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Query on OSPF Router ID

 
 
rajesh.premachandran@gmail.com
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      03-04-2005
Hi

I had a regarding OSPF Router ID concept. Assume that we have a router
with three WAN interfaces s0/0, s0/1 and s0/2 and OSPF is configured on
all the three interfaces. Let the IP address configured for the
interfaces be
s0/0 --- 10.1.0.1/32
s0/1 --- 11.1.0.1/32
s0/2 --- 12.1.0.1/32

By default the RID in this case would be 12.1.0.1 since this is the
highest interface IP address.

Let us assume that the link connected to S0/2 is down. In this case
will the OSPF work. That is all the other interfaces are up. Will those
interfaces exchange rotuing information(and hello packets).
Or will the router be considered to be down since interface S0/2(Router
ID) is down.

If the OSPF works what will be the RID in the hello packet.

Regards
Raj

 
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Ivan Ostreš
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      03-04-2005
In article <(E-Mail Removed) .com>,
http://www.velocityreviews.com/forums/(E-Mail Removed) says...
> Hi
>
> I had a regarding OSPF Router ID concept. Assume that we have a router
> with three WAN interfaces s0/0, s0/1 and s0/2 and OSPF is configured on
> all the three interfaces. Let the IP address configured for the
> interfaces be
> s0/0 --- 10.1.0.1/32
> s0/1 --- 11.1.0.1/32
> s0/2 --- 12.1.0.1/32
>
> By default the RID in this case would be 12.1.0.1 since this is the
> highest interface IP address.
>


Correct.

> Let us assume that the link connected to S0/2 is down. In this case
> will the OSPF work. That is all the other interfaces are up. Will those
> interfaces exchange rotuing information(and hello packets).
> Or will the router be considered to be down since interface S0/2(Router
> ID) is down.
>


If interface was down during OSPF startup, then the router-id is the
next biggest address (11.1.0.1 in this case). If s0/2 was up when ospf
was starting and later it was shut, the OSPF will use it's initial
(12.1.0.1) router-id until process was cleared (clear ip ospf proc) or
until router is reloaded.

The thing is that RID does not have to be any real IP address on your
network. You can specify just any RID using router-id command but it has
to be uniqe in your OSPF routing domain.

And just to mention.. ospf will not start if all interfaces on the
router are down.


--
-Ivan.

*** Use Rot13 to see my eMail address ***
 
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Victor Cappuccio
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      03-04-2005
Hola Rajesh

Try to configure a loopback to avoid your Problem.
http://www.cisco.com/univercd/cc/td/....htm#wp1001369

Saludos,
Victor Cappuccio


"(E-Mail Removed)" <(E-Mail Removed)> wrote in message news:<(E-Mail Removed) roups.com>...
> Hi
>
> I had a regarding OSPF Router ID concept. Assume that we have a router
> with three WAN interfaces s0/0, s0/1 and s0/2 and OSPF is configured on
> all the three interfaces. Let the IP address configured for the
> interfaces be
> s0/0 --- 10.1.0.1/32
> s0/1 --- 11.1.0.1/32
> s0/2 --- 12.1.0.1/32
>
> By default the RID in this case would be 12.1.0.1 since this is the
> highest interface IP address.
>
> Let us assume that the link connected to S0/2 is down. In this case
> will the OSPF work. That is all the other interfaces are up. Will those
> interfaces exchange rotuing information(and hello packets).
> Or will the router be considered to be down since interface S0/2(Router
> ID) is down.
>
> If the OSPF works what will be the RID in the hello packet.
>
> Regards
> Raj

 
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ws00sw@gmail.com
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      03-04-2005

OSPF like BGP always preffer the highest loopback address rather than a
highest physical interface address if they are configured.

 
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Toby
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      03-04-2005

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Hi
>
> I had a regarding OSPF Router ID concept. Assume that we have a router
> with three WAN interfaces s0/0, s0/1 and s0/2 and OSPF is configured on
> all the three interfaces. Let the IP address configured for the
> interfaces be
> s0/0 --- 10.1.0.1/32
> s0/1 --- 11.1.0.1/32
> s0/2 --- 12.1.0.1/32
>
> By default the RID in this case would be 12.1.0.1 since this is the
> highest interface IP address.
>
> Let us assume that the link connected to S0/2 is down. In this case
> will the OSPF work. That is all the other interfaces are up. Will those
> interfaces exchange rotuing information(and hello packets).
> Or will the router be considered to be down since interface S0/2(Router
> ID) is down.
>
> If the OSPF works what will be the RID in the hello packet.
>
> Regards
> Raj
>

Hi Raj

The OSPF router-id is not a network IP address but a unique number within an
OSPF area allocated to a router in a OSPF area to enable all routers within
that area to build a topological view of the area.

