Overview

The aggregate-address command for aggregating BGP routes on Cisco routers has many options. The options are divided into two categories.

  • What to do with the attributes of the aggregate route
  • How to handle pre-aggregation routes

The advertise-map is an option for “what to do with the attributes of the aggregate route”. And it is an option to be used with as-set.

The as-set allows the aggregate route to inherit the attributes of the pre-aggregate route. The AS number and community of “all” pre-aggregation routes will then be inherited by the aggregate route. With advetise-map, only the attributes of a particular pre-aggregate route can be inherited to the aggregate route.

Related article

For more information about inheritance of attributes to aggregate routes by as-set, see the following article.

N-Study
aggregte-address command : as-set opiton | N-Study
https://www.n-study.com/en/bgp-detail/aggregate-address-as-set/
The as-set is an option for “what to do with the attributes of the aggregate route”. The as-set option allows the aggregate route to inherit attributes of the pre-aggregate routes.

aggregate-address command : advertise-map

The advertise-map is an option to be used with as-set. The as-set allows the aggregate route to inherit the attributes of the pre-aggregate route. The AS number and community of “all” pre-aggregation routes will then be inherited by the aggregate route. With advetise-map, only the attributes of a particular pre-aggregate route can be inherited to the aggregate route.

The following figure shows how advertise-map allows the aggregate route 192.168.0.0/22 to inherit the attributes of the pre-aggregate routes 192.168.1.0/24 and 192.168.3.0/24.

Figure: aggregate-address option advertise-map
Figure: aggregate-address option advertise-map

The advertise-map option must first be configured as-set. Then, a route map is specified. The attributes of the pre-aggregation routes that are permitted in the route-map will be inherited by the aggregate route.

aggregate-address advertise-map

(config)#router bgp <AS>
(config-router)#aggregate-address <network-address> <subnetmask> as-set advertise-map <route-map-name>

<AS>: AS number
<network-address>: Network address of the aggregate route
<subnetmask>: Subnet mask of the aggregate route
<route-map-name>: route-map name

The router configuration in the previous figure is as follows.

advertise-map configuration example

access-list 1 permit 192.168.1.0
access-list 1 permit 192.168.3.0
!
route-map ADV-MAP permit 10
 match ip address 1
!
router bgp 
 aggregate-address 192.168.0.0 255.255.252.0 as-set advertise-map ADV-MAP

The route-map specified by the advertise-map option does not require “set” action. The match condition in the route map matches the pre-aggregation routes that you want to inherit.

aggregate-address advertise-map option configuration example

Network diagram and initial configuration

Consider the advertise-map option of aggregate-address on R1 in the following network diagram.

Figure: aggregate-address as-set/advertise-map option configuration example network diagram
Figure: aggregate-address as-set/advertise-map option configuration example network diagram

The configurations related to BGP for R1 to R4 before aggregate-address is configured are as follows.

R1 Initial Configuration

On R1, an EBGP neighbor is established with AS2 (R2), AS3 (R3), and AS4 (R4), and 100.1.1.0/24 to 100.1.4.0/24 is advertised with the network command.

R1 Initial Configuration

interface Loopback0
 ip address 100.1.2.1 255.255.255.0 secondary
 ip address 100.1.3.1 255.255.255.0 secondary
 ip address 100.1.4.1 255.255.255.0 secondary
 ip address 100.1.1.1 255.255.255.0
!
router bgp 1
 network 100.1.1.0 mask 255.255.255.0
 network 100.1.2.0 mask 255.255.255.0
 network 100.1.3.0 mask 255.255.255.0
 network 100.1.4.0 mask 255.255.255.0
 neighbor 12.12.12.2 remote-as 2
 neighbor 12.12.12.2 send-community
 neighbor 13.13.13.3 remote-as 3
 neighbor 13.13.13.3 send-community
 neighbor 14.14.14.4 remote-as 4
 neighbor 14.14.14.4 send-community
 ip bgp-community new-format

R2 Initial Configuration

R2 has established EBGP with AS1 (R1) and advertises routes 100.2.1.0/24 through 100.2.4.0/24 with COMMUNITY. The COMMUNITY of each route is as follows.

100.2.1.0/24 2:1000
100.2.2.0/24 2:2000
100.2.3.0/24 2:3000
100.2.4.0/24 2:4000

R2 Initial Configuration

interface Loopback0
 ip address 100.2.2.1 255.255.255.0 secondary
 ip address 100.2.3.1 255.255.255.0 secondary
 ip address 100.2.4.1 255.255.255.0 secondary
 ip address 100.2.1.1 255.255.255.0
!
router bgp 2
 network 100.2.1.0 mask 255.255.255.0
 network 100.2.2.0 mask 255.255.255.0
 network 100.2.3.0 mask 255.255.255.0
 network 100.2.4.0 mask 255.255.255.0
 neighbor 12.12.12.1 remote-as 1
 neighbor 12.12.12.1 send-community
 neighbor 12.12.12.1 route-map COMMUNITY
 ip bgp-community new-format
!
route-map COMMUNITY permit 10
 match ip address 1
 set community 2:1000
route-map COMMUNITY permit 20
 match ip address 2
 set community 2:2000
route-map COMMUNITY permit 30
 match ip address 3
 set community 2:3000
route-map COMMUNITY permit 40
 match ip address 4
 set community 2:4000
route-map COMMUNITY permit 1000
!
access-list 1 permit 100.2.1.0
access-list 2 permit 100.2.2.0
access-list 3 permit 100.2.3.0
access-list 4 permit 100.2.4.0

