Re: BGP TTL

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)
>>> >
>>> >
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>>
>

-- 
With Warmest Regards,
CCIE KID
CCIE#29992 (Security)
Blogs and organic groups at http://www.ccie.net