Avaya Configuring IP Services Bedienungsanleitung Seite 39

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IP Concepts, Terminology, and Features
114065 Rev. A 1-11
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.
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.
You can configure a preference value in the range of 1 to 16 for RIP, BGP, EGP,
OSFP external, and static routes. The preference of direct routes and OSPF
intra-area and interarea routes cannot be user-configured.
To assign a preference to a route learned by RIP, OSPF, BGP, and EGP, you
configure an accept policy for the route. If an incoming route matches the policy,
IP assigns the preference value you specify to the route and considers the route for
possible inclusion in the routing table.
Route Weights
Route-weight calculation is an internal tool that IP uses to facilitate selection of
the best route among alternative routes to the same destination. Route-selection
criteria are encoded into the route weight in a way that allows IP to compare
routes simply by comparing their weight values, regardless of route sources.
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