Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) — Question 190

R1 and R2 are configured as eBGP neighbors. R1 is in AS100 and R2 is in AS200. R2 is advertising these 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 blocking all subnets of 172.16.0.0/16 major network with a mask lower than 23 from coming in.
Which set of configurations accomplishes the task on R1?

Answer options

• 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

Correct answer: B

Explanation

The correct answer, B, uses a prefix-list to deny any subnet of 172.16.0.0/16 that has a prefix length less than or equal to 23, effectively blocking the specified subnets. Option A incorrectly uses 'ge 23', which would deny subnets with a prefix length greater than or equal to 23, not the intended subnets. Option C fails to specify the correct prefix length requirement and would not block the necessary subnets. Option D uses an access-list, which is not the appropriate method for filtering BGP updates in this scenario.