Information Technology Reference
In-Depth Information
Example 9-4 shows the router configuration for assigning the prefix origin communities.
Prefix Origin Community Assignment
Example 9-4
route-map cust_inbound permit 10
set community 100:3000 additive
!
route-map peer_inbound permit 10
set community 100:2000 additive
!
route-map transit_inbound permit 10
set community 100:1000 additive
!
These communities can be used to filter upstream prefix advertisements by permitting
the advertisement of only prefixes with the community <ASN>:3000. This prevents the
ISP from advertising peering routes over the transit connection and transit routes to peers.
If the ISP does not filter prefix advertisements between the transit and peering connections,
the ISP ends up providing transit service for its peers. Example 9-5 shows the outbound
configuration for filtering based on the prefix origin communities.
Outbound Community-Based Filtering
Example 9-5
route-map peer_outbound permit 10
match community 1
!
route-map peer_outbound deny 20
!
route-map transit_outbound permit 10
match community 1
!
route-map transit_outbound deny 20
!
ip community-list 1 permit 100:3000
ip community-list 1 deny
!
The configurations provided in Examples 9-4 and 9-5 will be built upon in the remainder
of this section. The resulting configuration will be a complete BGP community design.
Dynamic Customer Policy
It is common for customers to multihome to one or more providers. Customers choose
providers for a variety of reasons and often have specific requirements for traffic policy.
A customer that is multihomed to the same provider might want traffic to load-balance
between the two connections or might want to use the connections in a primary and backup
