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DGS-3312SR
Инструкция по эксплуатации Страница: 149

D-Link DGS-3312SR [149/290] Adjacencies

DGS-3312SR Stackable Gigabit Layer 3 Switch

Neighbors

Routers that are connected to the same area or segment become neighbors in that area. Neighbors are elected via the Hello

protocol. IP multicast is used to send out Hello packets to other routers on the segment. Routers become neighbors when

they see themselves listed in a Hello packet sent by another router on the same segment. In this way, two-way

Of course, the interfaces should belong to the same subnet and have the same subnet mask.

rs on

password before they

e Hello interval specifies the length of time, in seconds, between the

on an OSPF interface. The dead interval is the number of seconds that a

been seen before its neighbors declare the OSPF router down. OSPF

on each segment in order to acknowledge each other’s existence on a

n multi-access segments. OSPF requires these intervals to be

exactly the same between any two neighbors. If any of these intervals are different, these routers will not

ar segment.

uters also have to have the same stub area flag in their Hello packets in

d router as the Backup Designated Router (BDR) on each

All other routers on the segment will then contact the

width required for link-state database updates.

Designated Router Election

The election of the DR and BDR is accomplished using the Hello protocol. The router with the highest OSPF priority on a

given multi-access segment will become the DR for that segment. In case of a tie, the router with the highest Router ID

wins. The default OSPF priority is 1. A priority of zero indicates a router that cannot be elected as the DR.

Building Adjacency

Two routers undergo a multi-step process in building the adjacency relationship. The following is a simplified description

of the steps required:

• Down − No information has been received from any router on the segment.

• Attempt − On non-broadcast multi-access networks (such as Frame Relay or X.25), this state indicates

that no recent information has been received from the neighbor. An effort should be m de to contact the

neighbor by sending Hello packets at the reduced rate set by the Poll Interval.

• Init − The interface a neighbor but bi-directional

communication has not yet been established.

• Two-way − Bi-directional communication with a neighbor has been established. The router has seen its

address in the Hello packets coming from a neighbor. At the end of this stage the DR and BDR election

would have been done. At the end of the Two-way stage, routers will decide whether to proceed in

building an adjacency or not. The decision is based on whether one of the routers is a DR or a BDR or

the link is a point-to-point or virtual link.

• Exstart − (Exchange Start) Routers establish the initial sequence number that is going to be used in the

information exchange packets. The sequence number insures that routers always get the most recent

information. One router will become the primary and the other will become secondary. The primary

router will poll the secondary for information.

• Exchange − Routers will describe their entire link-state database by sending database description packets.

communication is guaranteed to be possible between any two neighbor routers.

Any two routers must meet the following conditions before the become neighbors:

• Area ID − Two routers having a common segment − their interfaces have to belong to the same area on

that segment.

• Authentication − OSPF allows for the configuration of a password for a specific area. Two route

the same segment and belonging to the same area must also have the same OSPF

can become neighbors.

• Hello and Dead Intervals − Th

hello packets that a router sends

router’s Hello packets have not

routers exchange Hello packets

segment and to elect a Designated Router o

become neighbors on a particul

• Stub Area Flag − any two ro

order to become neighbors.

Adjacencies

Adjacent routers go beyond the simple Hello exchange and participate in the link-state database exchange process. OSPF

elects one router as the Designated Router (DR) and a secon

multi-access segment (the BDR is a backup in case of a DR failure).

DR for link-state database updates and exchanges. This limits the band

has detected a Hello packet coming from

136

D-Link DGS-3312SR [149/290] Adjacencies

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