Avaya Configuring IP Services Bedienungsanleitung PDF herunterladen (Seite 269)

100

101

Configuring EGP Services

6-9

Figure 6-4. Neighbor Reachability Is Established with Both Routers in the

UP State

Network Reachability Phase

This portion of EGP is responsible for determining which networks are reachable

through two EGP neighbors; that is, it provides the network reachability

information. This information provides a list of gateways, the networks those

gateways can reach, and their associated distances.

Two neighbors determine network reachability by exchanging Poll messages and

Routing Update responses as described below:

• Poll Message

The active neighbor sends a Poll message to a passive neighbor that it already

knows to be reachable. The Poll message requests routing information from

the passive neighbor.

• Routing Update Response

The Routing Update Response is the message that contains the routing

information (the list of gateways on the common network, the networks they

can reach, and associated distances). Both active and passive neighbors can

send Routing Update messages. The active neighbor usually sends a Routing

Update Response after it sends a Poll message. The passive neighbor usually

sends a Routing Update Response in response to a Poll message.

ASN = 50 Hello = 30 Poll = 120

Mode = Passive

Router A

ASN = 51 Hello = 30

Poll = 240

Mode = Active

Router B

State = UP

Active Router B

transitions to the

UP state upon request

of the third I-H-U.

Third

I-H-U

Third

Hello Command

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