Title

The BGP Cisco ACI must be configured to use the maximum prefixes feature to protect against route table flooding and prefix de-aggregation attacks. (Cat II impact)

Discussion

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. Maximum prefix limits on peer connections combined with aggressive prefix-size filtering of customers' reachability advertisements will effectively mitigate the de-aggregation risk. BGP maximum prefix must be used on all eBGP routers to limit the number of prefixes that it should receive from a particular neighbor, whether customer or peering AS. Consider each neighbor and how many routes they should be advertising and set a threshold slightly higher than the number expected.

Check Content

Verify the BGP configuration for each tenant: ip route protocol BGP View the BGP peer configuration maximum prefix value: neighbor 10.0.0.1 maximum-prefix nnnnnnn If the router is not configured to control the number of prefixes received from each peer to protect against route table flooding and prefix de-aggregation attacks, this is a finding.

Fix Text

Configure the router to use the maximum prefixes feature to protect against route table flooding and prefix de-aggregation attacks as shown in the example below: For each BGP peer, use the command "neighbor <peer-ip> maximum-prefix <number of prefixes>" within the BGP configuration section, where <peer-ip> is the IP address of the BGP peer and <number of prefixes> is the desired maximum prefix limit to be set; the default maximum prefix limit is typically 20,000 prefixes.

Additional Identifiers

Rule ID: SV-272088r1114096_rule

Vulnerability ID: V-272088

Group Title: SRG-NET-000362-RTR-000117

Expert Comments

CCIs tied to check.

Controls tied to check. These are derived from the CCIs shown above.