Avaya Configuring IP Services Manual do Utilizador descarregar pdf (Página 266)

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101

Configuring IP Services

6-6

Neighbor Reachability Phase

This portion of EGP is responsible for monitoring and maintaining an established

EGP neighbor relationship between two routers. Its purpose is to ensure that the

neighbors are operational and can provide reliable network reachability

information.

Two neighbors will be able to exchange network reachability information only if

they are both in the UP state and know that they are both in the UP state. This is

the point at which neighbor reachability is positively determined.

Whether a router is in the UP or DOWN state is indicated in the status ﬁeld of the

Hello and I-H-U messages. Following is a description of these two messages. Poll

and Update messages are also sent during the neighbor reachability phase, but

they will be discussed in the next section.

• Neighbor Reachability Hello Message

This message is sent by the active neighbor to the passive neighbor to

determine whether the passive neighbor is functioning. The frequency of the

active router’s Hello message transmissions is dictated by the passive router’s

conﬁgured Hello interval. The passive router speciﬁes, in the Neighbor

Acquisition Conﬁrm response, an interval at which it is willing to respond to

Hello messages. The active router can send Hello messages less frequently

than the speciﬁed Hello interval, but not more frequently. The passive

neighbor determines reachability by the status ﬁeld in the active neighbor’s

Hello message.

• Neighbor Reachability I-H-U Response

This response is sent by the passive neighbor in response to a Hello message.

If the status ﬁeld in the I-H-U is UP, then the active neighbor determines that

the passive neighbor is reachable.

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