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-221111	CISC-RT-000570	SV-221111r622190_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 NX-OS Switch RTR Security Technical Implementation Guide	2021-03-29

Details

Check Text ( C-22826r409822_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 below: router bgp xx router-id 10.1.1.1 neighbor x.1.12.2 remote-as xx password 3 7b07d1b3023056a9 address-family ipv4 unicast prefix-list FILTER_PREFIX_LENGTH in neighbor x.2.44.4 remote-as xx password 3 f07a10cb41db8bb6f8f0a340049a9b02 address-family ipv4 unicast 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-22826r409822_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 below:

router bgp xx
router-id 10.1.1.1
neighbor x.1.12.2 remote-as xx
password 3 7b07d1b3023056a9
address-family ipv4 unicast
prefix-list FILTER_PREFIX_LENGTH in
neighbor x.2.44.4 remote-as xx
password 3 f07a10cb41db8bb6f8f0a340049a9b02
address-family ipv4 unicast
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-22815r409823_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-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

Fix Text (F-22815r409823_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-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