Chapter 19
| Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
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Routes are assigned a metric equal to the sum of all metrics for each interface
link in the route.
This router uses a default cost of 1 for all ports. Therefore, if you install a
10 Gigabit module, you need to reset the cost for all of the 1 Gbps ports to a
value greater than 1 to reflect the actual interface bandwidth.
◆ Router Priority – Sets the interface priority for this router. (Range: 0-255;
Default: 1)
This priority determines the designated router (DR) and backup designated
router (BDR) for each OSPF area. The DR forms an active adjacency to all other
routers in the area to exchange routing topology information. If for any reason
the DR fails, the BDR takes over this role.
Set the priority to zero to prevent a router from being elected as a DR or BDR. If
set to any value other than zero, the router with the highest priority becomes
the DR and the router with the next highest priority becomes the BDR. If two or
more routers are set to the same highest priority, the router with the higher ID
will be elected.
If a DR already exists for an area when this interface comes up, the new router
will accept the current DR regardless of its own priority. The DR will not change
until the next time the election process is initiated.
Configure router priority for multi-access networks only and not for point-to-
point networks.
◆ Hello Interval – Sets the interval between sending hello packets on an
interface. This interval must be set to the same value for all routers on the
network. (Range: 1-65535 seconds; Default: 10)
Hello packets are used to inform other routers that the sending router is still
active. Setting the hello interval to a smaller value can reduce the delay in
detecting topological changes, but will increase routing traffic.
◆ Dead Interval – Sets the interval at which hello packets are not seen before
neighbors declare the router down. This interval must be set to the same value
for all routers on the network. (Range: 1-65535 seconds; Default: 40, or 4 times
the Hello Interval)
The dead-interval is advertised in the router's hello packets. It must be a
multiple of hello-interval and be the same for all routers on a specific network.
◆ Transmit Delay – Sets the estimated time to send a link-state update packet
over an interface. (Range: 1-65535 seconds; Default: 1 second)
LSAs have their age incremented by this delay before transmission. You should
consider both the transmission and propagation delays for an interface when
estimating this delay. Set the transmit delay according to link speed, using
larger values for lower-speed links.
If this delay is not added, the time required to transmit an LSA over the link is
not taken into consideration by the routing process. On slow links, the router
may send packets more quickly than devices can receive them. To avoid this
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