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Community Member

ospf state

Hi,

some confusion on ospf..1.it normally chooses the router-id if one is configured to denote the neigbor-id ,we have used the Lo0

address as the router-id, but will it take the router-id to denote as neighbor id even though the lo0 address in not included in

opsf networks?or is that ospf will take it as it has been configured to take it as router-id?

2.for the DR/BDR election, opsf normally choses the router with router-id or highest loopback.I have this 2 routers connected to each

other over a /30 network.A has 10.86.0.1/30 & B has 10.86.0.2/30 on the interfaces.Both are in area 0.

A has router-id 10.48.251.3 & B has router-id 10.86.251.1.Both have lo0 configured as router-id.

Following is the output on these:

on A

Neighbor ID Pri State Dead Time Address

10.86.251.1 1 FULL/BDR 00:00:37 10.86.0.2

on B

Neighbor ID Pri State Dead Time Address

10.48.251.3 1 FULL/DR 00:00:34 10.86.0.1

Why does it chose A as DR over B even though the interfaces involved with both for opsf, has higher address on B (10.86.0.2) & even

router-id for B 10.86.251.1 is higher than A? Or is it also because when ospf process is initiated, it will take the first corresponding

address from the relevant interfaces which actually comes in up/up state first?

3) Both are having pri 1 as can be seen, is it fine that both are on pri 1. Will it cause any problems in operation of ospf..like if the ospf process restarts due to some link issue..will this affect it to get into full state?

Should one of them be configured to be in pri 0 state?What is the basis to determine which one should be put in pri 0 when configured manually?

Thanks in advance!

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: ospf state

Hello Sunny,

the network segment is of type broadcast there is one DR and one BDR.

One very important note about OSPF DR/BDR election is the following: it is very conservative:

if RB has been configured first for OSPF and started to send out hellos on the link but RA wasn't an OSPF speaker after 40 seconds RB promotes itself to DR on the segment.

(the wait time that you can see in sh ip ospf interface)

When RA starts to send its own hellos on the segment it discovers that a DR is already present and so it can only become BDR.

In real networks sometimes happen that the router with the highest uptime is the DR in a segment even if it is not the one with the best parameters.

My guess is that you have configured RB first. Then you have moved to RA.

You can do the following test:

shut down both router interfaces wait one minute then reenable them together a new election occurs and this time the router with the highest OSPF RID should win.

Hope to help

Giuseppe

Hall of Fame Super Silver

Re: ospf state

Hello Sunny,

a priority of 0 makes a router interface not eligible for DR or BDR role.

The rule for DR/BDR election choices are:

of all concurrent routers in the segment the one with the highest priority is elected DR.

If there is a tie (two or more routers with the same OSPF priority) the highest OSPF RID is elected DR.

Going back to your case if you configure

ip ospf priority 0 on the fly on current DR it should resign its role and the other router becomes DR.

But the router with ospf pri 0 cannot be BDR so it should be seen as DRother from the other router.

Edit:

about when to use ip ospf pri 0:

on NBMA networks like a FR hub and spokes on all spokes.

on a logical NBMA network like DMVPN the same on spokes.

You don't need to use it in a normal lan segment.

Hope to help

Giuseppe

5 REPLIES

Re: ospf state

1. In choosing its router ID, OSPF will choose the highest IP address of any loopback interfaces configured on the router. If there are no loopbacks configured, it will choose the highest IP address of any interface configured on the router. It does not matter whether these interfaces are part of OSPF.

2. You said both your routers have Lo0 configured as router ID. This doesn't look true, though. Your Router B's ID is the same as the address of the physical interface to Router A. Are you certain that Router B has any loopback interfaces configured on it?

What type of interfaces are these? A /30 subnet mask is usually used on serial point-to-point links. DR/BDR elections don't occur on point-to-point links.

I'd like to see your configuration; as well as the output of the show ip ospf interface command.

Hall of Fame Super Silver

Re: ospf state

Hello Sunny,

the network segment is of type broadcast there is one DR and one BDR.

One very important note about OSPF DR/BDR election is the following: it is very conservative:

if RB has been configured first for OSPF and started to send out hellos on the link but RA wasn't an OSPF speaker after 40 seconds RB promotes itself to DR on the segment.

(the wait time that you can see in sh ip ospf interface)

When RA starts to send its own hellos on the segment it discovers that a DR is already present and so it can only become BDR.

In real networks sometimes happen that the router with the highest uptime is the DR in a segment even if it is not the one with the best parameters.

My guess is that you have configured RB first. Then you have moved to RA.

You can do the following test:

shut down both router interfaces wait one minute then reenable them together a new election occurs and this time the router with the highest OSPF RID should win.

Hope to help

Giuseppe

Community Member

Re: ospf state

Thanks for getting this clarified for me!

It became more clearer to me..also i understand how important the forum is with good people helping each other with knowledge,no wonder why i prefer to keep coming back for any queries on to this forums!

Thanks a Ton!!!

Community Member

Re: ospf state

Any words on my query relating to priority?

Thanks!!

Hall of Fame Super Silver

Re: ospf state

Hello Sunny,

a priority of 0 makes a router interface not eligible for DR or BDR role.

The rule for DR/BDR election choices are:

of all concurrent routers in the segment the one with the highest priority is elected DR.

If there is a tie (two or more routers with the same OSPF priority) the highest OSPF RID is elected DR.

Going back to your case if you configure

ip ospf priority 0 on the fly on current DR it should resign its role and the other router becomes DR.

But the router with ospf pri 0 cannot be BDR so it should be seen as DRother from the other router.

Edit:

about when to use ip ospf pri 0:

on NBMA networks like a FR hub and spokes on all spokes.

on a logical NBMA network like DMVPN the same on spokes.

You don't need to use it in a normal lan segment.

Hope to help

Giuseppe

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