Changing the aggregates attributes

In some situations, changing the aggregate's attributes is required. The example in this post shows one scenario in which this could be useful.

As you have already seen, the aggregate can carry information about the individual elements if configured with the AS_SET option. If one or more of the routes forming the AS_SET aggregate are configured with no-export community attribute, the aggregate itself will carry the same attribute. his will prevent the aggregate from being exported. To remedy this situation, you can modify the community attribute of the aggregate by using what Cisco calls an attribute map, another form of a route map that applies only to aggregates.

Setup: As Figure 11-13 in Chapter 11 of Internet Routing Architectures.

First we configure RTC to send the Community: no-export attribute

RTC(config)#router bgp 1
RTC(config-router)#no neighb 192.68.6.1 send-comm
RTC(config-router)#neighb 172.16.20.2 send-comm
RTC(config-router)#neighb 172.16.20.2 route-map SETCOMMUNITY out
RTC(config-router)#exit
RTC(config)#acces
RTC(config)#access-list 1 permit 192.68.11.0 0.0.0.255
RTC(config)#route-map SETCOMMUNITY
RTC(config-route-map)#route-map SETCOMMUNITY permit 10
RTC(config-route-map)#match ip add 1
RTC(config-route-map)#set community no-export
RTC(config-route-map)#route-map SETCOMMUNITY permit 20
RTC(config-route-map)#

Before we go any futher, make sure RTA is picking up this attribute.

RTA>sh ip bgp 192.68.0.0
BGP routing table entry for 192.68.0.0/16, version 14
Paths: (1 available, best #1, table Default-IP-Routing-Table, not advertised to EBGP peer)
Flag: 0x880
Advertised to non peer-group peers:
172.16.1.2
{2,1}, (aggregated by 3 172.16.2.254)
0.0.0.0 from 0.0.0.0 (172.16.2.254)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, best
Community: no-export

"Community: no-export" confirms that it is, but this is not what we want so configure RTA to set up a attribute-map to specify the attribute to send ie; "community: none".

RTA# conf t
Enter configuration commands, one per line. End with CNTL/Z.
RTA(config)#route-map SET_ATTRIBUTE permit 10
RTA(config-route-map)#set community none
RTA(config-route-map)#router bgp 3
RTA(config-router)#agg 192.68.0.0 255.255.0.0 as-set attribute-map SET_ATTRIBUTE
RTA(config-router)#end
RTA#

Have a look now on RTA!

RTA#sh ip bgp 192.68.0.0
BGP routing table entry for 192.68.0.0/16, version 15
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to non peer-group peers:
172.16.1.2 172.16.20.1 172.16.220.2
{2,1}, (aggregated by 3 172.16.2.254)
0.0.0.0 from 0.0.0.0 (172.16.2.254)
Origin IGP, localpref 100, weight 32768, valid, aggregated, local, best
RTA#

No longer do we have the "community: no export" attribute.