DR and BDR Election Procedure

DR and BDR Election Procedure
Each OSPF interface (multi-access networks only) possesses a configurable router priority. The
Cisco default is 1. If you don’t want a router interface to participate in the DR/BDR election,
set the router priority to 0 using the ip ospf priority command in interface configuration
mode. Here is a sample (the priority field is highlighted for ease of identification):
RouterA#show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 172.16.22.14/24, Area 0
Process ID 100, Router ID 172.16.246.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 172.16.247.1, Interface address 172.16.22.9
Backup Designated router (ID) 172.16.246.1, Interface address 172.16.22.14
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:08
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 172.16.22.101
Adjacent with neighbor 172.16.247.1 (Designated Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
Youngest key id is 10
RouterA#
This value is key when electing the DR and BDR. Let’s go through the steps that occur in any
given router when the DR and BDR are elected for a specific multi-access network of which it
is a member:
1. The local router creates a list of eligible neighbors. The eligible neighbors are those with
which the local router has entered at least a 2Way state. The local router adds itself to this
list and to all lists that are formed from this list in subsequent steps, for which it qualifies.
2. From this list, all routers whose participating interfaces have a router priority of 0 are
removed. These routers will be among the DROthers on this network.
3. The local router makes note of the current DR and BDR values for later comparison.
4. A list of all routers not claiming to be the DR (their own Hello packets do not list them as
the DR) is compiled from the list resulting from step 2.
5. The local router will select the BDR from the list in step 4, based on the following criteria
in order:
 If one or more of the routers in the list have declared themselves the BDR, then the one
of these with the highest priority is selected to be the BDR.
 If all router priorities are equal, the router with the highest router ID becomes the BDR.
 If no router in the resulting list from step 4 has declared itself the BDR, then the router
with the highest router priority is selected to be the BDR.
Open Shortest Path First 151
 If all routers have the same router priority, then the router with the highest router ID is
selected to be the BDR.
6. A list of all routers claiming to be the DR (their own Hello packets list them as the DR) is
compiled from the list resulting from step 2.
7. The local router will select the DR from the list in step 6, based on the following criteria in order:
 The router with the highest router priority is selected to be the DR.
 If all router priorities are equal, the router with the highest router ID is selected to be the DR.
 If the resulting list from step 6 is empty, meaning that no router has declared itself the
DR, then the BDR that was selected in step 5 becomes the DR.
8. If the local router’s DR status has been altered by the preceding steps, either causing it to
become, or cease to be, the DR, based on a comparison to the results of step 3, then it will
repeat steps 4 through 7. This serves to make sure that the local router does not declare
itself both the DR and the BDR, because both may be declared by default. If selected to be
the DR, the local router will definitely not make the BDR list the next time around in step 4.
Conversely, if dethroned as the DR, the local router can become eligible to be the BDR the
next time around, by making the list in step 4.
You should remember that the previous process occurs independently for each router interface
when a router becomes active on a segment for which it does not detect a current DR. If a
DR and BDR already exist on the segment, any new interfaces accept the DR and BDR regardless
of their own router ID or router priority. This minimizes changes on the segment, which can
otherwise generate new router and network LSAs, causing a need for the entire routing domain
to reconverge, which leads to temporary instability.
To further the example, if initially there is only one OSPF router interface active on the segment,
it becomes the DR. The next router would become the BDR. Barring some event that
causes a router’s interface state machine to trigger a new election, all subsequent initializing
interfaces on the multi-access network would accept the existing DR and BDR and form adjacencies
with them. In other words, OSPF does not allow preempting for the DR even if another
router becomes active that has a higher router priority or router ID. This allows for greater network
stability because a router with a higher priority, oscillating from up to down, will not
affect the router selected as the DR.