Refer to the Exhibit.
In the exhibit, R2 is receiving external routing information for the 192.168.16.0/24 prefix and is
redistributing it into IS-IS. R1 has a policy that leaks the 192.168.16.0/24 route into Area 49.1111.
R3 has a policy that leaks the 192.168.16.0/24 route into Area 49.2222. However, the IS-IS
version of the route does not appear in R2’s routing table.
Why does R3’s route leaking policy appear not to be working?
The Up/Down bit is set to down for the prefix.
The external flag is set for the prefix.
You can only leak routes from Level 2 to Level 1.
R2 already has better routing information for the prefix.
One Comment on “Why does R3’s route leaking policy appear not to be working?”
When you want to modify the default leaking policies, you might trigger routing loops. For example a /32 leaked from L2 to L1 then leaked back from L1 to L2 and so on. This is not possible because ISIS includes a specific flag (per IP reachability – aka per prefix) which informs if a prefix has been leaked. This flag is called the Distribution bit (aka UP/DOWN bit). When a route is leaked by a policy from the Level 2 to the Level 1, the UP/DOWN bit of this route is set to DOWN which means that the route comes from the Level 2. Then a DOWN route can never be leaked back from L1 to L2.