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ABF on XR with multiple next hops

10.10.0.1--  BE20.4000-ASR9k--------Router------192.168.10.1-------Router----10.20.0.1

                                           |-------------Router---------192.168.20.1---------|

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.

SAMPLE CONFIGURATION:

router static
 address-family ipv4 unicast
  0.0.0.0/0 192.168.100.1
  192.168.20.0/24 Null0 254

ipv4 access-list test
 10 permit ipv4 host 10.10.0.1 host 10.20.0.1 nexthop1 ipv4 192.168.10.1 nexthop2 ipv4 192.168.20.1

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 192.168.10.0/28 is learnt via OSPF
Network 192.168.20.0/24 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 192.168.10.1

Scenario B)
Network 192.168.10.0/28 vanishes from OSPF
Network 192.168.20.0/24 is learnt via iBGP

In the second scenario the first next hop will be not there and CEF will resolve to second nexthop
192.168.20.1

Scenario C)
Network 192.168.10.0/28 is learnt via OSPF
Network 192.168.20.0/24 vanishes from iBGP

Since the first next hop is reachable, next hop 192.168.10.1 will be  taken
Scenario D)
Network 192.168.10.0/28 vanishes from OSPF
Network 192.168.20.0/24 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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