Thanks Bob, I think that was the most (and maybe first) informative tweet ever! :)
-------- Original Message -------- Subject: Re: [OSL | CCIE_RS] TTL on EIGRP multicast packets set to 2? From: Bob McCouch <[email protected]> Date: Fri, August 03, 2012 6:33 pm To: Mike McPhee <[email protected]> Cc: "<[email protected]>" <[email protected]>, IPexpert Online Study List <[email protected]> It occurred to me to pose this question to Donnie Savage at Cisco via Twitter, who was heavily involved with EIGRP development. Here was his reply: "@diivious: @BobMcCouch a packet could be sent multicast/unicast/both. for efficiency its never 'rebuilt' ttl is set so to not route if sent unicast" He didn't specifically address why 2 and not 1, but I suspect that may be to deal with FR hub/spoke. If you traceroute spoke-to-spoke with a Cisco router acting as a frame switch, you will see two hops. It seems a router will decrement TTL even on a switched frame in that case. Pretty sure I've had to enable BGP multihop for this same scenario. Bob -- Sent from my iPhone, please excuse any typos. On Aug 1, 2012, at 8:51 PM, Mike McPhee <[email protected]> wrote: > Could TTL=2 be to allow transit to the loop back interface and avoid the drop > after hitting an interface? > > Mike > > On Aug 1, 2012, at 7:24 PM, <[email protected]> wrote: > >> Thanks for the replies, interesting reading... especially the RFC :) >> >> I'm still not clear on exactly why the TTL is two, this may be something >> I just let go for now. Usually when I don't understand why something is >> the way it is, it means I am missing something fundamental... >> >> Doug >> >> -------- Original Message -------- >> Subject: Re: [OSL | CCIE_RS] TTL on EIGRP multicast packets set to 2? >> From: Bal Birdy <[email protected]> >> Date: Wed, August 01, 2012 1:27 am >> To: Joe Sanchez <[email protected]> >> Cc: [email protected], IPexpert Online Study List >> <[email protected]> >> >> Doug, >> >> The multicast IP for EIGRP is 224.0.0.10. This is within the Multicast >> Permanent address range which states that 224.0.0.0 - 224.0.0.255 are >> non routable (so local subnet only). The Multicast Permanent address >> range also includes 224.0.1.0- 224.0.1.255, which are routable, but this >> is out of scope of your question. >> >> The TTL set to 1, is a method of multicast TTL scoping to limit packets >> being routed out of your administrative domain, but I dont think this is >> the purpose here. >> >> B >> >> On Wed, Aug 1, 2012 at 2:31 PM, Joe Sanchez <[email protected]> wrote: >> Doug,, >> >> There have been lots of statements about EIGRP that have been >> misleading at >> best. Take a look at the link below and see another discussion about >> this >> topic: >> >> http://www.groupstudy.com/archives/ccielab/200506/msg00687.html >> http://www.mail-archive.com/[email protected]/msg72839.html >> https://supportforums.cisco.com/thread/211406 >> https://learningnetwork.cisco.com/thread/40817 >> >> In times like these, I like to refer back to this RFC which sum's it >> all up. >> >> http://tools.ietf.org/rfc/rfc1925.txt >> >> >> Joe Sanchez >> >> >> >> On Tue, Jul 31, 2012 at 9:02 PM, <[email protected]> wrote: >> >>> My understanding of EIGRP multicast packets was that the TTL is set >> to >>> one, and that this is the mechanism that prevents the packet from >> being >>> routed off the local subnet. >>> >>> However, doing a packet capture reveals that the TTL on EIGRP >> multicast >>> packets is actually 2. Some research hints that the same may be true >> for >>> RIP and OSPF, although I haven't verified. The Wireshark expert note >>> refers to RFC 3171, but that RFC doesn't mention TTL. Here is the >> actual >>> text of the note: >>> >>> "Time To Live" != 1 for a packet sent to the Local Network Control >> Block >>> (see RFC 3171) >>> >>> So what is the purpose of setting these TTL's to 2? Obviously, they >> are >>> not getting routed off the subnet, so some other mechanism is being >> used >>> to prevent this. >>> >>> Thanks, >>> >>> Doug >>> _______________________________________________ >>> For more information regarding industry leading CCIE Lab training, >> please >>> visit www.ipexpert.com >>> >>> Are you a CCNP or CCIE and looking for a job? Check out >>> www.PlatinumPlacement.com >>> >>> http://onlinestudylist.com/mailman/listinfo/ccie_rs >>> >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, >> please visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com >> >> http://onlinestudylist.com/mailman/listinfo/ccie_rs >> _______________________________________________ >> For more information regarding industry leading CCIE Lab training, please >> visit www.ipexpert.com >> >> Are you a CCNP or CCIE and looking for a job? Check out >> www.PlatinumPlacement.com >> >> http://onlinestudylist.com/mailman/listinfo/ccie_rs > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > > http://onlinestudylist.com/mailman/listinfo/ccie_rs _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
