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The Cisco BGP router must be configured to reject inbound route advertisements for any prefixes belonging to the local autonomous system (AS).


Overview

Finding ID Version Rule ID IA Controls Severity
V-96597 CISC-RT-000500 SV-105735r2_rule Medium
Description
Accepting route advertisements belonging to the local AS can result in traffic looping or being black holed, or at a minimum using a non-optimized path.
STIG Date
Cisco IOS Router RTR Security Technical Implementation Guide 2020-06-30

Details

Check Text ( C-95433r2_chk )
Review the router configuration to verify that it will reject routes belonging to the local AS.

Step 1: Verify a prefix list has been configured containing prefixes belonging to the local AS. In the example below x.13.1.0/24 is the global address space allocated to the local AS.

ip prefix-list PREFIX_FILTER seq 5 deny 0.0.0.0/8 le 32



ip prefix-list PREFIX_FILTER seq 74 deny x.13.1.0/24 le 32
ip prefix-list PREFIX_FILTER seq 75 permit 0.0.0.0/0 ge 8

Step 2: Verify that the prefix list has been applied to all external BGP peers as shown in the example below.

router bgp xx
no synchronization
bgp log-neighbor-changes
neighbor x.1.1.9 remote-as yy
neighbor x.1.1.9 prefix-list PREFIX_FILTER in
neighbor x.2.1.7 remote-as zz
neighbor x.2.1.7 prefix-list PREFIX_FILTER in

If the router is not configured to reject inbound route advertisements belonging to the local AS, this is a finding.
Fix Text (F-102271r2_fix)
Configure the router to reject inbound route advertisements for any prefixes belonging to the local AS.

Step 1: Add to the prefix filter list those prefixes belonging to the local autonomous system.

R1(config)#ip prefix-list PREFIX_FILTER seq 74 deny x.13.1.0/24 le 32

Step 2: If not already completed to be compliant with previous requirement, apply the prefix list filter inbound to each external BGP neighbor as shown in the example.

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