Avaya Configuring IP, ARP, RIP, and OSPF Services Instrukcja Użytkownika Pobierz pdf (Strona 41)

IP Concepts, Terminology, and Features

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Router Discovery Protocol

Before a host can send IP datagrams beyond its directly attached subnet, the host

must discover the address of at least one operational router on that subnet. Router

Discovery is an extension of the Internet Control Message Protocol (ICMP) that

enables hosts attached to multicast or broadcast networks to discover the IP

addresses of their neighboring routers.

Routers configured with Router Discovery periodically multicast or broadcast a

route advertisement from each of their interfaces, announcing the IP address or

addresses of that interface. Hosts discover the addresses of their neighboring

routers by listening for these advertisements. Hosts will use the router with the

highest preference level as a gateway.

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

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

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

BGP, or EGP. (Information about adjacent hosts is maintained in a separate table.)

When a routing table contains multiple routes to the same destination, IP uses

(among other information) a preference value to determine which route to select.

Preference values range from 1 through 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 value of 16.

You can configure a preference value from 1 through 16 for RIP, BGP, EGP, OSFP

external, and static routes. You cannot configure the preference value of direct

routes and OSPF intra-area and interarea routes.

To assign a preference value to a route learned by RIP, OSPF, BGP, or EGP, you

configure an accept policy for the route (see “

IP Routing Policies and Filters” on

page 1-21). 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.

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Avaya Configuring IP, ARP, RIP, and OSPF Services Instrukcja Użytkownika Strona 41