OSPF RIDs confusion?!

Question:

Your configuring a router with both physical and logical interfaces.
What would determine the OSPF Router ID?

Answers:

A: Highest IP address of any Logical Interface
B: Highest IP address of an Physical Interface
C: Lowest IP address of any Logical Interface

Lowest IP address of an Physical Interface

I choose "A" (Highest IP address of any Logical Interface) because if im not mistaken a logical interface that has a higher IP of any physical interface will become the RID. Am i correct?

It is saying "B" is correct. (Highest IP address of an Physical Interface)

If not can someone please explain this too me?

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Comments

pr3d4t0r

Ιf loopback interface is configured then OSPF will select it. If not highest ip on logical if.

steve-o87

Ιf loopback interface is configured then OSPF will select it. If not highest ip on logical if.

Loopback is a logical interface

OSPF will use the highest loopback address if it is configured because it will never go down - I say this because you could have multiple loopback addresses. If no loopback interface is configured then the highest ip on any physical interface.

Danman32

Official statement in Cisco courseware:

Router ID:
Default: The highest IP address on an active interface at the moment of OSPF process startup.
Can be overridden by a loopback interface: Highest IP address of any active loopback interface.

It says active interface, doesn't mention logical or physical. However a physical interface can be split into several logical interfaces, such as for WAN or trunking.

unwritt3n

yeah tis all good now i get it...

Thanks guys..

I knew i was right that an Logical Interface with the highest IP will always overide any other int being another logical or physical.

darkuser

it often helps to learn from the documentation .....

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm#wp1000871

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fipr_c/ipcprt2/1cfospf.htm#wp1001369

BubbaJ

Realize, too, that adding a Loopback interface won't change the router ID unless the OSPF process (or the router) is restarted. This can cause hidden problems which is why it is a good practice to specifically set the router ID with the router-id command.

unwritt3n

BubbaJ wrote:

Realize, too, that adding a Loopback interface won't change the router ID unless the OSPF process (or the router) is restarted. This can cause hidden problems which is why it is a good practice to specifically set the router ID with the router-id command.

Just say u had a s0/0 ip with 10.0.0.1/24
Then u configure a Loopback0 with 192.168.10.2/24

The S0/0 would still be the RID because no reboot has been done, but what if u go into the s0/0 int and shut it down, let the router see 192.168.10.2/24 is now the higest, then bring S0/0 back up.

I know this would prob be more work then just typing "reload" or power-cycling the router, but would that way still work?

Just something I thought.

BubbaJ

unwritt3n wrote:

Just say u had a s0/0 ip with 10.0.0.1/24
Then u configure a Loopback0 with 192.168.10.2/24

The S0/0 would still be the RID because no reboot has been done, but what if u go into the s0/0 int and shut it down, let the router see 192.168.10.2/24 is now the higest, then bring S0/0 back up.

I know this would prob be more work then just typing "reload" or power-cycling the router, but would that way still work?

Just something I thought.

OSPF will keep the router ID, whether or not the interface goes down, until the OSPF process is restarted (manually with clear ip ospf i]pid[/i process or by a reload).

Older versions of IOS will not even change the router ID with the router-id command until the OSPF process is restarted. Newer versions will.

unwritt3n

BubbaJ wrote:

unwritt3n wrote:

Just say u had a s0/0 ip with 10.0.0.1/24
Then u configure a Loopback0 with 192.168.10.2/24

The S0/0 would still be the RID because no reboot has been done, but what if u go into the s0/0 int and shut it down, let the router see 192.168.10.2/24 is now the higest, then bring S0/0 back up.

I know this would prob be more work then just typing "reload" or power-cycling the router, but would that way still work?

Just something I thought.

OSPF will keep the router ID, whether or not the interface goes down, until the OSPF process is restarted (manually with clear ip ospf i]pid[/i process or by a reload).

Older versions of IOS will not even change the router ID with the router-id command until the OSPF process is restarted. Newer versions will.

Ahh ok, thanks alot mate for clearing that up!! cheers