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V-269851

CAT II (Medium)

The Dell OS10 BGP router must be configured to reject inbound route advertisements for any prefixes belonging to the local autonomous system (AS).

Rule ID

SV-269851r1137895_rule

STIG

Dell OS10 Switch Router Security Technical Implementation Guide

Version

V1R2

CCIs

CCI-001368

Discussion

Accepting route advertisements belonging to the local AS can result in traffic looping or being black holed, or at a minimum using a nonoptimized path.

Check Content

Review the router configuration to verify that it will reject routes belonging to the local AS.

The prefix filter must be referenced inbound on the appropriate BGP neighbor statements.

Step 1: Verify a prefix list has been configured containing the local AS prefixes.

ip prefix-list PREFIX_FILTER seq 5 deny 0.0.0.0/8 le 32
...
...
ip prefix-list PREFIX_FILTER seq 73 deny 20.10.10.0/24 le 32
ip prefix-list PREFIX_FILTER seq 74 deny 40.10.10.0/24 le 32
ip prefix-list PREFIX_FILTER seq 75 permit 0.0.0.0/0 ge 8

Step 2: Verify the route map applied to the external neighbors references the configured prefix list shown above.

!
route-map PREFIX_FILTER_MAP permit 10
 match ip address prefix-list PREFIX_FILTER
 
!
router bgp 10
 !
 template ebgp
  !
  address-family ipv4 unicast
   route-map PREFIX_FILTER_MAP in
 !
 neighbor 123.1.1.10
  !
  address-family ipv4 unicast
   route-map PREFIX_FILTER_MAP in

If the router is not configured to reject inbound route advertisements belonging to the local AS, this is a finding.

Fix Text

Ensure all eBGP routers are configured 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.

OS10(config)# ip prefix-list PREFIX_FILTER seq 73 deny 20.10.10.0/24 le 32
OS10(config)# ip prefix-list PREFIX_FILTER seq 74 deny 40.10.10.0/24 le 32

Step 2: Configure the route map referencing the configured prefix list.

OS10(config)# route-map PREFIX_FILTER_MAP 10
OS10(config-route-map)# match ip address prefix-list PREFIX_FILTER
OS10(config-route-map)# exit

Step 3: Apply the route-map inbound to each external BGP neighbor.

OS10(config)# router bgp 10
OS10(config-router-bgp-10)# neighbor 123.1.1.10
OS10(config-router-neighbor)# address-family ipv4 unicast
OS10(config-router-bgp-neighbor-af)# route-map PREFIX_FILTER_MAP in
OS10(config-router-bgp-neighbor-af)# exit
OS10(config-router-neighbor)# exit
OS10(config-router-bgp-10)# template ebgp
OS10(config-router-template)# address-family ipv4 unicast
OS10(config-router-bgp-template-af)# route-map PREFIX_FILTER_MAP in
OS10(config-router-bgp-template-af)# exit
OS10(config-router-template)# exit
OS10(config-router-bgp-10)# exit