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

[Reza Sharifi]

Have a look at this doc:

http://blog.ipspace.net/2008/10/ospf-ignores-subnet-mask-mismatch-on.html

HTH

[siddhartham]

Thanks Reza, I already read that post- that didn't explain the reason for the exception-- Does anyone why the point to point links have an exception?

Siddhartha

[siddhartham]

anyone?

Sidd

[Julio Carvajal]

Hello Sid,

What do you mean by it did not explain the reason for the execption??

I mean Reza already told u bud (some kudos to reza)

The RFC clearly states:

However, there is one exception to the above rule: on point-to-point networks and on virtual links, the Network Mask in the received Hello Packet should be ignored.

And back to you are scenario, what's the default encapsulation for a serial interface PPP cause it's a point to point link.

That being said the expection goes there

Rate all of the helpful posts!!!

Regards,

Jcarvaja

Follow me on http://laguiadelnetworking.com

[Jon Marshall]

edited      

[Jon Marshall]

Sidd

Why doesn't the link provide the explanation ?  It states that the RFC for OSPFv2 makes a specific exception for virtual and point-to-point networks and that exception is that the subnet mask does not need to match. It also says that Cisco strictly adhere to the RFC so they have implemented OSPF that way.

Am i misunderstanding your question ?

Jon

[siddhartham]

Hi Jon, I meant to ask what is the technical reason behind that exception.

Sidd

[Jon Marshall]

Sidd

Sorry, i was posting as you replied -

I think the technical reason is that on a point-to-point link if a router sends a packet, and providing the link is up, the only place that packet can end up is at the router at the other end. So the subnet mask is not really an issue.

Compare that to multi-access networks where the subnet mask very much matters in terms of which adjacencies are formed.

Jon

[siddhartham]

Thanks Jon, I am having hard time in understanding. Whther its a P to P link or multi access link the LSAs will always be sent to the layer 2 mac address calculated from either 224.0.0.5 or 224.0.0.6 address right? why does the layer3 address or subnet mask matter( as long as they have reachability)

Please correct me if I am wrong

Siddhartha

[Richard Burts · Accepted solution]

Siddhartha

But on point to point serial link (which is what the RFC is oriented to) there is not any MAC address.

My understanding of the reason for the exception for point to point links is that some types of OSPF messages are sent to the unicast address (acknowledgements, retransmission of LSA, etc) of the neighbor. For broadcast (Ethernet) networks if the subnet mask does not match it is quite possible that arp will not work correctly and that neighbors might not be able to get to the neighbor unicast address. So there was a requirement for the subnet mask to match to be sure that unicast communication would work. But with point to point the subnet mask makes no difference. With point to point you just send the packet out the interface and it always gets to the right destination. So there is not a need for subnet mask to match on point to point interfaces.

HTH

Rick

[siddhartham]

make sense- thanks Richard and Jon

Siddhartha

[Jon Marshall]

edited

[Jon Marshall · Accepted solution]

Lets say you have an ethernet segment with 3 routers attached R1, R2 and R3.

R1 = 192.168.5.10 255.255.255.240

R2 = 192.168.5.11 255.255.255.240

R3 = 192.168.5.17 255.255.255.240 

note that R3 is actually in a different network (192.168.5.16/28) and the administrator has incorrectly assigned the IP .17

If the network mask was not checked then these 3 routers would form adjacencies because each would see the multicast packets.  But R1 and R2 will both think R3 is on a different network and R3 will think R1 and R2 are on different networks. They will not be able to send unicast packets to R3 because they will think they have to route them.

In addition what would happen if R3 was elected DR. The DR generates network LSAs (type 2) and it would generate one for the 192.168.5.16/28 network but it is actually connected to the 192.168.5.0/28 network.  I don't know how this would affect the SPF calculation but there would be clearly be incorrect information being used.

Jon

[siddhartham]

what happens if

R1 = 192.168.5.10 255.255.255.240

R2 = 192.168.5.11 255.255.255.240

R3 = 192.168.5.17 255.255.255.0

Siddhartha