10G Core Routing Switch User Manual
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DR is not designated manually; instead, it is elected by all the routers on the segment. Routers with the priorities > 0 in
the network are eligible “candidates”. Among all the routers self-declared to be the DR, the one with the highest
priority will be elected. If two routers have the same priority, the one with the highest router ID will be elected as the
DR. Votes are the hello packets. Each router writes the expected DR in the packet and sends it to all the other routers
on the segment. If two routers attached to the same segment concurrently declare themselves to be the DR, choose
the one with higher priority. If the priorities are the same, choose the one with greater router ID. If the priority of a
router is 0, it will not be elected as DR or BDR.If DR fails due to some faults, the routers on the network must elect a
new DR and synchronize with the new DR. The process will take a relatively long time, during which, the route
calculation is incorrect. In order to speed up this process, OSPF puts forward the concept of BDR. In fact, BDR is a
backup for DR. DR and BDR are elected in the meantime. The adjacencies are also established between the BDR and all
the routers on the segment, and routing information is also exchanged between them. When the DR fails, the BDR will
become the DR instantly. Since no re-election is needed and the adjacencies have already been established, the
process is very short. But in this case,a new BDR should be elected. Although it will also take a quite long period of time,
it will not exert any influence upon the route calculation.
But please note:
The DR on the network is not necessarily the router with the highest priority. Likewise, the BDR is not necessarily
the router with the second highest priority. If a new router is added after DR and BDR election, it is impossible for
the router to become the DR even if it has the highest priority.
DR is based on the router interface in a certain segment. Maybe a router is a DR on one interface, but can be a
BDR or DR Other on the other interface.
Beginning in privileged EXEC mode, follow these steps to set the Interface Priority for DR Election.
Command
Purpose
Step 1
config terminal
Enter global configuration mode.
Step 2
Interface
ifName
Enter interface configuration mode.
Step 3
ip ospf priority
priority_num
Configure the interface with a priority for DR election.
By default, the priority of the Interface is 1 in the DR
election. The value can be taken from 0 to 255.
Step 4
exit
Return to global configuration mode.
Step 5
exit
Return to privileged EXEC mode.
Step 6
show ip ospf interface [
vlan-id
]
Verify your entries.
Step 7
Copy
running-config
startup-config
(Optional) Save your entries in the configuration file.
To restore the default interface priority, use
no ip ospf priority
interface configuration command.
Setting the Interval of Hello Packet Transmission
Hello packets are a kind of most frequently used packets, which are periodically sent to the adjacent router for
discovering and maintaining the adjacency, and for electing DR and BDR. The user can set the hello timer.
According to RFC2328, the consistency of hello intervals between network neighbors should be kept. The hello interval
value is in inverse proportion to the route convergence rate and network load.
