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2.8.- The Designated (and
Backup) Router |
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Every broadcast or NBMA network has a Designated Router
(DR). The DR main functions are: |
- The DR originates a network-LSA on behalf of the
network. This LSA lists the set of routers currently attached to
the network. The Link State ID for this LSA is the DR's
IP interface address.
- The DR becomes adjacent to all other routers on the network.
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The DR is elected by the Hello Protocol. A router's Hello
packet contains its Router Priority, which is configurable on a
per-interface basis. When a router's interface becomes functional, it
checks to see whether there is currently a DR. If there is, it
accepts that DR, regardless of its Router Priority. Otherwise,
it becomes itself DR if it has the highest Router Priority on
the network. |
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The DR is the endpoint of many adjacencies. To optimize the flooding
procedure, the DR multicast its Link State Update packets to
the address AllSPFRouters, rather than sending separate packets over
each adjacency. |
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Router nodes are labelled with their Router ID. Transit networks are
labelled with the IP address of their DR. Then, when the DR
changes, this will cause the network and all its routers to originate new
LSAs. Until the Link-State Database converges, some temporary
loss of connectivity occur. This may result in ICMP unreachable
messages being sent in response to data traffic. For that reason, the
DR should change only infrequently. Router Priority should be
configured to elect the most reliable router on a network as DR. |
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The Backup Designated Router |
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In order to make the transition to a new DR smoother, there is a
Backup Designated Router (BDR) for each broadcast and NBMA
network. The BDR is also adjacent to all routers on the network, and
becomes DR when the previous DR fails. I there were no BDR,
when a new DR became necessary, new adjacencies and link-state
database synchronization should be required, which can potentially take
quite a long time. During this time, the network would not be available for
transit data traffic. The BDR obviates the need to form these
adjacencies, since they already exist. This means the period of disruption
last only as long as it takes to flood the new LSAs which announce
the new DR. |
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The BDR does not generate a network-LSA for the network. If it
did, the transition would be even faster. However, this is a tradeoff
between database size and speed of convergence when the DR fails. The
BDR is also elected by the Hello Protocol. Each Hello
packet has a field that specifies the BDR for the network. |
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