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		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	2024-06-06

Details

Check Text ( C-22746r408887_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.

Check Text ( C-22746r408887_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

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