Elections occurs in a multi-access network for becoming
DR (Designated Router)
BDR (Backup Designated Router)
Router A won the election as DR and B router gets promoted to BDR, one network can have only one DR and only one BDR, the remaining routers will become DR others. DR (Designated Router)
BDR (Backup Designated Router)
Election in OSPF is stable one election is finalized it will never change, if they change then it means that our databases will keep on changing.
The Election of DR and BDR depends upon two factors
- Priority
- Router ID
The router with highest priority wins the election and the router with second highest Priority will become BDR all others becomes DR Others.
Waiting state is for DR and BDR only and not for DR Others
I. First of all Priority will be checked Router with highest priority wins election and one with second highest priority becomes BDR.
II. If Priority is equal router with highest Router-ID wins DR election and router with second highest Router-ID becomes BDR and rest of the router becomes DR other.
Now even if any router with higher priority or higher router-id will come, doesn’t matter OSPF elections are stable because stability is very critical requirement on networks.
How to make Proper high end router your DR and BDR?
I. You must complete configurations within 40 seconds in order to make the appropriate router with highest priority, DR for your network.
II. You must clear OSPF process in order to organize OSPF Elections once again.
Router# clear ip ospf process
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Now Elections will take place once again
Highly recommended not to use this command in live environment, it will give you down time
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§ What if DR goes down?
BDR will get promoted to DR and the router with the third highest priority will take place of BDR.
§ What if DR comes up once again?
Once the elections are finalized the elections will not occur again until you issue clear OSPF process command and the coming router will become DR Other.
§ Now if the new DR goes down?
BDR will become DR and the elections will occur for the BDR.
§ What is role of DR and BDR?
OSPF didn’t broadcasts its packets, it multicasts, on the other hand in the case of RIP it broadcasts its packets, to ensure debug rip’s packets
As a result you’ll see rip send it’s packets to 255.255.255.255, but when you debug OSPF packets you’ll see it sends it’s packets on two multicast address.
I) 224.0.0.5
II) 224.0.0.6
How many networks a router can have?
It depends upon the Number of interfaces, because every interface represents one network.
At the same time, it is possible that we’ve made some loop back interfaces then every loop back interface will also have a different network so it can’t be answered accurately.
It can have 15 or even 100 networks, so when we say that a router has 15 networks then it will send all its networks to its neighbors for performing convergence.
One possibility is that router broadcast its networks and sends it to everybody which is not at all efficient.
OSPF multicasts, it is kind of centralized communication.
§ When a new router comes up in a network it talks to group of DR’s, DR and BDR are responsible for informing everybody about network.
§ Why DR and BDR?
If there are 100 routers and everybody is sending it’s advertisements to everybody it will eat up all the bandwidth and it will take a long time to converge,
DR and BDR handles the convergence, OSPF running routers multicast network updates to DR and BDR instead of broadcasting.
§ The very first process in OSPF is DR and BDR election.
Because DR and BDR are responsible for communication, to manage the database of LSA it’s another responsibility of DR and BDR.
§ Every network has its own DR and BDR provided if it’s multi-access network
How many DR and BDR are there in this Topology?
There could be three DR and BDR in this topology
No DR/BDR elections on serial (point to point) links they will talk directly to each other.
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How many DR and BDR are there in this Topology?
There are four networks in topology and each network will have its own DR and BDR if it’s multi-access network, two are serial so there could be two DR and BDR.
How many DR and BDR are there in this Topology?
One DR one BDR and one DR other
To check which interfaces are running OSPF
Router# show ip ospf interface
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R1#show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 200.100.1.1/24, Area 0, Attached via Network Statement
Process ID 1, Router ID 200.100.1.1, Network Type BROADCAST, Cost: 1
Topology-MTID Cost Disabled Shutdown Topology Name
0 1 no no Base
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) 200.100.1.3, Interface address 200.100.1.3
Backup Designated router (ID) 200.100.1.4, Interface address 200.100.1.4
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:02
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 2
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 3, Adjacent neighbor count is 2
Adjacent with neighbor 200.100.1.3 (Designated Router)
Adjacent with neighbor 200.100.1.4 (Backup Designated Router)
Suppress hello for 0 neighbor(s)
A router is DR / BDR / DR other at interface level not as a whole, a router can be DR / BDR / DR Other at the same time at different interfaces.
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Practice LAB
I. Configure that router with lowest priority first so that it can become DR
II. Now configure that router second priority so that it can become BDR
III. And now configure the highest priority router to become DR other
IV. Clear OSPF process to see the fair elections result
For any correction, improvement or suggestion please do not hesitate to reach out at usman@cyberswats.com
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