V-221031

Discussion

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

Check Content

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

Step 1: Verify that a route filter has been configured to reject prefixes longer than /24, or the least significant prefixes issued to the customers as shown in the example below:

ip prefix-list FILTER_PREFIX_LENGTH seq 5 permit 0.0.0.0/0 ge 8 le 24
ip prefix-list FILTER_PREFIX_LENGTH seq 10 deny 0.0.0.0/0 le 32

Step 2: Verify that prefix filtering has been applied to each eBGP peer as shown in the example:

router bgp xx
 neighbor x.1.1.9 remote-as yy
 neighbor x.1.1.9 prefix-list FILTER_PREFIX_LENGTH in
 neighbor x.2.1.7 remote-as zz
 neighbor x.2.1.7 prefix-list FILTER_PREFIX_LENGTH in

If the switch 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.