ospf

[amms68958]

HI,

How does OSPF builds database for ip unnumbered interfaces?

Thanks in advance,

Thomas.

[Georg Pauwen]

Hello Thomas,

check this document, it should answer your question:

OSPF Routers Connected by an Unnumbered Serial Link

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9e0.shtml

Regards,

GP

[amms68958]

Hi GP,

Thanks for your reply.BUt I have some doubts.

In the example shown in the link which I gave ,in the output of sh ip ospf database router 1.1.1.1 ,it gives,

" (Link ID) Neighboring Router ID: 2.2.2.2

(Link Data) Router Interface address: 0.0.0.12

!--- In the case of unnumbered link, use the MIB II IfIndex

!--- value. This value usually starts with 0. "

My doubt is

1) Why is the router interface addaress shown as 0.0.0.12

2) Someone told me that OSPF router uses SNMP ,in building of database in the case of ip unnumbered links.Pls correct me if I am wrong.If yes ,pls explain how this process works..

3) If possible,pls let me know in which section of OSPF RFC ,this topic is covered.

Thanks in advance,

THomas.

[Kevin Dorrell]

Thomas,

AFAIK, the topic of unnumbered interfaces is not covered in the RFC2328 except in passing, e.g. at the top of page 9 and page 59. The addressing thing is on page 127. It does not use SNMP dircetly, but it does use the same indexing scheme that SNMP uses, just as a convention. Have a look through the document for the word "unnumbered". Here is the RFC, for refernce:

http://www.ietf.org/rfc/rfc2328.txt?number=2328

The crux of it is that on a point-to-point link, the IP address of the link is not used in any case. Instead it uses the OSPF multicast address 224.0.0.5. (There is no DR, so the .6 address is not used in this case.)

I think that on an unnumbered link, you will not find the "nominal" 0.0.0.11 addresses in your routing table because they are not really addresses at all. Like the RID of the router, they are only really "tags" that look a bit like IP addresses but are not. After all, if the router bases the "IP address" of the link on the SNMP ifindex, it is always well possible that you will get duplicates in the network. (Remember that on OSPF there is no requirement to be able to reach the IP address that looks like the RID of the router.)

Does this help?

Kevin Dorrell

Luxembourg

[amms68958]

Hi Kevin,

Thanks for your reply.As always,It is very helpful.

Just a couple more clarifications.

1) To avoid ip address duplication on ip unnumbered links,"tags" are used to identify a link.RThis tag value can be from 0.0.0.0 and it increments for another link.Pls correct me if I am wrong.

2) regding the last sentence in your post "Remember that on OSPF there is no requirement to be able to reach the IP address that looks like the RID of the router".I think I didnt completely got your point on this.

Thanks

Thomas.

[Kevin Dorrell]

Thomas,

That is what I understand from the document. Internally to the router, each interface (real and virtual) has a unique index number, starting at 1 and working upwards. It is this index number that SNMP uses when interrogating interfaces - "get me the error count on interface index=3", except that the technical term is "instance" rather than "index". I don't know how you can read these index numbers in IOS ... in CatOS it is show port ifindex ... but if you have a Network Management station you will see the index number of each interface. The thing about these index numbers is that once they have been allocated, they are pretty much cast in stone. They survive reboot, plugging and unplugging cards, creating virtual interfaces etc. etc. Once an interface has an index number, it is for life. So, AFAIK, OSPF just puts 0.0.0. on the front of the index number to identify the interface (if you have up to 255 of them).

Regarding my last sentence: it is a fact about OSPF that often raises a few eyebrows, that the RID of the router does not necessarily need to be a reachable IP address for OSPF to work. The RID is just that, an identifier - not an IP address. OK, so it is generated from the IP addresses 'cos that way it has a fighting chance of being unique. But it does not really behave like an IP address. This is bourn out by the fact that if the interface that lent its IP address as an RID goes down, the OSPF carries on working. (That was not always true in the past, but it is now.)

Similarly, the nominal 0.0.0.11 "IP address" of an unnumbered point-to-point interface does not really behave like an IP address. I don't think you can ping it ... well I suppose you could, but I don't think it would answer. it is just an identifier.

As an experiment, try show ip ospf interface for your unnumbered. If you could post the result, I would be interested to see it as well, and it would save me time in the lab.

Kevin Dorrell

Luxembourg

[amms68958]

Thanks Kevin,

Your post is really informative.

Unfortunately I am not able to test this,because I am not having any spare router avaiable with me.

Thanks,

Thomas.