it gets complicated, routing protocols versus ip packets. first of all, if I understand correctly, all ip routing protocols use ip headers. The routing protocol packet is the payload, and not an entity unto itself. I have seen traces of OSPF packets showing IP TTL of various values. Someone shared with me some traces to validate something I suspected - that the OSPF virtual link packet has an initial TTL of 255. My theory is that it has to be deliberately set high because there is no predicting the number of hops a virtual link will traverse.
The eBGP multihop command sets the IP TTL to something greater than the native BGP TTL of 1. EIGRP? Don't know. Was merely speculating. But consider - where else might the "hop limit" occur? The EIGRP header has no field indicating hop count that I can see. My source is the Rad Com World of Protocols book. Yes, RIP and RIPv2 contain within the RIP packet ( not the IP header ) a field in which metric / hopcount is carried. This leads me to believe that RIP does nothing to manipulate the IP TTL value. The others appear to do just that, however. Chuck ""Priscilla Oppenheimer"" wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > At 08:05 PM 2/21/02, Chuck wrote: > >to augment the other answers, the IP hop count is really the IP TTL value. > >It can never exceed 255 > > You're confusing two issues. > > Remember the router has two jobs: forwarding packets and learning the > topology. Hop count has to do with the latter and affects what goes in the > routing table. The IP TTL causes a router to drop a packet before > forwarding if the TTL becomes zero. > > > >EIGRP defaults to 100 hops, so I would expect that the routing packet IP TTL > >is set at 100 at that point. > > Routing packets only go to neighbors. The IP TTL should be set to one or > two. This has nothing to do with hop count which will be later in the > packet in the distance vectors. > > > >Well ( checking the sniffer trace that Priscilla so thoughtfully supplied a > >couple of days ago ) I'm seeing the IP TTL as 2. Still, maybe there is an > >adjustment made. After all, the (E)IGRP metric includes end to end metrics. > >hhmmm... ( looking over Priscilla's trace again ) way down there I see an > >EIGRP hop count 0 line. > > The router was advertising a directly-connected network. > > > >the IP TTL is still really the only thing that makes sense in terms of the > >way IP works. > > In terms of forwarding maybe. You better reconsider routing protocols > though... > > Priscilla > > > >Anyone? > > > >Chuck > > > >""Steven A. Ridder"" wrote in message > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > Anyone know why there is a hop-count in EIGRP? It has a 1 byte value, > but > > > it doesn't limit the number of hops and it looks like routers don't use > it > > > in their calculations. Why is it there? > > > > > > -- > > > RFC 1149 Compliant. > ________________________ > > Priscilla Oppenheimer > http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=36179&t=36082 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
