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New Member

Regarding sending OSPF packets over ptp interface

The section 8.1 of RFC2328 says like as follows:

"On physical point-to-point networks, the IP destination is always set to the address AllSPFRouters"

On the contrary, the section 8.2 says like as follows:

"The packet's IP destination address must be the IP address of the receiving interface, or one of the IP multicast addresses AllSPFRouters or AllDRouters."

What happens if a router receives a OSPF packet using unicast packet over ptp interface? Do we have to discard the unicast packet since OSPF router is supposed to send a multicast packet over ptp interface? Or do we have to just allow to receive a unicast packet over ptp interface? Which one is complying with the standard? How bout Cisco router?

Thanks in advance.

5 REPLIES
Bronze

Re: Regarding sending OSPF packets over ptp interface

The paragraph after the one in 8.1 that you reference goes on to state:

"Retransmissions of Link State Update packets are ALWAYS sent directly to the neighbor. On multi-access networks, this means that retransmissions should be sent to the neighbor's IP address."

The section of 8.2 that you quoted is referring to a router receiving a packet at the IP level, and making the decision of whether or not to pass the packet up to OSPF. Once the packet is passed to OSPF, OSPF will handle the packet as necessary. In the case of a ptp interface, a packet is not likely to be received with the AllDRouters destination, but it's not up to the IP process to reject the packet based on this, so it will pass the packet to OSPF. OSPF will likely reject the packet in the unlikely event that it has the AllDRouters destination address. As you point out, the router will receive most of its OSPF packets with the AllSFPRouters address. The process must also allow for the router to receive retransmissions of Link Statue Updated packets with its unicast address as the destination.

I hope this clears things up.

Mark

New Member

Re: Regarding sending OSPF packets over ptp interface

To summarize your answer, OSPF router should be able to accept an Link State Update packet with either unicast or AllSFPRouters as the destination over ptp interface. Am I right? By the way, I'm wondering if Cisco router sends a OSPF Link State Update (except for retransmisson) with AllSFPRouters or unicast destination over ptp interface?

New Member

Re: Regarding sending OSPF packets over ptp interface

As a matter of fact, we found that Alcatel 7670 just discards an OSPF Link State Update with unicast destination received over a point-to-point interface, therby not being able to establish neighbor with a RTA that sends out an Link State Update with unicast address over ptp interface, while RTA accepts an OSPF Link State Update with either unicast or AllSFPRouters as the destination. Which one is wrong? Both of them are wrong?

Bronze

Re: Regarding sending OSPF packets over ptp interface

What is RTA? Is it Cisco?

RFC 2328 calls for all OSPF packets over a ptp link to be sent to AllSPFRouters, with the exception of LSU retransmissions. I would say that a router that enforces this, and rejects LSUs sent to the unicast address, is following the rules, but the router that will accept LSUs to the unicast is not incorrect, just giving more leeway. The rule applies to the sender, and I have seen nothing that indicates that a receiver is obligated to enforce the rules.

Bronze

Re: Regarding sending OSPF packets over ptp interface

You are correct, the router should accept Link State Updates with the AllSPFRouters destination, and Link State Update retransmissions with its unicast address as the destination. The router sends LSUs with the AllSPFRouters destination on ptp interfaces.

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