OSPF Hello Parameters- SM Question

[siddhartham]

I observed a interesting behaviour while I was setting up a LAB yesterday- OSPF on serial links/Point to Point links ignore the mismatched subnet mask and form a full adj while the same config on a ethernet/broadcast links throws out an error saying mismatched Hello paremeters-

Can someone explain me the reason for this behaviour

Serial link

R1-s0/0-10.0.0.1 255.255.255.252---10.0.0.2 255.255.255.0-s0/0-R2-----Forms a Fulll adj

Ethernet Link

R1-F0/0-10.0.0.1 255.255.255.252---10.0.0.2 255.255.255.0-F0/0-R2----Doesnt reach 2way state because of Hello parameter(Subnet Mask) mismatch

Sidd

[Jon Marshall · Accepted solution]

From R3's persepctive R1 and R2 are on the same subnet. From R1 and R2s perspective R3 is on a different subnet. So you would get similiar issues in terms of unicasting etc.

Note that the bit about a DR being elected and bad LSA information is a best guess in that i can't actually test it (obviously as the adjacencies won't form) but i could see how it might cause problems.

Certainly as botth Rick and myself pointed out unicasting of LSA etc. would be an issue.

Jon

[siddhartham]

Thank you both !

Siddhartha

[Richard Burts · Accepted solution]

In this case R1 and R2 will believe that R3 is in a different subnet. So they will not send an arp request for R3 but will want some routing entry for how to get to that subnet.

In the scenario suggested by Jon R1 and R2 think that R3 is remote and R3 believes that R1 and R2 are remote. Your scenario seems to address the issue from the perspective of R3 which will now believe that R1 and R2 are local and will arp for them. However it is highly likely that R1 and R2 will reject the arp request from R3 because it came from a remote IP (and the Cisco behavior is to reject an arp request from a device whose source address is not on the local subnet). So the result in your scenario is still that the routers do not communicate.

HTH

Rick

[Jon Marshall]

Rick

To save me having to dig out my TCP/IP routing books do you think in the example i gave that R3 could end up as the DR and that this would indeed cause problems with the LSAs propogated and the subsequent SPF calculation ?

I agree on the unicasting bit but i'm just wondering whether it would actually create a corrupt LSA database among those three routers.

Jon

[Richard Burts]

Jon

It is indeed an interesting scenario and not clear exactly what the outcome would be. But I believe that the bottom line is that forming adjacencies would fail and the OSPF database would not be populated. I believe that the scenario might go like this:

- R3 becomes DR.

- R3 sends multicast hello message announcing that it is DR.

- multicast message reaches R1 and R2.

- R1 and R2 send hello messages and begin the process of attempting to form adjacencies with R3.

- R3 would try to send data base updates to R1 and R2 (and I am pretty sure that the database update would be unicast). Assuming that the database updates are unicast here is where things break because I am pretty sure that R3 arp for R1 and R2 did not work.

- if the database update did get to R1 and R2 then they would send acknowledgements to R3. And things pretty certainly break here because R1 and R2 believe that R3 is in a remote subnet and they have no route to that subnet. So they can not send unicast acknowledgement.

This has certainly been an interesting thought process

HTH

Rick

[Jon Marshall]

Rick

So i guess it would never get to the SPF calculation ie. either database updates or acks for those would be unicast and so would never get there. Thanks for clarifying and saving me having to pour through my books.

It has indeed been interesting and it's one of the things that makes CSC so worthwhile.

Jon

[Richard Burts]

Jon

Agree with you 100%

HTH

Rick