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.
Overview
| Finding ID | Version | Rule ID | IA Controls | Severity |
| V-221031 | CISC-RT-000570 | SV-221031r856417_rule | CCI-002385 | low |
| Description | ||||
| 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. | ||||
| STIG | Date | |||
| Cisco IOS XE Switch RTR Security Technical Implementation Guide | 2025-05-20 | |||
Details
Check Text (C-221031r856417_chk)
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.
Fix Text (F-22735r408888_fix)
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