ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
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available path, the peer keeps a copy of it in its routing table so that if path
information for that prefix changes (such as if the 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path is withdrawn), it can be used to calculate a new best available path.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, E G P a n d I n c o m p l e t e .
◆ 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.