Re: BGP TTL

I though we are the experts or in some cases wanta be experts (me:)

I am pretty sure CCIEKID was right the way he explained it.

Two Words

Trace Route

Rob

On Sat, Oct 22, 2011 at 4:25 PM, Imran Ali <immrccie_at_gmail.com> wrote:

> my point was simple ....
>
> if a device is forwarding any packets ....like a transit router....
>
> afer making all checkums , cef lookup;s, encriptions , adjacency table
> lookps , NAT , statefull , qos decisions if the device later on drops the
> packet owing to LOW ttl value. wastes the cpu cycles used in previous
> operations......
>
> you may be correct ...but only an expert can explain correct order of
> operations ...and palcing the ttl value checking operation first or second
> makes good use of resources.
>
> this may also depondant on IOS code or palform or vendor ...
>
>
> so let the experts guide us to correct order of operations .
>
>
>
> On Sat, Oct 22, 2011 at 8:05 AM, CCIE KID <eliteccie_at_gmail.com> wrote:
>
> > Hi Imran,
> >
> > My question is all IGP control plane packets are send with TTL of 1. Lets
> > go with ur logic
> >
> >
> > *pcket comes in with a ttl value of 1*
> >
> > *=> routers looks at the cef table to find an entry and determines the
> > exit interface *
> >
> > *=> The above step may involve many cef table lookup's (recursive
> > lookup)
> >
> > *
> > *=> Now the packet is destined to itslef *( its own locaaly connected IP
> > Address ), so there is nothing related to next hop or outgoing interface
> or
> > outgoing encapsulation.
> >
> > *=> So there will be no decrement in the TTL vlaue. so packets with a TTL
> > of 1 cannot be dropped if it sees the destined address is itself ( local
> > router ) which receives the packet . ( Example is any IGP update packet )
> .
> > It doesnt go beyond that particular router.
> >
> > **=> So there will be two checksums will be created on routers. When a
> > packet hits the router , it will generate a checksum to check that the
> > packet has not been scrambled and does all the lookup and sends it to the
> > outgoing interface and there will be a decrement in TTL and again the new
> > checksum will be created ( because the TTL has been changed) and attached
> to
> > the IP Header and send it out of the outgoing interface
> >
> > **=> *So thats my point. Correct me if i am wrong in the above statement
> > and justify ur statement
> >
> > THank You
> >
> > *
> > **
> >
> > *
> >
> >
> >
> > On Sat, Oct 22, 2011 at 1:15 AM, Imran Ali <immrccie_at_gmail.com> wrote:
> >
> >>
> >>
> >> On Fri, Oct 21, 2011 at 3:06 PM, Routing Freak <routingfreak_at_gmail.com
> >wrote:
> >>
> >>> Then buddy all OSPF update packets are sent with a TTL of 1 to their
> >>> directly connected neighbors , so when the neighbors receive the packet
> ,
> >>> decrement the TTL to 0 and drop the packet ahh. Naa this will not
> happen
> >>>
> >>> TTL decrement will happen only on outbound .
> >>>
> >>>
> >>> *=> pcket comes in with a ttl value of 1*
> >> **
> >> *=> routers looks at the cef table to find an entry and determines the
> >> exit interface *
> >> **
> >> *=> The above step may involve many cef table lookup's (recursive
> >> lookup)*
> >> **
> >> *=>now looks at the adjacency table to find the layer 2 add of next hop
> *
> >> **
> >> *=> now he builds the layer 2 header *
> >> **
> >> *=> poor router now decriments the TTL value , and drops the packet . *
> >> **
> >> *it make more and more sense , first to find out if the packet have
> >> enough TTL value ,THEN he can proceed with other forwarding operations
> ....
> >> *
> >>
> >>
> >>
> >>
> >>
> >>> On Fri, Oct 21, 2011 at 5:20 PM, Imran Ali <immrccie_at_gmail.com> wrote:
> >>>
> >>>> Hi kid,
> >>>>
> >>>> regardsless of data plane or control pale TTL value is reduced .
> >>>>
> >>>> The moment any interface receives a packet the TTL will be reduced by
> 1
> >>>> .
> >>>>
> >>>> On Fri, Oct 21, 2011 at 2:25 PM, CCIE KID <eliteccie_at_gmail.com>
> wrote:
> >>>>
> >>>> > Hi fellas,
> >>>> >
> >>>> > I am working on BGP and i find in sh ip bgp neighbor command, the
> >>>> incoming
> >>>> > TTL and the outgoing TTL . I understood the outgoing TTL is the one
> >>>> when
> >>>> > the
> >>>> > control plane packets are generated , the TTL in the IP Header will
> be
> >>>> 255.
> >>>> > I find the incoming TTL to 0 . Can anyone explain me what is
> actually
> >>>> meant
> >>>> > by incoming TTL.
> >>>>
> >>>>
> >>>>
> >>>> > My understanding is the incoming TTL is the when the
> >>>> > neighbor send u any BGP Control plane packet, the local router is
> >>>> expecting
> >>>> > this TTL value in the IP HEader. Am i right ?
> >>>> > So the incoming TTL valus is 0 in my case..
> >>>> >
> >>>>
> >>>> regardless of any operation , the TTL value is always present in ip
> >>>> header
> >>>> :-
> >>>>
> >>>> >
> >>>> > Router drops a packet with a TTL value of 0. So when the control
> plane
> >>>> > packet comes. it should be of TTL of 1.
> >>>> > TTL will be only decremented on the outgoing interface and not the
> >>>> incoming
> >>>> > interface. THe genreal logic says that the TTL of all control plane
> >>>> packets
> >>>> > of all IGPS will be 1 and so when a router receives this control
> plane
> >>>> > packet , it is destined to itself , it will update the OSPF or EIGRP
> >>>> RIB
> >>>> > and
> >>>> > in turn do the local computation for the best path .
> >>>> >
> >>>> > My question is whether the where does the TTL decrement happen ? My
> >>>> answer
> >>>> > is outbound and for any other control plane packet it will be
> destined
> >>>> to
> >>>> > itslef , so there will be no decrement in the TTL
> >>>> >
> >>>> > --
> >>>> > With Warmest Regards,
> >>>> >
> >>>> > CCIE KID
> >>>> > CCIE#29992 (Security)
> >>>> >
> >>>> >
> >>>> > Blogs and organic groups at http://www.ccie.net
> >>>> >
> >>>> >
> >>>>
> _______________________________________________________________________
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> >>>>
> >>>>
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> _______________________________________________________________________
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> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>
> >
> >
> > --
> > With Warmest Regards,
> >
> > CCIE KID
> > CCIE#29992 (Security)
>
>
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>
> _______________________________________________________________________
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Received on Sat Oct 22 2011 - 18:06:15 ART