QuestionQ221

Layer 3 Technologies

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Refer to the exhibit. The Customer Edge router needs to use AS 100 as the preferred ISP for all external routes, with ISP-2 as the backup.

Customer-Edge -

route-map SETAS  
set as-path prepend 111  
!  
router bgp 64555  
neighbor 192.168.1.1 remote-as 100  
neighbor 192.168.2.2 remote-as 200  
neighbor 192.168.2.2 route-map SETAS in  

After this configuration, all backup routes have disappeared from the BGP table on the Customer Edge router. Which configuration set resolves the issue on the Customer Edge router?

  • A route-map SETASset as-path prepend 200!router bgp 64555neighbor 192.168.1.1 remote-as 100neighbor 192.168.2.2 remote-as 200neighbor 192.168.2.2 route-map SETAS in
  • B route-map SETASset as-path prepend 111!router bgp 64555neighbor 192.168.1.1 remote-as 100neighbor 192.168.2.2 remote-as 200neighbor 192.168.2.2 route-map SETAS out
  • C route-map SETASset as-path prepend 200!router bgp 64555neighbor 192.168.1.1 remote-as 100neighbor 192.168.2.2 remote-as 200neighbor 192.168.2.2 route-map SETAS out
  • D route-map SETASset as-path prepend 111!router bgp 64555neighbor 192.168.2.2 remote-as 100neighbor 192.168.1.1 remote-as 200neighbor 192.168.1.1 route-map SETAS in
Explanation

BGP selects a route with a shorter AS_PATH when earlier best-path attributes are equal. Applying an inbound route map to the ISP-2 neighbor and prepending AS 200 increases the received backup path length, causing equivalent paths learned through AS 100 to be preferred while ISP-2 routes remain available as backup paths. Cisco documents that set as-path prepend modifies the AS path and can be applied in inbound or outbound BGP route maps.

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