BGP Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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BGP Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

Policy-based aggregate route management

Updated: June 9, 2026

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Explains configuring aggregate routes and marking specific routes as aggregate contributors to control route aggregation and advertisement via route policies.

Policy-based aggregate route management is a feature that

  • allows you to configure aggregate routes in the routing table

  • marks specific routes as aggregate contributors for a specific destination route via route policy, and

  • enables you to set and modify aggregate contributors to a route aggregate address.

Aggregate contributor behavior in BGP

Table 1. Feature History Table

Feature Name

Release

Description

Configuring an Aggregate Contributor

Release 7.5.4

You can now configure aggregate routes in the routing table and mark specific routes as aggregate contributors for a specific destination route via route policy. This allows you to set the aggregate contributors to a route aggregate address and modify these routes. You can then use the BGP route policy to tag BGP prefixes before announcing them to the rest of the global network.

Earlier, there was no mechanism to identify a more specific route contributing to an aggregate and mark them as aggregate contributors.

This feature introduces these changes:

Route aggregation in BGP combines several specific routes into one route. You can configure the aggregate routes in the BGP routing table and mark specific routes as aggregate contributors for a specific destination route via route policy.

For example, if you have three prefixes 1.1.1.2/32, 1.1.1.3/32, and 1.1.1.4/32 in the routing table, BGP aggregates them by an aggregate route 1.1.1.0/24 and advertises that route to a peer. Before Cisco IOS XR Release 7.5.4, you could advertise to a peer the more specific route addresses along with the aggregate route (the 1.1.1.X’s) or the aggregate route, 1.1.1.0/24.

You can now mark specific routes (the 1.1.1.X’s) as aggregate contributors for a specific destination route. This allows you to set the aggregate contributors to a route aggregate address and modify these routes. After setting an aggregate contributor, you also have the option to set BGP attributes (for example, cost community, next-hop, BGP multiple exit discriminator) to the aggregate contributor. This checks the integrity of BGP updates in BGP update messages and optimizes reaction when detecting invalid attributes. You can then apply the inbound policy and the outbound policy to the neighbors.

Restrictions of policy-based aggregate route management

This feature is applicable only for the following Address Family Indicators (AFIs):

  • IPv4 unicast

  • IPv6 unicast

Configure BGP aggregate contributors

Define a route policy to mark routes as aggregate contributors and associate it with a specific aggregate address route in BGP.

This task guides you through creating a route policy that identifies routes as aggregate contributors and then applying this policy within BGP to an aggregate address. This allows for fine-grained control over how BGP aggregates and advertises routes.

Procedure

1.

Define the aggregate contributor route policy.

Example:

Router# configure
Router(config)# route-policy aggregate-policy1
Router(config-rpl)# set aggregate-contributor
Router(config-rpl)# end
Router(config-rpl)# commit
2.

Associate the route policy with an aggregate address in BGP.

Example:

Router# configure
Router(config)# router bgp 100
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# aggregate-address 250.2.2.0/24 route-policy aggregate-policy1
Router(config-bgp-af)# commit
3.

Verify the running configuration.

Example:

Router# show running-config
route-policy aggregate-policy1
   set aggregate-contributor
end-policy
!

router bgp 100
  address-family ipv4 unicast
    aggregate-address 250.2.2.0/24 route-policy aggregate-policy1
  !
 !
!
4.

Verify the BGP routing table entry for a specific prefix to confirm it is an aggregate contributor.

Example:


Router#show bgp 250.2.2.1/32

BGP routing table entry for 250.2.2.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 247          247
Last Modified: Dec  1 09:00:20.000 for 01:11:55
Paths: (1 available, best #1)
Net is an aggregate-contributor
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.2 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192.168.0.5     10.10.10.1      
  Path #1: Received by speaker 0
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.2 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192.168.0.5     10.10.10.1      
  105
    13.0.1.1 from 13.0.1.1 (13.0.1.1)
      Origin IGP, localpref 100, valid, external, best, group-best
      Received Path ID 0, Local Path ID 1, version 247
      Community: 20:20
      Origin-AS validity: (disabled)
5.

Display all aggregate contributors of a specific BGP address.

Example:


Router#show bgp 250.2.2.0/24 aggregate-contributors 

BGP router identifier 192.168.0.2, local AS number 100
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0xe0000000   RD version: 247
BGP main routing table version 247
BGP NSR Initial initsync version 22 (Reached)
BGP NSR/ISSU Sync-Group versions 247/0
BGP scan interval 60 secs

Status codes: s suppressed, d damped, h history, * valid, > best
              i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network            Next Hop            Metric LocPrf Weight Path
*> 250.2.2.1/32       13.0.1.1                               0 105 i
*> 250.2.2.2/32       13.0.2.1                               0 105 i
*> 250.2.2.3/32       13.0.3.1                               0 101 i
*> 250.2.2.4/32       13.0.4.1                               0 101 i
*> 250.2.2.5/32       13.0.5.1                               0 102 i
*> 250.2.2.6/32       13.0.6.1                               0 102 i
*> 250.2.2.7/32       13.0.7.1                               0 103 i
*> 250.2.2.8/32       13.0.8.1                               0 103 i
*> 250.2.2.9/32       13.0.9.1                               0 104 i
*> 250.2.2.10/32      13.0.10.1                              0 104 i
*> 250.2.2.11/32      0.0.0.0                  0         32768 ?
*> 250.2.2.12/32      0.0.0.0                  0         32768 ?
*> 250.2.2.13/32      0.0.0.0                  0         32768 ?

