Re: BGP TTL

Hi Carlos,thank u for ur explanation. Is there any commands which can be
used to tweak the minimum incoming TTL connections in BGP .

Thank you

On Fri, Oct 21, 2011 at 5:53 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar>wrote:

> TTL serves loop control as a primary function. Data plane.
> It does not matter much when exactly it is evaluated/decremented,
> but that it is decremented on each hop and eventually dropped
> when it gets to 0.
>
> Second function (added value) is scoping. I.e. you can control how far
> this packet will get changing the initial value. So routing protocols
> use this feature to get an added security that messages are only
> processed by neighbours, by setting TTL to 1.
> BGP changes that when you do multihop, obviously.
>
> Now, from a security standpoint, someone might want to talk to your router
> when in fact it should not. Here's a third application of TTL:
> I'd like to call it reverse scoping. By enforcing a minimum TTL above 0
> (say m), you are in fact restricting anyone that is farther than 255-m
> from being able to talk to you. That's inbound TTL.
>
> Talk about reusing a feature :)
>
> -Carlos
>
> CCIE KID @ 21/10/2011 8:25 -0300 dixit:
>
> 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
>>
>>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina
>

-- 
With Warmest Regards,
CCIE KID
CCIE#29992 (Security)
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