Re: BGP TTL

Dan Shechter @ 23/10/2011 08:49 -0300 dixit:
> Since we can't have a look at the IOS code, we can only guess.
>
> Base on BSD and linux code, TTL is checked and decremented in the ip_forward
> function, which is not called when the destination is the router it self.
>
> We can _maybe_ see such behavior with IOS, as packets going to one of the
> router's interfaces are not subject to PBR.
>
> Even sending traceroute to a router's Lo0 with TTL=0 will get you ICMP
> port unreachable (not TTL expired).
>
> I _think_ that BGP actually tests the TTL and not the net/routing code.

That is for sure, since it has its own semantics for that test.
Same thing happens with multicast scoping too.

-Carlos
>
> HTH,
> Dan #13685 (RS/Sec/SP)
> The CCIE troubleshooting blog: http://dans-net.com
>
>
> On Fri, Oct 21, 2011 at 1:25 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..
>>
>> 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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-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net