UUNET BGP Community Usage

UUNET supports a user configurable routing policy by allowing customers to fine tune their routing announcements via the BGP community attribute. The following information outlines the policy and demonstrates how customers can implement the routing policy of their choice.

Customers now can set values which modify the BGP attributes "Local Preference" and "AS Path Length" as well as control which route(s) should be advertised to UUNET peers (ISPs). These controls are customer implemented by advertising routes tagged with the appropriate BGP community and when received UUNET will act on the community value as shown below.

The attribute "LOCAL_PREF" is used to set a route as "preferred" or "least preferred" over all others. Since BGP selects the route with the highest local preference a route can be made "preferred" by setting the "LOCAL_PREF" value higher than the default value of 100. Also, any route can be made to be "least preferred" by setting the "LOCAL_PREF" value lower than the default value of 100. The "LOCAL_PREF" setting is non-transitive and affects routes only in the AS in which it is set and does not pass to external BGP peers. The table below defines the values and actions for the "LOCAL_PREF" attribute.

Customers may modify the UUNET "LOCAL_PREF" value by tagging their route announcements with ONE of the communities defined below. Each route should be tagged with only one value for "LOCAL_PREF." Once UUNET receives this route announcement tagged with the appropriate community string for modifying "LOCAL_PREF", the attribute will be set accordingly. "LOCAL_PREF" is set immediately on the UUNET router to which the customer attaches. Please refer to the table below for the community string values that UUNET will accept for setting "LOCAL_PREF."

A case may arise where 2 or more paths exist for the same destination with the same "LOCAL_PREF" value. The next configurable option in BGP route selection process is to lengthen the AS PATH. The route with the shortest AS PATH LENGTH is preferred. The table below defines the values and actions for AS PATH LENGTH.

The AS PATH LENGTH is adjusted at the egress or UUNET peering points to all peers. The table below defines the community value for setting AS PATH LENGTH and the total path length announced to peers.

UUNET maintains multiple autonomous systems with each one providing continental coverage. AS 701 is used in North America, AS 702 provides coverage for Europe, and AS 703 is growing in Asia. Routes UUNET hears from customers are advertised to all UUNET autonomous systems as well as UUNET peers. However, customers have the ability to change this in two ways. Routes can be contained to UUNET's AS's and not announced to peers or routes can be contained to a UUNET continental AS and not announced to peers. This is accomplished by filtering routes based on a community value filtering at UUNET's egress points. The table below defines the community values a customer may send to instruct UUNET to filter route(s) to its peers.

Sample Cisco Configurations

If for example, a customer is multi-homed to UUNET and maintains a web farm and wants all traffic destined for the web farm netowrk (191.255.1.0/24) to use the connection via Router A and all other traffic to use the connection via Router B, this could be achieved by modifying LOCAL_PREF:

Router A
    !
    ip bgp-community new-format
    router bgp ASxx
    neighbor 157.130.x.y remote-as 701
    neighbor 157.130.x.y version 4
    neighbor 157.130.x.y route-map to-UUNET out
    neighbor 157.130.x.y send-community
    !
    ip as-path access-list 1 permit .*
    !
    ip address 101 permit ip 191.255.1.0 0.255.255.255 255.255.255.0 
0.255.255.255
    !
    route-map to-UUNET permit 10
     match ip address 101
     set community 701:120
    !
    route-map to-UUNET permit 20
     match as path 1
     set community 701:80
    !

The above example would set the value of LOCAL_PREF to 120 for all networks listed in access list 101 and all other networks from ASxx would be sent with a LOCAL-PREF value of 80.

Router B
    !
    ip bgp-community new-format
    router bgp ASxx
    neighbor 157.130.x.z remote-as 701
    neighbor 157.130.x.z version 4
    neighbor 157.130.x.z route-map to-UUNET out
    neighbor 157.130.x.z send-community
    !
    !
    ip as-path access-list 1 permit .*
    !
    ip address 101 permit ip 191.255.1.0 0.255.255.255 255.255.255.0 
0.255.255.255
    !
    route-map to-UUNET permit 10
     match ip address 101
     set community 701:80
    !
    route-map to-UUNET permit 20
     match as path 1
     set community 701:120
    !

The above example would set the value of LOCAL_PREF to 80 for all networks listed in access list 101 and all other networks from ASxx would be sent with a LOCAL-PREF value of 120. Technically, its not necessary to set the LOCAL-PREF to 120 for the rest of the networks, using the default of 100 would work as well. But by setting the LOCAL_PREf high makes for a more explicit routing policy. So looking at the the routing announcements from both A and B, UUNET would select Router A as the primary path for the customer's web farm and Router B for all others destinations to this customer.

If for example, a customer is connected to UUNET and a local exchange point and wants all of its peers at the exchange points to prefer its direct connection over a alternative path advertised by UUNET but still wants to use UUNET to reach UUNET customers and other peers, this situation is best solved by modifying the AS PATH LENGTH. The customers router configuration might look like this:

Router A
    !
    ip bgp-community new-format
    router bgp ASxx
    neighbor 157.130.x.y remote-as 701
    neighbor 157.130.x.y version 4
    neighbor 157.130.x.y route-map from-UUNET in
    neighbor 157.130.x.y route-map to-UUNET out
    neighbor 157.130.x.y send-community
    !
    ip community-list 2 deny 705:666
    ip community-list 2 permit
    !
    ip as-path access-list 1 permit .*
    !
    route-map to-UUNET permit 10
     match as-path 1
     set community 701:2
    !
    route-map from-UUNET permit 10
     match community 2
     set community 701:80
    !

Questions on this material should be directed to a Senior.