The Cisco BGP router must be configured to limit the prefix size on any inbound route advertisement to /24 or the least significant prefixes issued to the customer.

The effects of prefix de-aggregation can degrade router performance due to the size of routing tables and also result in black-holing legitimate traffic. Initiated by an attacker or a misconfigured router, prefix de-aggregation occurs when the announcement of a large prefix is fragmented into a collection of smaller prefix announcements.

This requirement is not applicable for the DODIN Backbone.

Review the router configuration to determine if it is compliant with this requirement.

Step 1: Verify that an inbound route policy has been configured for each CE router as shown in the example below.

router bgp xx address-family ipv4 unicast ! neighbor x.12.4.14 remote-as 64514 address-family ipv4 unicast route-policy FILTER_LONG_PREFIXES in ! ! neighbor x.12.4.16 remote-as 64516 address-family ipv4 unicast route-policy FILTER_LONG_PREFIXES in !

Step 2: Verify that the route policy permits only routes from each CE router with a prefix length of 24 or shorter or the least significant prefixes issued to the customer as shown in the example below.

route-policy FILTER_LONG_PREFIXES if destination in PREFIX_LENGTH then pass else drop endif end-policy

Note: The inbound route policy to filter customer prefixes can be nested with the above route policy as shown in the example below.

route-policy CUST1_INBOUND_FILTER apply CUST1_FILTER apply FILTER_64514_ROUTES apply FILTER_LONG_PREFIXES end-policy

Step 3: Review the prefix set referenced in the route policy above to determine if it only allows a prefix length 24 or shorter.

prefix-set PREFIX_LENGTH 0.0.0.0/0 ge 8 le 24 end-set

If the router is not configured to limit the prefix size on any inbound route advertisement to /24 or the least significant prefixes issued to the customer, this is a finding.

This requirement is not applicable for the DODIN Backbone.

Configure the router to limit the prefix size on any route advertisement to /24 or the least significant prefixes issued to the customer.

Step 1: Configure a prefix set to not include prefixes are longer than /24.

RP/0/0/CPU0:R2(config)#prefix-set PREFIX_LENGTH RP/0/0/CPU0:R2(config-pfx)#0.0.0.0/0 ge 8 le 24 RP/0/0/CPU0:R2(config-pfx)#end-set

Step 2: Configure a route policy to only accept prefixes that are /24 or shorter as shown in the example below.

RP/0/0/CPU0:R2(config)#route-policy FILTER_LONG_PREFIXES RP/0/0/CPU0:R2(config-rpl)#if destination in PREFIX_LENGTH then RP/0/0/CPU0:R2(config-rpl-if)#pass RP/0/0/CPU0:R2(config-rpl-if)#else RP/0/0/CPU0:R2(config-rpl-else)#drop RP/0/0/CPU0:R2(config-rpl-else)#endif RP/0/0/CPU0:R2(config-rpl)#end-policy

Step 3: Apply the route policy above inbound with each peering CE router as shown in the example below.

RP/0/0/CPU0:R2(config)#router bgp xx RP/0/0/CPU0:R2(config-bgp)#neighbor x.12.4.14 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy route-policy FILTER_LONG_PREFIXES in RP/0/0/CPU0:R2(config-bgp)#neighbor x.12.4.16 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy FILTER_LONG_PREFIXES in RP/0/0/CPU0:R2(config-bgp-nbr-af)#end