Yes, I was wrong about it not being in the IP header. The ttl is in the ip
header. RIP updates are transported over UDP port 520. Now, what do you
think the ttl for a RIP update should be, Nimesh? And why?
Nimesh Vakharia wrote:
> 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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