Chapter 50
| IP Routing Commands
Border Gateway Protocol (BGPv4)
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current best available path is withdrawn), it can be used to calculate a new best
available path.
BGP cannot detect routes and provide reachability information. To ensure that each
iBGP peer knows how to reach other, each peer must run some sort of Interior
Gateway Protocol (such as static routes, direct routes, RIP or OSPF) which provides
neighbor IP addresses. In order to avoid routing loops, an iBGP speaker cannot
advertise prefixes it has learned from one iGBP peer to another neighboring iBGP
peer.
BGP Routing Basics Both RIP and OSPF attach a metric, or cost, to each path. These protocols rely on
every router attaching the same meaning to each metric, allowing consistent
calculation of routes. However, after routing policies are put in place, routers may
value some metrics differently, invalidating the basic assumptions up which RIP
and OSPF are based. This makes it unrealistic to run a distance-vector AS-level
protocol
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.
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