Processed 13 prefixes, 13 paths

Set BGP attributes to the aggregate contributor

Setting BGP attributes to the aggregate contributor is an optional step that

  • allows you to set or modify BGP attributes, or example, cost community, next-hop, BGP multiple exit discriminator, to the aggregate contributor

  • checks the integrity of BGP updates in BGP update messages, and

  • optimizes reaction when detecting invalid attributes.

Setting the aggregate contributor to a specific aggregate address route (aggregate-address address/mask-length route-policy aggregate-route-policy-name ) on a router (Router1) sets the aggregate contributor to the more specific routes on that router (Router1) only. The aggregate contributor then can be used in neighbor In and Out policy to match the BGP prefix with the aggregate contributor on the same router. However, you cannot match the BGP prefix with the aggregate contributor on a remote BGP node (Router2) because the aggregate contributor is not set in the remote node by default.

You can use this optional step to configure BGP attributes for an aggregate contributor via an inbound policy or an outbound policy.

Procedure

1.

Configure BGP attributes for an aggregate contributor via an inbound policy in Router1.

Example:

Router1# config
Router1(config)# route-policy set_comm_in
Router1(config-rpl)# if aggregate-contributor then
Router1(config-rpl-if)# set community (20:20) additive
Router1(config-rpl-if)# pass
Router1(config-rpl-if)# else
Router1(config-rpl-else)# drop
Router1(config-rpl-else)# endif
Router1(config-rpl)# end-policy
Router1(config)#
Router1(config)# router bgp 100
Router1(config-bgp)# neighbor 13.0.1.1
Router1(config-bgp-nbr)# remote-as 105
Router1(config-bgp-nbr)# address-family ipv4 unicast
Router1(config-bgp-nbr-af)# route-policy set_comm_in in
Router1(config-bgp-nbr-af)# commit
2.

Configure BGP attributes for an aggregate contributor via an outbound policy in Router1.

Example:

Router1# config
Router1(config)# route-policy set_comm_out
Router1(config-rpl)#  if aggregate-contributor then
Router1(config-rpl-if)# set extcommunity rt(200:200) additive
Router1(config-rpl-if)# pass
Router1(config-rpl-if)# else
Router1(config-rpl-if)# set extcommunity rt(500:500) additive
Router1(config-rpl-else)# pass
Router1(config-rpl-else)# endif
Router1(config-rpl)#end-policy
Router1(config)#
Router1(config)# router bgp 100
Router1(config-bgp)# neighbor 192.168.0.5
Router1(config-bgp-nbr)# remote-as 100
Router1(config-bgp-nbr)# address-family ipv4 unicast
Router1(config-bgp-nbr-af)# route-policy set_extcomm_out out
Router1(config-bgp-nbr-af)# commit
3.

Verify the running configuration.

  • For inbound policy: 
    
route-policy set_comm_in
  if aggregate-contributor then
     set community (20:20) additive
     pass
   else
      drop
   endif
 end-policy
 !
router bgp 100
   neighbor 13.0.1.1
   remote-as 105
     address-family ipv4 unicast
        route-policy set_comm_in in
  • For outbound policy: 
    
route-policy set_extcomm_out
  if aggregate-contributor then
     set extcommunity rt (200:200) additive
     pass
  else
    set extcommunity rt (500:500) additive
    pass
  endif
end-policy
!

router bgp 100
  neighbor 192.168.0.5
  remote-as 100
    address-family ipv4 unicast
      route-policy set_extcomm_out out
    !
!
4.

Verify the BGP routing table entry on Router1 for prefix 250.2.2.1/32 to confirm it is an aggregate contributor and has the expected community attribute.

Example:

Router1#show bgp 250.2.2.1/32
Thu Dec  1 10:12:15.374 EST
BGP routing table entry for 250.2.2.1/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 247          247
Last Modified: Dec  1 09:00:20.000 for 01:11:55
Paths: (1 available, best #1)
Net is an aggregate-contributor
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.2 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192.168.0.5     10.10.10.1      
  Path #1: Received by speaker 0
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.2 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    192.168.0.5     10.10.10.1      
  105
    13.0.1.1 from 13.0.1.1 (13.0.1.1)
      Origin IGP, localpref 100, valid, external, best, group-best
      Received Path ID 0, Local Path ID 1, version 247
      Community: 20:20
      Origin-AS validity: (disabled)

This example shows the BGP attribute (community) being set to aggregate contributor 250.2.2.1/32 via inbound policy. The neighbor inbound policy matches with an aggregate contributor. If it matches, then the route is added with the Community 20:20.
5.

Verify the BGP routing table entry on Router2 for prefix 250.2.2.4/32 to confirm it has the expected extended community attribute.

Example:

Router2#show bgp ipv4 u 250.2.2.4/32
Thu Dec  8 15:08:40.672 EST
BGP routing table entry for 250.2.2.4/32
Versions:
  Process           bRIB/RIB  SendTblVer
  Speaker                 174          174
Last Modified: Dec  8 15:06:53.000 for 00:01:47
Paths: (1 available, best #1)
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.1 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    12.12.12.4      
  Path #1: Received by speaker 0
  Advertised IPv4 Unicast paths to update-groups (with more than one peer):
    0.1 
  Advertised IPv4 Unicast paths to peers (in unique update groups):
    12.12.12.4      
  101, (Received from a RR-client)
    13.0.4.1 (metric 2) from 192.168.0.2 (192.168.0.2)
      OC-RIB Attribute-Index 0
      Origin IGP, localpref 100, valid, internal, best, group-best
      Received Path ID 1, Local Path ID 1, version 174
      Extended community: RT:200:200

This example shows the BGP attribute (extended-community) being set to aggregate contributor 250.2.2.4/32 via outbound policy. The neighbor outbound policy matches with an aggregate contributor. If it matches, then the route is added with the Extended community: RT:200:200. This attribute is set before the router (Router2) receives the route.