Refer to the exhibit. Excessive routes are flooding from network 150.0.0.0 into AS100. Internet traffic between AS400 and AS300 is working normally. No route controlling mechanism is applied on…

Question

Exhibits

Options

• AR2#router bgp 200 address-family ipv4 unicast neighbor 12.12.12.2 remote-as 100 neighbor 23.23.23.2 remote-as 300 neighbor 12.12.12.2 filter-list 1 out exit-address-family ! route-map PREPEND permit 10 set as-path prepend 100 100
• BR2#router bgp 200 neighbor 12.12.12.2 remote-as 100 neighbor 23.23.23.2 remote-as 300 neighbor 12.12.12.2 filter-list 1 out ip as-path access-list 1 deny ^400$ ip as-path access-list 1 permit .*
• CR1#router bgp 100 neighbor 12.12.12.1 remote-as 200 neighbor 12.12.12.1 route-map SET-LOCAL-PREF in route-map SET-LOCAL-PREF permit 10 match ip address 2 set local-preference 700 route-map SET-LOCAL-PREF permit 20 access-list 2 permit 150.0.0.0 0.255.255.255 access-list 2 deny any
• DR2#router bgp 200 neighbor 12.12.12.2 route-map FLOODING out ip as-path access-list 1 permit ^400_ route-map FLOODING permit 10 match as-path 1 set metric 50000

• A
  4% (2)
• B
  61% (28)
• C
  24% (11)
• D
  11% (5)

Explanation

Option B correctly filters outbound BGP advertisements from R2 (AS200) to AS100 by applying filter-list 1 out on the neighbor facing AS100 (12.12.12.2). The AS-path access-list denies routes whose entire AS path is ^400$ (originated solely in AS400) while permitting everything else, which stops 150.0.0.0 routes from flooding into AS100 without disrupting AS400↔AS300 traffic.

Why the distractors fail:

• A defines a route-map with AS-path prepending but never applies it to a neighbor, and is missing the actual ip as-path access-list definition - the filter-list has no backing ACL, making it incomplete and non-functional.
• C sets local-preference on R1 (AS100) for incoming routes - local preference controls which exit path the local AS prefers, it does not filter or block route advertisements from being received.
• D applies a route-map that sets a high MED (metric 50000) on AS400 routes going to AS100 - MED influences path preference, it does not suppress advertisement of the routes themselves, so flooding continues.

Memory tip: To block routes from being advertised to a neighbor, you need a filter-list or route-map with an explicit deny action - anything that only modifies attributes (prepend, local-pref, MED) leaves the route in the table. The regex anchor pair ^X$ means "AS path is exactly X with no other ASes," perfect for catching routes that originated directly in a specific AS.

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