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Routing & Switching · Advanced

IS-IS Route Leaking

Advertising selected Level 2 prefixes into Level 1 so L1 routers choose optimal exits instead of the nearest default.

Definition

Leaked routes carry the up/down bit to prevent them from being re-advertised back into Level 2, avoiding loops. Route leaking is also needed so PE loopbacks are visible for MPLS LSPs across areas.

Example

redistribute isis ip level-2 into level-1 route-map LEAK

Exam tip

The up/down bit marks leaked prefixes; it prevents L2-to-L1-to-L2 loops.

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