Chapter 29
| IP Routing Commands
Border Gateway Protocol (BGPv4)
– 910 –
BGP uses a path vector routing approach, which is roughly based on a distance-
vector approach, where the cost between two adjacent ASes is implicitly assumed
to be a single hop. The shortest path from an AS to a remote AS is therefore the
path with the shortest number or AS hops. Just note that each AS may be
comprised of multiple routers or networks that a packet traverses as it crosses the
associated route to the destination, so the AS hop count does not equal to the
number of routers along that path.
Path Attributes
The key information passed along with the path vector in routing messages include
the following attributes:
◆ ORIGIN – This attribute indicates how the network of BGP routers first learned
of a route, and is set by the first BGP router to introduce the routes to its peers.
There are three methods for injected a prefix into an update message: IGP, EGP
and Incomplete.
◆ AS_PATH – This attribute lists the autonomous systems that make up the path
to the routes’ destination. Each entry contains a series of path segments. Each
path segment begins with a 1 for SETS or a 2 for SEQUENCES, where a SET
indicates that it is an aggregate prefix which was derived from multiple ASes.
◆ NEXT_HOP – This attribute indicates the IP address of the router that should be
used as the next hop to reach the router’ destination. This address is normally
that of the router sending the BGP message, but a BGP router may advertise a
route on behalf of another router.
◆ MULTI_EXIT_DISC (MED) – The multi-exit discriminator attribute lets an
autonomous system set a preference for different routes when there are
multiple external links to a neighboring AS. Selection is normally based on the
exit point with the lowest metric.
◆ WEIGHT – This attribute is used locally by a router to select a path when
multiple paths are available for a prefix.
◆ LOCAL_PREF – This local preference attribute is similar to that of the MED, but
within an AS. It sets a metric which is used between BGP speakers within an AS.
It can help in selecting an outgoing BGP when an AS has connectivity to
multiple ASes or multiple BGP routes even with the same next hop AS.
◆ ATOMIC_AGGREGATE – This attribute indicates that the routes were created by
aggregating more specific routes. More specific routes may exist for some the
these longer prefixes, but the router chose not to send them, so as to reduce
the size for the AS path parameters.
◆ AGGRATOR – This is an optional attribute that identifies the AS and router that
originally aggregated the routes.
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