Detections
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1.46 CISC-RT-000570

Information

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.

GROUP ID: V-221031
RULE ID: SV-221031r856417

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://workbench.cisecurity.org/benchmarks/26260

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, Vuln-ID|V-221031

Plugin: Cisco

Control ID: d30b0949a54e30e188bdeaa7c7c404bbd887e164b8365322340880294a7f1c7c