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Forum Replies

  1. I’m sorry but i don’t really agree with what you say. Your example starts with a multicast link-local ip address as destination. It has to be with TTL1.

    When you establish static neighborship instead, your packet becomes a unicast.

    The answer to this i guess, is programmers laziness. Is easier to keep the packet as is and just change the destination ip address rather than changing ip address and ttl. From a “c” developer point of view, you will not need to define a “ttl variable” and “set up a “setsockopt” call” … less code, less errors can be made.

  2. Hi Andrea,

    Thanks for your message. I think it’s common practice to set the TTL of link local multicast packets to 1 but it’s not written down in any standard. Only thing I could find about this is a draft: http://mirror.physik-pool.tu-berlin.de/pub/ietf/ietf-tools.html/draft-asati-eckert-multicast-local-ttl-00.html.

    The TTL of eigrp packets (unicast or multicast) is 2 so the only thing I could think of was the spoke to spoke communication as the hub decrements the TTL by 2.

    Rene

  3. Rene is correct. Just because the neighbor statement is used with an IGP does not modify it’s TTL. Whether RIP or EIGRP are sending their hellos/updates via multicast or unicast, the TTL is 2 in both cases. For OSPF, PIM, and LDP, it’s always 1.

    If you used the neighbor statement with OSPF, the TTL remains 1. So, simply by modifying the behavior of the IGP does not modify the packet’s TTL.

    The reason behind this is to still maintain neighborships across NBMA networks, including DMVPN, for distance vector routing protocols WITHOUT having to disable split horizon

    ... Continue reading in our forum

  4. With any of the OSPF “point-to-xxxx” network types, the spoke routers will become neighbors with the Hub router.

    When one spoke router advertises something to the hub then the hub will create a new IP packet with a LSA and sends it to the other spoke. This IP packet will have a TTL of 1.

  5. Hello Rene,

    I captured packets in wireshark for eigrp over framerelay for both dynamic and static neighbor. But the TTL is always 1. So in case of static neighbor spoke to spoke neighbor ship did not ever happen. Also HUB sending the message to spoke “Time to Live Exceeded”. My spoke to spoke inverse arp is working. I am using 7200 IOS 15.2 router. Please have a look at this.

    Thanks

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