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The Cisco BGP switch 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-221104 | CISC-RT-000500 | SV-221104r622190_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 NX OS Switch RTR Security Technical Implementation Guide | 2023-02-15 |
Details
| Check Text ( C-22819r409801_chk ) |
|---|
| Review the switch 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 router-id 10.1.1.1 neighbor x.1.12.2 remote-as xx password 3 7b07d1b3023056a9 address-family ipv4 unicast prefix-list PREFIX_FILTER in neighbor x.2.44.4 remote-as xx password 3 f07a10cb41db8bb6f8f0a340049a9b02 address-family ipv4 unicast prefix-list PREFIX_FILTER in If the switch is not configured to reject inbound route advertisements belonging to the local AS, this is a finding. |
| Fix Text (F-22808r409802_fix) |
|---|
| Configure the switch 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. SW1(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 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 PREFIX_FILTER 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 PREFIX_FILTER in SW1(config-router-neighbor-af)# end |