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The Cisco BGP router 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-96963 CISC-RT-000570 SV-106101r2_rule Low
Description
The effects of prefix de-aggregation can degrade router performance due to the size of routing tables and also result in black-holing legitimate traffic. Initiated by an attacker or a misconfigured router, 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 Router RTR Security Technical Implementation Guide 2019-12-20

Details

Check Text ( C-95799r2_chk )
This requirement is not applicable for the DODIN Backbone.

Review the router 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 router 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-102643r2_fix)
This requirement is not applicable for the DODIN Backbone.

Configure the router 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.

R1(config)#ip prefix-list FILTER_PREFIX_LENGTH permit 0.0.0.0/0 ge 8 le 24
R1(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.

R1(config)#router bgp xx
R1(config-router)#neighbor x.1.1.9 prefix-list FILTER_PREFIX_LENGTH in
R1(config-router)#neighbor x.2.1.7 prefix-list FILTER_PREFIX_LENGTH in