300-410 ENARSI: Implementing Cisco Enterprise Advanced Routing and ServicesDemo

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QuestionQ168

Layer 3 Technologies

R1 and R2 are configured as eBGP neighbors. R1 resides in AS100, and R2 resides in AS200. R2 advertises the following networks to R1:

  • 172.16.16.0/20
  • 172.16.3.0/24
  • 172.16.4.0/24
  • 192.168.1.0/24
  • 192.168.2.0/24
  • 172.16.0.0/16

The network administrator on R1 must improve convergence by preventing all subnets of the 172.16.0.0/16 major network with a mask lower than 23 from being received.

Which configuration set accomplishes this task on R1?

  • A ip prefix-list PL-1 deny 172.16.0.0/16 ge 23 ip prefix-list PL-1 permit 0.0.0.0/0 le 32 ! router bgp 100 neighbor 192.168.100.2 remote-as 200 neighbor 192.168.100.2 prefix-list PL-1 in
  • B ip prefix-list PL-1 deny 172.16.0.0/16 le 23 ip prefix-list PL-1 permit 0.0.0.0/0 le 32 ! router bgp 100 neighbor 192.168.100.2 remote-as 200 neighbor 192.168.100.2 prefix-list PL-1 in
  • C ip prefix-list PL-1 deny 172.16.0.0/16 ip prefix-list PL-1 permit 0.0.0.0/0 ! router bgp 100 neighbor 192.168.100.2 remote-as 200 neighbor 192.168.100.2 prefix-list PL-1 in
  • D access-list 1 deny 172.16.0.0 0.0.254.255 access-list 1 permit any ! router bgp 100 neighbor 192.168.100.2 remote-as 200 neighbor 192.168.100.2 distribute-list 1 in
Explanation

A prefix-list entry for 172.16.0.0/16 le 23 matches prefixes within 172.16.0.0/16 with lengths from /16 through /23, so a deny entry filters the required less-specific routes on inbound BGP updates. The following permit 0.0.0.0/0 le 32 entry permits all remaining routes, including the /24 prefixes and unrelated networks. Cisco documents that le defines the upper bound of the matched prefix-length range.

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