ISIS Operation


Attached bit sent from L1/L2 router towards L1 routers to instruct them to generate a default route for reachability to Level 2
Level 1 routes generated with up bit set and are sent into Level 2 via Level1/2 router. Routes with up bit set are not sent downwards again
to other Level 1 areas
Level 2 routes that require leaking into Level 1 require a routing policy specifying routes from Level 2 towards Level 1. This policy sets
the up/down bit to down to allow the routes into Level 1. The routes are not sent back up to Level 2 when the down bit is set to down, this
prevents a loop from forming across Level 2
Externally redistributed routes are not sent from the Level they are injected into other levels by default. A routing policy is required to
leak the routes between Levels

lab@r6> show isis database r7 detail IS-IS level 1 link-state database:

r7.00-00 Sequence: 0x3, Checksum: 0x7247, Lifetime: 539 secs

IS neighbor: IP prefix: IP prefix: IP prefix:

r7.02 Metric: Metric: Metric: Metric:10
0 Internal 10 Internal 10 Internal

r7.02-00 Sequence: 0x1, Checksum: 0xfdbc, Lifetime: 539 secs

IS neighbor: r6.00 Metric: 0 IS neighbor: r7.00 Metric: 0

IS-IS level 2 link-state database:

This display confirms that r7 is acting as the pseudonode for the subnet, as indi- cated by its generation
of an LSP with a non-zero selector field. Because all level 2 IS-IS processing has been disabled in r6 and r7, we find
that the level 2 database is empty as expected.