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what is difference between designated router and backup
designated router and explain feature of both router.
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Answer Posted / jitu
A designated router (DR) is the router interface elected
among all routers on a particular multiaccess network
segment, generally assumed to be broadcast multiaccess.
Special techniques, often vendor-dependent, may be needed to
support the DR function on nonbroadcast multiaccess (NBMA)
media. It is usually wise to configure the individual
virtual circuits of a NBMA subnet as individual
point-to-point lines; the techniques used are
implementation-dependent.
Do not confuse the DR with an OSPF router type. A given
physical router can have some interfaces that are designated
(DR), others that are backup designated (BDR), and others
that are non-designated. If no router is DR or BDR on a
given subnet, the DR is first elected, and then a second
election is held if there is more than one BDR. [7] The DR
is elected based on the following default criteria:
* If the priority setting on a OSPF router is set to 0,
that means it can NEVER become a DR or BDR (Backup
Designated Router).
* When a DR fails and the BDR takes over, there is
another election to see who becomes the replacement BDR.
* The router sending the Hello packets with the highest
priority wins the election.
* If two or more routers tie with the highest priority
setting, the router sending the Hello with the highest RID
(Router ID) wins. NOTE: a RID is the highest logical
(loopback) IP address configured on a router, if no
logical/loopback IP address is set then the Router uses the
highest IP address configured on its active interfaces.
(e.g. 192.168.0.1 would be higher than 10.1.1.2).
* Usually the router with the second highest priority
number becomes the BDR.
* The priority values range between 0 - 255[8], with a
higher value increasing its chances of becoming DR or BDR.
* IF a HIGHER priority OSPF router comes online AFTER
the election has taken place, it will not become DR or BDR
until (at least) the DR and BDR fail.
* If the current DR 'goes down' the current BDR becomes
the new DR and a new election takes place to find another
BDR. If the new DR then 'goes down' and the original DR is
now available, it then becomes DR again, but no change is
made to the current BDR.
DR's exist for the purpose of reducing network traffic by
providing a source for routing updates, the DR maintains a
complete topology table of the network and sends the updates
to the other routers via multicast. All routers in an area
will form a slave/master relationship with the DR. They will
form adjacencies with the DR and BDR only. Every time a
router sends an update, it sends it to the DR and BDR on the
multicast address 224.0.0.6. The DR will then send the
update out to all other routers in the area, to the
multicast address 224.0.0.5. This way all the routers do not
have to constantly update each other, and can rather get all
their updates from a single source. The use of multicasting
further reduces the network load. DRs and BDRs are always
setup/elected on Broadcast networks (Ethernet). DR's can
also be elected on NBMA (Non-Broadcast Multi-Access)
networks such as Frame Relay or ATM. DRs or BDRs are not
elected on point-to-point links (such as a point-to-point
WAN connection) because the two routers on either sides of
the link must become fully adjacent and the bandwidth
between them cannot be further optimize
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