When routers first come up on a
network, they perform the Hello process, as discussed earlier. In addition, they
must elect a Designated Router and Backup Designated Router (DR and BDR) to
forward network information to all participating OSPF routers. The DR and BDR
add value to the network in the following ways:
- Reduce routing update traffic --- The DR and BDR act as a central point
of contact for link-state information exchanged on a given network; therefore,
each router must establish an adjacency with the DR/BDR, as shown in the
figure. Instead of each router exchanging link-state information with every
other router on the segment, each router sends information about itself in a
link-state format (known as a Link State Advertisement, or LSA) to the DR and
BDR. The DR sends each router's link-state information to all other routers in
the network. This flooding process significantly reduces the
router-related traffic on a segment.
- Manage link-state synchronization --- The DR and BDR ensure that the other
routers on the network have the same link-state information about the
internetwork. In this way, the number of routing errors is reduced.
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Lab
Activity |
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In this lab,
you will learn how to configure OSPF timers.
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The BDR does not perform any DR functions when the DR is operating. Instead,
it receives all information, but allows the DR to perform the forwarding and
synchronization tasks. The BDR performs DR tasks only if the DR fails.
To elect a DR and BDR, the routers view each other's priority value during
the Hello packet exchange process, as shown in the figure. The priorities are
listed as P=x where x is the priority setting. The router with priority 3 is the
highest priority number and will become the Designated Router. The router with
the next highest priority (priority 2) is the BDR. Routers with a priority of 0
cannot become a Designated Router or a Backup Designated Router. The following
conditions are used to determine which routers are elected as DR and BDR:
- The router with the highest set priority value is the DR.
- The router with the second highest set priority value is the BDR.
- The default value for OSPF interface priority is 1. In case of a tie, the
router's Router ID is used as a tie breaker. Thus, when two routers'
priority values are equal, the router with the highest ID number becomes the
DR.
- If a router with a higher priority value is added to the network, the DR
and BDR do not change. The only time a DR or BDR will change is if
one of them goes down. If the DR goes down, the BDR takes over as the DR and
a new BDR is elected. If the BDR goes down, a new BDR is elected.
- To determine whether the DR is down, the BDR sets a timer. This is a
reliability feature. If the BDR does not hear the DR forwarding link-state
advertisements (LSAs) before the timer expires, the BDR assumes the DR
is out of service and the BDR becomes the new DR.
In a multiaccess environment, each network segment, as shown in the
main Figure,
will have its own DR and BDR. Therefore, a router that is connected to multiple
networks can be a DR on one segment and a regular router on another segment. How
neighbors are perceived in other network topologies is discussed next.
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