Nokia Network Voyager, Reference Manual

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6
Configuring Routing
282 Voyager Reference Guide
An AS uses an interior gateway protocol and common metrics to route
packets within an AS; it uses an exterior routing protocol to route packets to
other ASes.
Note
This implementation supports only BGP version 4.
BGP sends update messages that consist of network number/AS path pairs.
The AS path contains the string of ASes through which the specified network
can be reached. An AS path has some structure in order to represent the
results of aggregating dissimilar routes. These update messages are sent over
TCP transport mechanism to ensure reliable delivery. BGP contrasts with
IGPs, which build their own reliability on top of a datagram service.
As a path-vector routing protocol, BGP limits the distribution of a router’s
reachability information to its peer or neighbor routers.
BGP Sessions (Internal and External)
BGP supports two basic types of sessions between neighbors: internal
(sometimes referred to as IBGP) and external (EBGP). Internal sessions run
between routers in the
same autonomous systems, while external sessions run
between routers in different autonomous systems. When sending routes to an
external peer, the local AS number is prepended to the AS path. Routes
received from an internal neighbor have, in general, the same AS path that the
route had when the originating internal neighbor received the route from an
external peer.
BGP sessions may or may not include a single metric (Multi-Exit
Discriminator or MED) in the path attributes. Smaller values of the metric are
preferred. These values are used to break ties between routes with equal
preference from the same neighbor AS.
Internal BGP sessions carry at least one metric in the path attributes that BGP
calls the local preference. The size of the metric is identical to the MED. The
use of these metrics is dependent on the type of internal protocol processing.