IS IS Route Leaking


Route leaking is a way to de-aggregation from the default route sent by the L1L2 router and solving the suboptimal routing if the L1L2 route is not optimal.

Route Leaking

  • This feature makes sense to use with multiple L1L2 routers.
  • Used to leak specific routes from L2 to L1. By default L1 will only get a default route and are considered to be stub area.
  • Route leaking can be used to select which L2 router to choose for certain routes, in a multiple L1L2 designs.
  • Configured using a redistribution, but instead of other protocols redistributes ISIS protocol.
  • Route-map did not work in the redistribution.
  • ACL extended only allowed for distribution list.

Figure 1 - Packet capture of ISIS LSP showing UP and DOWN bits set.

  • There is a loop prevention mechanism implemented.
    • L1 routes with UP bit set - are natively advertised and eligible for leaking.
    • L1 routes with DOWN bit set - are already leaked and are not leaked back to L2 area. L1L2 sets it to down during leaking, if any prefixes are received with down bit set, the loop prevention mechanism stops them.
  • Leaked routes are displayed as interarea in the routing table with ia flag:

i ia [115/168] via, 00:04:05, GigabitEthernet0/0

Relevant Commands

redistribute isis ip level-2 into level-1 distribute-list

R1(config-router)#        redistribute isis [instance] ip level-2 into level-1 distribute-list  (ext acl)

show clns protocol

show clns protocol

Show isis database level-1 detail

show isis database level-1 R3.00-00 detail

Additional Resources

IS-IS Route Leaking - Amits Cisco Zone KB
IS-IS Route Leaking Overview - Cisco


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