— Exhibit –
— Exhibit —
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?
A.
The Up/Down bit is set to down for the prefix.
B.
The external flag is set for the prefix.
C.
You can only leak routes from Level 2 to Level 1.
D.
R2 already has better routing information for the prefix.
Explanation:
http://inetzeroblog.com/isis-training-and-junos-configuratio/
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.