Re: ibgp/ebgp AD

Thanks guys, that put things into perspective

Atle

On Sun, Jan 23, 2011 at 10:02 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar> wrote:
> One way to look into it is to think that IGPs are good at internal
> things and EGPs should be good at external things.
>
> So in IGPs, internal, local is good. But in EGPs, local might not
> be so. This relates to what is called hot potato (vs cold potato).
> You want to get rid of the traffic ASAP so if at a given place
> you have an eBGP learned route and an iBGP one, eBGP means going out
> now and is preferred (hot potato).
> iBGP learned route means that the exit to the external network is
> in another router you still have to transit to using your internal
> network.
>
> This is related to OSPF E1 and E2 types somehow. E1 means external metric is
> compatible with OSPF, so the metric increases with your
> internal network transit. This resolves into hot potato policy.
> E2, on the ohter way, stays constant inside your network, meaning
> it's ok to traverse all your network to the best (at the edge) path,
> which is a cold potato variant.
>
> HTH,
> -Carlos
>
> P.S.
> In IGPs, internal routes are preferred because you have complete knowledge
> there, as opposed to a summarization done at an ABR, IMHO.
> I would not call that "common knowledge" though. It's just part
> of the algorithm that warrants loop free convergence.
>
> Atle Xrn Hardarson @ 23/01/2011 16:59 -0300 dixit:
>>
>> Hi experts,
>>
>> It is common knowledge that a routing protocol and platform *should*
>> prefer internal routes before external routes.
>> Like, OSPF prefers Intra Area Routes before Inter Area Routes, and
>> EIGRP prefers Internal (AD 90) before External (AD 170) routes...
>>
>> So then to my question - *why* does eBGP have a lower AD then iBGP?
>> (20 vs 200)? As in "why did Cisco design it that way"?
>>
>> I tried looking it up, but having problems finding my answer. I really
>> enjoy reading protocol history, and knowing why things were designed
>> the way they were, it helps me learn...but this one I can't find.
>>
>> Thx
>>
>> Atle
>>
>>
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>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar> LW7 EQI Argentina

Blogs and organic groups at http://www.ccie.net
Received on Sun Jan 23 2011 - 22:12:58 ART