The Cisco BGP router must be configured to reject outbound route advertisements for any prefixes belonging to the IP core.
Severity | Group ID | Group Title | Version | Rule ID | Date | STIG Version |
|---|---|---|---|---|---|---|
| medium | V-216781 | SRG-NET-000205-RTR-000006 | CISC-RT-000530 | SV-216781r531087_rule | 2024-08-22 | 3 |
| Description |
|---|
| Outbound route advertisements belonging to the core can result in traffic either looping or being black holed, or at a minimum, using a non-optimized path. |
| ℹ️ Check |
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| Step 1: verify that an outbound route policy has been configured for each external neighbor as shown in the example below. router bgp xx address-family ipv4 unicast ! neighbor x.1.23.3 remote-as yy address-family ipv4 unicast route-policy BGP_FILTER_OUTBOUND out ! ! neighbor x.1.24.4 remote-as zz address-family ipv4 unicast route-policy BGP_FILTER_OUTBOUND out ! ! Step 2: Review the route policy to determine if it is filtering at a minimum IP core prefixes as shown in the example below. route-policy BGP_FILTER_OUTBOUND if destination in CORE_PREFIX then drop else pass endif end-policy Step 3: Review the prefix set referenced in the route policy above to determine if it includes the IP core prefix as shown in the example below. prefix-set CORE_PREFIX 10.1.1.0/24 le 32 end-set If the router is not configured to reject outbound route advertisements for prefixes belonging to the IP core, this is a finding. |
| ✔️ Fix |
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| Step 1: Configure a prefix set containing the IP core prefix as shown below. RP/0/0/CPU0:R2(config)#prefix-set Step 2: Configure a prefix set containing the current Bogon prefixes as shown below. RP/0/0/CPU0:R2(config)#prefix-set CORE_PREFIX RP/0/0/CPU0:R2(config-pfx)#10.1.1.0/24 le 32 RP/0/0/CPU0:R2(config-pfx)#end-set Step 3: Configure the route policy to drop route advertisements for IP core prefixes as shown in the example below. RP/0/0/CPU0:R2(config)#route-policy BGP_FILTER_OUTBOUND RP/0/0/CPU0:R2(config-rpl)#if destination in CORE_PREFIX then RP/0/0/CPU0:R2(config-rpl-if)#drop RP/0/0/CPU0:R2(config-rpl-if)#else RP/0/0/CPU0:R2(config-rpl-else)#pass RP/0/0/CPU0:R2(config-rpl-else)#endif RP/0/0/CPU0:R2(config-rpl)#end-policy Step 4: Apply the route policy to each external BGP neighbor as shown in the example. RP/0/0/CPU0:R2(config)#router bgp xx RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.23.3 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER_OUTBOUND out RP/0/0/CPU0:R2(config-bgp)#neighbor x.1.24.4 RP/0/0/CPU0:R2(config-bgp-nbr)#address-family ipv4 unicast RP/0/0/CPU0:R2(config-bgp-nbr-af)#route-policy BGP_FILTER_ OUTBOUND out |