R3 Initial Configuration

R3 has established an EBGP neighbor with AS1 (R1) and advertises routes 100.3.1.0/24 through 100.3.4.0/24 with COMMUNITY. The COMMUNITY of each route is as follows

100.3.1.0/24 3:1000
100.3.2.0/24 3:2000
100.3.3.0/24 3:3000
100.3.4.0/24 3:4000

R3 Initial Configuration

interface Loopback0
 ip address 100.3.2.1 255.255.255.0 secondary
 ip address 100.3.3.1 255.255.255.0 secondary
 ip address 100.3.4.1 255.255.255.0 secondary
 ip address 100.3.1.1 255.255.255.0
!
router bgp 3
 network 100.3.1.0 mask 255.255.255.0
 network 100.3.2.0 mask 255.255.255.0
 network 100.3.3.0 mask 255.255.255.0
 network 100.3.4.0 mask 255.255.255.0
 neighbor 13.13.13.1 remote-as 1
 neighbor 13.13.13.1 send-community
 neighbor 13.13.13.1 route-map COMMUNITY
 ip bgp-community new-format
!
route-map COMMUNITY permit 10
 match ip address 1
 set community 3:1000
route-map COMMUNITY permit 20
 match ip address 2
 set community 3:2000
route-map COMMUNITY permit 30
 match ip address 3
 set community 3:3000
route-map COMMUNITY permit 40
 match ip address 4
 set community 3:4000
route-map COMMUNITY permit 1000
!
access-list 1 permit 100.3.1.0
access-list 2 permit 100.3.2.0
access-list 3 permit 100.3.3.0
access-list 4 permit 100.3.4.0

R4 Initial Configuration

R4 establishes an EBGP neighbor with AS1 (R1).

R4 Initial Configuration

router bgp 4
 neighbor 14.14.14.1 remote-as 4
 neighbor 14.14.14.1 send-community
 ip bgp-community new-format

aggregate-address command as-set option

First, add the as-set option to the aggregate-address command on R1. Also, configure summary-only to consider only aggregate route.

R1 aggregate-address as-set

router bgp 1
 aggregate-address 100.0.0.0 255.0.0.0 as-set summary-only

Then the aggregate route 100.0.0.0/8 is created and advertised with the AS numbers and communities of all the pre-aggregate routes inherited.

R1 show ip bgp 100.0.0.0

R1#sh ip bgp 100.0.0.0
BGP routing table entry for 100.0.0.0/8, version 27
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to non peer-group peers:
12.12.12.2 13.13.13.3 14.14.14.4
{2,3}, (aggregated by 1 100.1.1.1)
0.0.0.0 from 0.0.0.0 (100.1.1.1)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, best
Community: 2:1000 2:2000 2:3000 2:4000 3:1000 3:2000 3:3000 3:4000
Figure: aggregate-address as-set option
Figure: aggregate-address as-set option

advertise-map configuration

aadvertise-map is used to make the aggregate route inherit “only” the attributes of a particular pre-aggregate route. After advertise-map, a route map is specified, and the attributes (AS number, community) of the routes that are permitted in the route map will be inherited by the aggregate route.

Let’s actually use advertise-map. Configure only the attributes of the next route to be inherited from the pre-aggregation routes.

  • 100.2.1.0/24 AS-PATH=2,Community=2:1000
  • 100.3.3.0/24 AS-PATH=3,Community=3:3000

First, create a route-map to permit the routes above. The name of the route-map is SPECIFIC_ROUTE and the match condition refers to ACL1.

R1 Creating a route-map to be applied with advertise-map

access-list 1 permit 100.2.1.0
access-list 1 permit 100.3.3.0
!
route-map SPECIFIC-ROUTE permit 10
 match ip address 1

Then, in BGP configuration mode, add the as-set and advertise-map options to aggregate-address.

R1 Add advertise-map to aggregate-address command

router bgp 1
 aggregate-address 100.0.0.0 255.0.0.0 as-set summary-only advertise-map SPECIFIC_ROUTE

This will cause 100.0.0.0/8 to inherit only the AS numbers and communities of 100.2.1.0/24 and 100.3.3.0/24. Let’s verify this with the BGP tables on R1 and R4.

R1 show ip bgp 100.0.0.0

R1#sh ip bgp 100.0.0.0
BGP routing table entry for 100.0.0.0/8, version 28
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to non peer-group peers:
12.12.12.2 13.13.13.3 14.14.14.4
{2,3}, (aggregated by 1 100.1.1.1)
0.0.0.0 from 0.0.0.0 (100.1.1.1)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, atomic-aggregate, best
Community: 2:1000 3:3000

R4 show ip bgp 100.0.0.0

R4#sh ip bgp 100.0.0.0
BGP routing table entry for 100.0.0.0/8, version 41
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
1 {2,3}, (aggregated by 1 100.1.1.1)
14.14.14.1 from 14.14.14.1 (100.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best
Community: 2:1000 3:3000
Figure: aggregate-address advertise-map option
Figure: aggregate-address advertise-map option

Summary

Points

  • advertise-map is an option for “what to do with the attributes of the aggregate route”. Used to make the aggregate route inherit the AS number and COMMUNITY of only particular pre-aggregate route.
  • After advertise-map, specify a route-map. Inherits the attributes of the route permitted in the route-map.

How the BGP works