CISC-RT-000570 - The Cisco BGP switch must be configured to limit the prefix size on any inbound route advertisement to /24, or the least significant prefixes issued to the customer.

Information

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.

Solution

Configure the switch 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 list to reject any prefix that is longer than /24.

SW1(config)#ip prefix-list FILTER_PREFIX_LENGTH permit 0.0.0.0/0 ge 8 le 24
SW1(config)#ip prefix-list FILTER_PREFIX_LENGTH deny 0.0.0.0/0 le 32

Step 2: Apply the prefix list to all eBGP peers as shown in the example below:

SW1(config)#router bgp xx
SW1(config-switch)#neighbor x.1.1.9 prefix-list FILTER_PREFIX_LENGTH in
SW1(config-switch)#neighbor x.2.1.7 prefix-list FILTER_PREFIX_LENGTH in

See Also

https://dl.dod.cyber.mil/wp-content/uploads/stigs/zip/U_Cisco_IOS-XE_Switch_Y24M07_STIG.zip

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-5, CAT|III, CCI|CCI-002385, Rule-ID|SV-221031r856417_rule, STIG-ID|CISC-RT-000570, STIG-Legacy|SV-110883, STIG-Legacy|V-101779, Vuln-ID|V-221031

Plugin: Cisco

Control ID: 8baa199c53880ae9c970b03d0d0bcc17113b7a03f48436c6140a8b7d4ab66d70