Avaya Configuring IP Services Manual do Utilizador descarregar pdf (Página 267)

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101

Configuring EGP Services

6-7

As stated previously, Hello messages and I-H-U messages are used to determine

neighbor reachability. A neighbor is reachable when it moves to the UP state,

which is indicated in the status ﬁeld of these two messages. A neighbor will move

to the UP state only when it has received a certain number of reachability

indicators within a speciﬁed time interval. Similarly, a neighbor will move to the

DOWN state when it has not received a certain number of reachability indicators

within that same speciﬁed time. These UP and DOWN state thresholds differ for

active and passive routers (see Table 6-2).

T is the agreed-upon Hello interval for this neighbor relationship. If, after 5 Hello

intervals, the number of reachability indicators is 3 for an active router or 1 for a

passive router, the neighbor is considered UP. If, after 5 Hello intervals, the

number of reachability indicators is 1 for an active router or zero for a passive

router, the neighbor is considered DOWN.

Figure 6-3 shows two routers that already have formed an EGP neighbor

relationship in the Neighbor Acquisition phase, and are now attempting to

determine neighbor reachability. Router B, the active neighbor, will use the Hello

and Poll intervals provided by Router A, the passive neighbor. The Hello interval

is 30 and the Poll interval is 120.

Table 6-2. UP and DOWN State Thresholds

Mode UP Threshold DOWN Threshold Speciﬁed Time Interval

Active 3 1 T x 5

Passive 1 0 T x 5

1 2 ... 262 263 264 265 266 267 268 269 270 271 272 ... 465 466

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