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V-216781

CAT II (Medium)

The Cisco BGP router must be configured to reject outbound route advertisements for any prefixes belonging to the IP core.

Rule ID

SV-216781r531087_rule

STIG

Cisco IOS XR Router RTR Security Technical Implementation Guide

Version

V3R3

CCIs

CCI-001097

Discussion

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.

Check Content

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 Text

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