Re: BGP TTL

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.

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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Received on Sun Oct 23 2011 - 13:49:51 ART