Local Preference Attribute

• Local preference is a well-known discretionary attribute that tells the routers in an AS which path is the preferred path to exit the AS.
• If an internal BGP speaker receives a multiple routes to a destination, the router compares the LOCAL_PREF attribute of the routes.
  • The path with the higher local preference is chosen.
• Local preference is exchanged only among routers in the same AS, among internal BGP neighbors; it is not passed to other autonomous system (ie other EBGP peers).

• In the figure above, AS100 receives advertisement for network 172.16.1.0/24 from two different points.
• As Router A receives the advertisement from Router C, Router A sets the LOCAL_PREF to 50.
• Likewise, when Router B receives the advertisement to the same network (172.16.1.0/24), Router B sets the LOCAL_PREF to 100.
• These local preference values will be exchanged between IBGP neighbors, Routers A and B.
• Based on the higher value LOCAL_PREF for Router B, Router B will be use as the exit point for AS 100 to reach network 172.16.1.0 in AS 200.

Multi-exit Discriminator (MED) Attribute

• Whereas the local preference attribute affects traffic leaving the AS, The MED attribute influences incoming traffic.
• Also called the metric. A lower metric is preferred. As is true with most “metrics”, the lowest metrics means the shortest distance, and thus the preferred one.
  • MED is set to 0 (zero) by default.
• This attribute is carried in EBGP updates and allows an AS to indicate to another AS its preferred incoming points.
• By default, a router compares the MED attribute only for paths from the neighbors in the same AS.
• The MED is exchanged between two directly connected autonomous systems only.
  • MEDs are not passed beyond the receiving AS.

• In the Figure above,  a subscriber in AS 200 is dual-homed to a single ISP (AS 100).
• Within AS 100, IBGP is being used between the routers. The MEDs from AS 200 are exchanged between these internal peers so that they both know which route to prefer.
• MEDs also do not go past beyond the receiving AS. IF AS 100 advertises 172.16.1.0/24 to another AS, for instance, it does not pass along the MED set by the originating AS; AS 200 in this case.
• Additionally, MEDs are not compared if two routes to the same destination are received from two different autonomous systems.
  • For example, 172.16.1.0/24 is advertised from AS 200 and another AS, the MEDs are not compared.
  • MEDs are meant only for a single AS (with multiple entry point) in order to compare which entry point to prefer.

Community Attribute

• Communities are optional transitive attributes that is designed to simplify policy enforcement. It is one way to filter incoming or outgoing routes.
• BGP communities allow routers to tag routes with a community indicator and allow other routers to make decisions based on that tag.
  • It Identifies a route as a member of some community of routes that share some common properties.
  • An example might be an ISP that assigns a particular COMMUNITY attribute to all of its customers’ routes. The ISP may then set its LOCAL_PREF attribute based on the COMMUNITY value instead of basing it on each inidividual route.
• The community attribute was originally a Cisco-speficific attribute. But now a RFC standard through RFC 1997.

Weight Attribute (Cisco Only)

• The weight attribute is a Cisco-specific attribute.
• It is configured locally on the router and is not communicated or propagated to other routers.
• The weight ha a value between 0 to 65,535.
  • By default, all routes generated by the local router have a weight of 32,768.
  • All routes learned from a peer have a weight of 0.
  • The higher the weight, the more preferable the route.
• The weight attribute applies when using one router with multiple exit points out of an AS.
  • Contrast it with the local preference attribute where it is used when two or more routers provide multiple exit points.

• In the figure above, Router A receives an advertisement for network 172.16.1.0/24 from Routers B and C.
  • Router A knows about more than one route to the same destination.
• The route coming from Router B has an associated weight of 50.
• The route coming from Router C has an associated weight of 100.
• Both paths for network 172.16.1.0 will be in the BGP routing table, with their respective weights.
• The route with a higher weight will be installed in the IP routing table.

Resources:

1. Border Gateway Protocol – Internetworking Technology Handbook – Cisco Systems
2. RFC 4451 – BGP MULTI_EXIT_DISC (MED) Considerations
3. RFC 1997 – BGP Communities Attribute
4. RFC 1998 -An Application of the BGP Community Attribute in Multi-home Routing

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet; Routing TCP/IP, Volume 1 (2nd Edition) (CCIE Professional Development) by Jeff Doyle and Jennifer Carroll; as well as following the links on the resources section of this entry.