isis advertise prefix
interface GigabitEthernet2
 no isis advertise prefix
interface GigabitEthernet3
 no isis advertise prefix

• 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.