BGP Attributes detailed

The ORIGIN Attribute

The ORIGIN attribute is a well-known mandatory attribute (Type Code 1) that indicates the origin of the routing update with respect to the autonomous system that originated it. BGP considers three types of origins:

  • IGP – via command network
  • EGP – old EGP protocol
  • Incomplete – redistributed into BGP

The AS-PATH

The AS_PATH attribute is a well-known mandatory attribute (Type Code 2) that contains a sequence of autonomous system numbers that represent the path a route has traversed. Internally to an AS, routes passed among BGP speakers leave the AS_PATH information intact; however, when sending routes to external BGP peers, the AS that originates the route adds its own AS number. Thereafter, each AS that receives the route and passes it on to other EBGP peers will prepend its AS number to the list. Prepending is the act of adding the AS number to the beginning of the list. The final list represents all the AS numbers that a route has traversed. The AS number of the AS that originated the route is at the end of the list.

AS_PATH information is one of the attributes BGP uses to determine the best route to take to reach a destination. In comparing two or more different routes, given that all higher-priority attributes are equal, a shorter AS_PATH is always preferred over a longer one. In case of a tie, other attributes are used to determine the best path to the destination.

The AS_PATH Manipulation

AS_PATH information is often manipulated to affect interdomain routing behavior. Because BGP prefers a shorter AS_PATH path to a longer one, network operators are tempted to change the path information by including dummy AS_PATH entries that would increase the path length to influence or deter the traffic trajectory. Cisco’s implementation lets a user prepend AS numbers at the beginning of an AS_PATH to make the path length longer.

The MULTI_EXIT_DISC Attribute

The BGP Multiexit Discriminator (MULTI_EXIT_DISC or MED) attribute is an optional nontransitive attribute (Type Code 4). It is a hint to external neighbors about the preferred path into an AS that has multiple entry points. The MED is also known as the external metric of a route. A lower MED value is preferred over a higher MED value. Unlike LOCAL_PREF, the MED attribute is exchanged between ASs, but a MED attribute that is received by an AS does not leave the AS. When an update enters the AS with a certain MED value, that value is used for decision making within the AS. When BGP passes the routing update to another AS, the MED is reset to 0 (unless the outgoing MED is explicitly set to a specific value). Thats why a MED is not much used to influence inbound routing. More used is the AS_PATH Prepending.

The COMMUNITY Attribute

Communities are used to simplify routing policies by identifying routes based on a logical property rather than an IP prefix or an AS number. A BGP speaker can use this attribute in conjunction with other attributes to control which routes to accept, prefer, and pass on to other BGP neighbors.

The COMMUNITY attribute (Type Code 8) is an optional transitive attribute. A community is not restricted to one network or one autonomous system; it has no physical boundaries. It is of variable length and consists of a set of 4-byte values. Communities in the range 0x00000000 through 0x0000FFFF and 0xFFFF0000 through 0xFFFFFFFF are reserved. These communities are well-known—that is, they have a global meaning. Here are some examples of well-known communities:

  • NO_EXPORT (0xFFFFFF01) – A route carrying this community value should not be advertised to peers outside an AS.
  • NO_ADVERTISE (0xFFFFFF02) – A route carrying this community value, when received, should not be advertised to any BGP peer.

A route can have more than one community attribute. A BGP speaker that sees multiple community attributes in a route can act based on one, some, or all of the attributes. A router has the option of adding or modifying community attributes before passing routes on to other internal and external peers.

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