Re: BGP TTL

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)
>> >
>> >
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Received on Fri Oct 21 2011 - 22:45:01 ART