OSPF RIDs confusion?!

unwritt3nunwritt3n Member Posts: 67 ■■□□□□□□□□
in CCNA & CCENT
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
D: 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?
Studying: 70-290, CCNP 1, CCSP 1
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Comments

  • pr3d4t0rpr3d4t0r Member Posts: 173
    Ιf loopback interface is configured then OSPF will select it. If not highest ip on logical if.
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  • steve-o87steve-o87 Member Posts: 274
    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.
    I am the lizard King. I can do anything.
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  • Danman32Danman32 Member Posts: 1,243
    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.
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  • unwritt3nunwritt3n Member Posts: 67 ■■□□□□□□□□
    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.
    Studying: 70-290, CCNP 1, CCSP 1
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  • darkuserdarkuser Member Posts: 620 ■■■□□□□□□□
    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
    rm -rf /
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  • BubbaJBubbaJ Member Posts: 323
    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.
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  • unwritt3nunwritt3n Member Posts: 67 ■■□□□□□□□□
    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.
    Studying: 70-290, CCNP 1, CCSP 1
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  • BubbaJBubbaJ Member Posts: 323
    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.
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  • unwritt3nunwritt3n Member Posts: 67 ■■□□□□□□□□
    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
    Studying: 70-290, CCNP 1, CCSP 1
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