What is IS-IS route leaking, and when do you need it?
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By default L1 routers get only a default route out of their area. Route leaking copies chosen L2 prefixes into the L1 LSP of an L1/L2 router, so L1 routers see specific routes. You need it when an area has two exits and you want the best exit per destination, and in MPLS networks where PE loopbacks must be known as exact /32s for LDP to build label switched paths. Leaked prefixes have the up/down bit set, which stops them being sent back up into L2 and causing a loop (RFC 5302).
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Configuration on IOS
ip prefix-list PE-LOOPBACKS seq 10 permit 10.255.0.0/16 ge 32
!
route-map LEAK-L2-TO-L1 permit 10
match ip address prefix-list PE-LOOPBACKS
!
router isis CORE
redistribute isis ip level-2 into level-1 route-map LEAK-L2-TO-L1IOS-XR uses propagate level 2 into level 1 route-policy for the same job.
What the L1 router sees afterwards
R1# show ip route isis | include ia
i ia 10.255.0.3/32 [115/30] via 10.1.12.2, 00:01:05, GigabitEthernet0/0/0ia marks an inter-area route that was leaked from L2.
Why MPLS cares
LDP only binds a label to a prefix that matches the routing table entry exactly. A remote PE's loopback reached through a default route has no label, so the LSP to it breaks and BGP VPN routes that use it as next hop fail. Leaking the /32 loopbacks fixes that.
They will ask next
What is the up/down bit for?
It marks a prefix that came down from L2 into L1. An L1/L2 router never advertises a prefix with that bit back into L2, so a leaked route cannot loop back up and compete with the real one.
Traps
- Leaking everything. It defeats the point of L1 areas. Leak only what needs a specific path, such as loopbacks.