Detections
Analytics

1.38 CISC-RT-000570

Warning! Audit Deprecated

This audit has been deprecated and will be removed in a future update.

View Next Audit Version

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-221111
RULE ID: SV-221111r999720

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-router)# neighbor x.1.12.2
SW1(config-router-neighbor)# address-family ipv4 unicast
SW1(config-router-neighbor-af)# prefix-list FILTER_PREFIX_LENGTH in
SW1(config-router-neighbor-af)# exit
SW1(config-router-neighbor)# exit
SW1(config-router)# neighbor x.2.44.4
SW1(config-router-neighbor)# address-family ipv4 unicast
SW1(config-router-neighbor-af)# prefix-list FILTER_PREFIX_LENGTH in
SW1(config-router-neighbor-af)# end

See Also

https://workbench.cisecurity.org/benchmarks/22581

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-5, CAT|III, CCI|CCI-002385, CSCv7|13.3, Rule-ID|SV-221111r999720_rule, STIG-ID|CISC-RT-000570, Vuln-ID|V-221111

Plugin: Cisco

Control ID: 881e6c7fcbc9325380650be5c0b4f1ff3e0910b40506cabf3cc0f993f311e287