It would be very interesting to see these traces. The switch must be doing
something very wierd. A UDP header consists of Source Port, Dest Port,
Length and the checksum... TTL's are usually L3 and a max TTL 2 indicates
somethings really screwed up.
Nimesh.
On Sun, 4 Jun 2000, Lance Simon wrote:
> Actually, if the switch is routing, it will decrement the hop or ttl
> value. And the ttl on a RIP update is in the UDP header. If you are
> not convinced I will send you the traces. And finally, as Travis
> stated, a RIP update should only have max ttl of 2.
> Lance
>
> Cormac Long wrote:
>
> > Not sure what the nature of the problem is here, but
> > here are a couple of points to note:
> >
> > 1. The TTL field is in the IP header and not the UDP
> > header.
> >
> > 2. The TTL is only decremented after the packet
> > crosses a router hop ( a switch hop does NOT count).
> >
> > 3. The inital TTL=15, and it gets decremented after
> > that as it crosses routers. This makes it surprising
> > that you're seeing TTL=1 or 2. It implies alot of
> > router hops.
> >
> > Cormac
> >
> > Cormac Long, CCSI#21600
> > http://www.cormaclong.com
> >
> > --- Lance Simon <[EMAIL PROTECTED]> wrote:
> > > Hi group!
> > >
> > > I am a lurker at best here, but today I saw
> > > something that really
> > > puzzled me. While I was looking at a trace file I
> > > noticed something
> > > unusual about the ttl values for RIP updates on a
> > > PacketEngines switch.
> > > This switch is connected to a Cat5000 and I had a
> > > sniffer in between the
> > >
> > > two.
> > > My understanding of RIP is that the ttl value = 2
> > > and that it is
> > > decremented as it enters a switch/router and then it
> > > is looked at.
> > > Therefore, a ttl=2 would become ttl=1 before the
> > > packet is even looked
> > > at. If the ttl=1 it would become ttl=0 and then,
> > > instead of being
> > > looked at, it would be discarded. Is this correct?
> > > First, let me say that the Packet Engines 2200
> > > switch is a very good box
> > >
> > > and it is communicating well with the Cat5k. The
> > > RIP updates are being
> > > handled well by both sides, but; when looking at
> > > the sniff, the ttl
> > > value from the packet Engines box is set to =1. How
> > > can this be?
> > > Do I totally misunderstand the UDP ttl value in
> > > relationship to RIP?
> > > Any insights would be helpful. BTW, I have got a
> > > call into a
> > > PacketEngines s/w engineer and am waiting for a
> > > response.
> > >
> > > Thanks,
> > >
> > > Lance
> > >
> > >
> > >
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> > =====
> > http://www.cormaclong.com
> >
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