There are several ways to allocate this "OSPF ROUTER-ID" but all forms use
a 32 bit dotted decimal format simular to an IP address.

This ID is taken at the time the OSPF process starts and follows the
following rules in order.

If a OSPF ROUTER-ID x.x.x.x command is issued then this will be the ID
The highest IP address of active loopback interfaces.
The highest active Physical interface IP address.

Once the OSPF Router-ID has been selected it does not change even if it was
allocated from a physical interface that has subsequently failed. The
reasoning here is to give stability to the rest of the area. i.e. other
routers do not have to re-build their topological database due to a
router-id change. i.e will just react to actual network failures etc.

Most network administrators tend to use the second option, the loopback
address method as most networks tend to use loopback interfaces to supply IP
addresses to multiple point-point interfaces "IP UNNUMBERED" to save on IP
addresses along with other uses, and as these loopback addresses have to be
unique anyway, why create a seperate OSPF ROUTER-ID manually.

Regards

Toby


 
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Toby
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      03-04-2005

"Toby" <(E-Mail Removed)> wrote in message
news:ri3Wd.169$(E-Mail Removed)...
>
> <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) oups.com...
>> Hi
>>
>> I had a regarding OSPF Router ID concept. Assume that we have a router
>> with three WAN interfaces s0/0, s0/1 and s0/2 and OSPF is configured on
>> all the three interfaces. Let the IP address configured for the
>> interfaces be
>> s0/0 --- 10.1.0.1/32
>> s0/1 --- 11.1.0.1/32
>> s0/2 --- 12.1.0.1/32
>>
>> By default the RID in this case would be 12.1.0.1 since this is the
>> highest interface IP address.
>>
>> Let us assume that the link connected to S0/2 is down. In this case
>> will the OSPF work. That is all the other interfaces are up. Will those
>> interfaces exchange rotuing information(and hello packets).
>> Or will the router be considered to be down since interface S0/2(Router
>> ID) is down.
>>
>> If the OSPF works what will be the RID in the hello packet.
>>
>> Regards
>> Raj
>>

> Hi Raj
>
> The OSPF router-id is not a network IP address but a unique number within
> an OSPF area allocated to a router in a OSPF area to enable all routers
> within that area to build a topological view of the area.
>
> There are several ways to allocate this "OSPF ROUTER-ID" but all forms
> use a 32 bit dotted decimal format simular to an IP address.
>
> This ID is taken at the time the OSPF process starts and follows the
> following rules in order.
>
> If a OSPF ROUTER-ID x.x.x.x command is issued then this will be the ID
> The highest IP address of active loopback interfaces.
> The highest active Physical interface IP address.
>
> Once the OSPF Router-ID has been selected it does not change even if it
> was allocated from a physical interface that has subsequently failed. The
> reasoning here is to give stability to the rest of the area. i.e. other
> routers do not have to re-build their topological database due to a
> router-id change. i.e will just react to actual network failures etc.
>
> Most network administrators tend to use the second option, the loopback
> address method as most networks tend to use loopback interfaces to supply
> IP addresses to multiple point-point interfaces "IP UNNUMBERED" to save on
> IP addresses along with other uses, and as these loopback addresses have
> to be unique anyway, why create a seperate OSPF ROUTER-ID manually.
>
> Regards
>
> Toby
>
>

So to continue to your actual question which I forgot during composing my
answer.

As you have not set the router-id manually or set up any loopback
interfaces, and assuming all three of your interfaces were up/up before you
started the ospf process then the router id would be 12.1.0.1 (taken from
S0/2)

As I said in my last post this is the id and just happens to be the same as
the highest ip address of your pysical interfaces so if the interface S0/2
goes down, this would not be a problem as far as the OSPF process is
concerned as this is just the router-id and although is the same number as
interface S0/2 it treated independantly. i.e. as stated before this id
would not change and OSPF would function correctly over your other 2 links.

Toby


 
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