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ABF on XR with multiple next hops  BE20.4000-ASR9k--------Router------


Lets take a scenario where ACL based forwarding (ABF) is configured with multiple next hops, and what happens if the next hops are up/down.

This document discusses different scenarios so that customers can understand how ABF works in different scenarios. ABF is a ingress only feature and implemented using ACL. ABF supports up to three next hops.


router static
 address-family ipv4 unicast Null0 254

ipv4 access-list test
 10 permit ipv4 host host nexthop1 ipv4 nexthop2 ipv4

interface Bundle-Ether20.4000
 ipv4 address x.x.x.x x.x.x.x
 encapsulation dot1q 4000
 ipv4 access-group test test

Scenario A)
Network is learnt via OSPF
Network is learnt via iBGP

In a scenario like this both the next hops are available in this case and based on CEF entry ABF will choose the first nexthop

Scenario B)
Network vanishes from OSPF
Network is learnt via iBGP

In the second scenario the first next hop will be not there and CEF will resolve to second nexthop

Scenario C)
Network is learnt via OSPF
Network vanishes from iBGP

Since the first next hop is reachable, next hop will be  taken
Scenario D)
Network vanishes from OSPF
Network vanishes from iBGP

In this scenario first next hop is not there and for the second next hop iBGP route is gone
but the static route with null0 will kick in and drop the packets.

ABF wil never use default route for deciding whether the next hop is reachable or not.

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Last update:
‎12-03-2012 02:03 PM
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