The route-map FILTER contains a single sequence, deny 10, which matches and denies the 1.1.1.0/24 prefix. However, all route-maps have an implicit, invisible deny all statement at the end. Consequently, any route that does not match sequence 10 (i.e., all routes other than 1.1.1.0/24) will be processed by this implicit deny and will also be dropped. This results in all routes from the neighbor being filtered.

To fix this, a second sequence must be added to explicitly permit all other routes. Adding route-map FILTER permit 20 with no match clause will match all routes that were not denied by sequence 10, thereby permitting them and restoring connectivity.

A. Add a second line in the access list to permit any: This would cause the deny 10 sequence in the route-map to match and deny all routes, not just the intended prefix.

B. Modify the route map to permit the access list instead of deny it: This would permit only 1.1.1.0/24, while all other routes would be blocked by the implicit deny at the end of the route-map.

C. Modify the access list to deny instead of permit it: A deny statement in an access list used by a match clause means the prefix is not considered a match, so it would fall to the implicit deny.

1. Cisco Systems

Inc. (2021). IP Routing: BGP Configuration Guide

Cisco IOS XE Cupertino 17.6.x. In the "Implementing BGP Route Maps" section

the documentation states

"If the route does not match any of the match commands in a route map

it is filtered (denied) by the implicit deny that exists at the end of every route map. To permit routes that are not explicitly denied

you must configure a permit statement without any match commands at the end of the route map." This directly supports the addition of a permit sequence to allow other routes.

2. Cisco Systems

Inc. (2023). Cisco IOS IP Routing: BGP Command Reference. Under the route-map command description

the behavior is detailed: "If you end the route-map statements without a permit or deny action

and a packet does not match any of the match statements

the packet is not forwarded." This confirms the implicit deny behavior.

3. Cisco Systems

Inc. (2023). Cisco IOS IP Routing: BGP Command Reference. The documentation for the match ip address command explains its interaction with access lists. It clarifies that the route-map sequence action (permit or deny) is taken only if the prefix matches a permit entry in the specified access list. This invalidates option C

as a deny in the ACL would prevent a match.