The Cisco BGP router must be configured to reject inbound route advertisements for any prefixes belonging to the local autonomous system (AS).

Accepting route advertisements belonging to the local AS can result in traffic looping or being black holed, or at a minimum using a non-optimized path.

Review the router configuration to verify that it will reject routes belonging to the local AS.

Step 1: verify that an inbound route policy has been configured for each external neighbor as shown in the example below.

router bgp xx address-family ipv4 unicast ! neighbor x.1.23.3 remote-as yy keychain YYY_KEY_CHAIN ttl-security address-family ipv4 unicast route-policy BGP_FILTER_INBOUND in ! ! neighbor x.1.24.4 remote-as zz keychain ZZZ_KEY_CHAIN ttl-security address-family ipv4 unicast route-policy BGP_FILTER_INBOUND in ! !

Step 2: Review the route policy to determine if it is filtering at a minimum local prefixes as shown in the example below.

route-policy BGP_FILTER_INBOUND if destination in LOCAL_PREFIX then drop else pass endif end-policy

Note: If bogons are also filtered per previous requirement, the route policy would look similar to the following example:

route-policy BGP_FILTER_INBOUND if destination in BOGON_PREFIXES then drop elseif destination in LOCAL_PREFIX then drop else pass endif end-policy

Step 3: Review the prefix set referenced in the route policy above to determine if it includes the local global prefix as shown in the example below.

prefix-set LOCAL_PREFIX x.13.1.0/24 le 32 end-set

If the router is not configured to reject inbound route advertisements belonging to the local AS, this is a finding.

Step 1: Configure a prefix set containing the current Bogon prefixes as shown below.

RP/0/0/CPU0:R2(config)#prefix-set Step 1: Configure a prefix set containing the current Bogon prefixes as shown below.

RP/0/0/CPU0:R2(config)#prefix-set LOCAL_PREFIX RP/0/0/CPU0:R2(config-pfx)#x.13.1.0/24 le 32 RP/0/0/CPU0:R2(config-pfx)#end-set

Step 2: Configure the route policy to drop routes with BOGON prefixes as shown in the example below.

RP/0/0/CPU0:R2(config)#route-policy BGP_FILTER_INBOUND RP/0/0/CPU0:R2(config-rpl)#if destination in LOCAL_PREFIX then RP/0/0/CPU0:R2(config-rpl-if)#drop RP/0/0/CPU0:R2(config-rpl-if)#else pass endif RRP/0/0/CPU0:R2(config-rpl)#end-policy RP/0/0/CPU0:R2(config)#exit

Step 3: Apply the route policy to each external BGP neighbor as shown in the example.

RP/0/0/CPU0:R2(config)#router bgp xx RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.23.3 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER_INBOUND in RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.24.4 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER_INBOUND in