Finding ID | Version | Rule ID | IA Controls | Severity |
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V-96597 | CISC-RT-000500 | SV-105735r2_rule | Medium |
Description |
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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 |
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Cisco IOS Router RTR Security Technical Implementation Guide | 2020-06-30 |
Check Text ( C-95433r2_chk ) |
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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) |
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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 |