Avaya Configuring IP Services Bedienungsanleitung PDF herunterladen (Seite 23)

IP Concepts, Terminology, and Features

1-11

Exterior Gateway Protocol (EGP)

The Exterior Gateway Protocol (EGP-2) is an exterior gateway protocol used to

exchange network reachability information between routers in different

autonomous systems. An IGP, such as RIP or OSPF, is used within an AS to

facilitate the communication of routing information with the autonomous system.

The routers that serve as the end points of a connection between two autonomous

systems run an exterior gateway protocol, such as EGP-2.

Routers establish EGP neighbor relationships in order to periodically exchange

reliable network reachability information. The router uses this information to

maintain a list of gateways, the networks the gateways can reach, and the

corresponding distances.

For a more detailed overview and EGP conﬁguration information, see Chapter 6.

Static Routes

You can manually conﬁgure a route to another network and enter the route in the

IP routing table. Such a route is called a

static route

For information about static routes and instructions on including a static route in

the routing table, see Chapter 2.

Route Preferences

The IP router maintains an internal routing table. When determining how to

forward a datagram, the IP router consults the table to determine the speciﬁc route

a datagram should take. A routing table can contain direct routes for the IP

router’s network interfaces, static routes, and the routes learned from RIP, OSPF,

BGP, and/or EGP, if enabled (information about adjacent hosts is maintained in a

separate table).

It is possible for a routing table to contain multiple routes to the same destination.

In such a situation, IP uses (among other information) a preference value to

determine which route to select. Preference values range from 1 to 16 (the higher

the number, the greater the preference).

By default, RIP, BGP, EGP, and OSPF external routes have a preference value of

1. Static routes, direct routes, and OSPF intra-area and interarea routes have a

default preference of 16.

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Avaya Configuring IP Services Bedienungsanleitung Seite 23