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TOPIC: election of DR and BDR in OSPF

election of DR and BDR in OSPF 6 years 6 months ago #33848

  • wakatana
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Hi all, I am trying to understand OSPF and I have one question regarding election of DR and BDR.

please refer to the topology at:
http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t22

As far as I know DR is the router with has highest IP address of loopback, if no loopback is set then the router which has highest (physical interface) IP address is the DR.

So my question is why is RTB elected for BDR ? (Second highest IP) There is RTD router which has higher IP than RTB, so RTD should be BDR.
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Re: election of DR and BDR in OSPF 6 years 6 months ago #33853

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Also could anyone please explain what is order in electing DR/BDR. I am experimenting with given topology but gives me no sense...

what is the order in electing DR/BDR ?? and what factors can affect electing BR/DBR I know one of those but what is the order ??

loopback IP address
normal IP address (physical interface)
Router(config-if)# ip ospf priority value
Router(config-router)# router-id value

Many thanks
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Re: election of DR and BDR in OSPF 6 years 6 months ago #33855

  • krik
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When an OSPF router becomes active and discovers its neighbors, it checks for an active DR and BDR. If a DR and BDR exist, the router accept them. If there is no BDR, an election is held in which the router with the highest priority becomes the BDR. If there is no active DR, the BDR is promoted to DR and a new election is held for the BDR.

It should be noted that the priority can influence an election, but will not override an active DR or BDR. If there are already DR and BDR on the LAN, they will remain DR and BDR until a failure.

The priority is the most important parameter in the DR/BDR election, the highest the best. Priority can range from 0 to 255. Cisco default is 1. A priority of 0 prevent the router to try to become DR/BDR.

If the priorities are equal then the router ID is used as tie breaker. Once again the highest the best.

Now the router ID is selected as follow, only move to the next step if the previous did not provide the router ID:
- configured router ID
- highest IP of any up/up loopback interface
- highest IP of any up/up non-loopback interface
- not defined and prevent ospf to start if none of the previous steps provided it

Note the router ID can only be changed by restarting the OSPF process.
Christophe Lemaire
www.exp-networks.be/blog/
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Re: election of DR and BDR in OSPF 6 years 6 months ago #33860

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Krik thanks for explain, could you please explain those scenarios:


1st:
Configured priority on RTF to 2 (so it will be DR, that is OK)
Q: why RTA is not BDR ? (RTA has configured loopback address)
BDR router is RTD (higher physical interface address) instead of RTA, why ???

2nd
Configured priority on RTF to 2 (so it will be DR, that is OK)
Configured router ID on RTB to 203.250.14.50 (so it will be be BDR, that is OK)
Seems everything works

3rd
Routers configured exactly according scenario (RTA with loopback, no priorities, no router IDs)
Q: RTD was elected for BR (why not RTA with configured loopback) ?




PS:

I treid this on Packet tracer 5.2.1

I have restarted OSPF process after change using:
Router# copy running startup
Router# reload

DR and BDR checked by issuing:
Router# sh ip ospf neighboor

on all routers and search those where are all states were FULL
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Re: election of DR and BDR in OSPF 6 years 6 months ago #33862

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Also noticed some strange thing I have router with IP address
fa0/0 203.250.15.1
fa0/1 203.250.14.2


when I issue:

Router# show ip ospf neighboor

on connected routers (no mather on which interface)

it gives me 203.250.14.2 in Neighboor ID, why ?? 203.250.15.1 is bigger than 203.250.14.2, so it should be RID (router ID)
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Re: election of DR and BDR in OSPF 6 years 6 months ago #33869

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Krik thanks for explain, could you please explain those scenarios:


1st:
Configured priority on RTF to 2 (so it will be DR, that is OK)
Q: why RTA is not BDR ? (RTA has configured loopback address)
BDR router is RTD (higher physical interface address) instead of RTA, why ???

RTD's higher physical interface IP (203.250.14.4) is simply greater than RTA's higher loopback interface IP (203.250.13.41)...
2nd
Configured priority on RTF to 2 (so it will be DR, that is OK)
Configured router ID on RTB to 203.250.14.50 (so it will be be BDR, that is OK)
Seems everything works
Ok here...
3rd
Routers configured exactly according scenario (RTA with loopback, no priorities, no router IDs)
Q: RTD was elected for BR (why not RTA with configured loopback) ?

I do not see any difference with point 1 here...
It is not because the router ID has been derived from a loopback interface that it is better than a router ID derived from a physical interface... Only the value matter. 203.250.13.41 is always smaler than 203.250.14.4 regardless how you got it...
PS:

I treid this on Packet tracer 5.2.1

I have restarted OSPF process after change using:
Router# copy running startup
Router# reload

DR and BDR checked by issuing:
Router# sh ip ospf neighboor

on all routers and search those where are all states were FULL

1. Reloading a router is not enough as another router will take the DR role. You should better clear all the OSPF processes on all the router at the sametime.

2. To check DR/BDR, it is easier to issue the "show ip ospf interface" command...

[code:1]Switch#sh ip ospf interface
Vlan2 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 101
Process ID 2, Router ID 10.1.1.1, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 255
[b]Designated Router (ID) 10.1.1.1[/b], Interface address 10.1.1.1
[b]Backup Designated router (ID) 10.1.1.12[/b], Interface address 10.1.1.12
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 0, maximum is 51
Last flood scan time is 0 msec, maximum is 16 msec
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 10.1.1.11
Adjacent with neighbor 10.1.1.12 (Backup Designated Router)
Suppress hello for 0 neighbor(s)[/code:1]
Also noticed some strange thing I have router with IP address
fa0/0 203.250.15.1
fa0/1 203.250.14.2


when I issue:

Router# show ip ospf neighboor

on connected routers (no mather on which interface)

it gives me 203.250.14.2 in Neighboor ID, why ?? 203.250.15.1 is bigger than 203.250.14.2, so it should be RID (router ID)

1. A router has only one RID, therefore if 203.250.14.2 has been selected, it will be the only RID used by the router regardless on which interface the neighborship is established.

2. Fa0/0 was probably down when the OSPF process started.
Christophe Lemaire
www.exp-networks.be/blog/
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