expectations for bgp peering?

Patrick W. Gilmore patrick at ianai.net
Wed Jan 21 05:35:17 UTC 2009

On Jan 21, 2009, at 12:25 AM, mike wrote:

> So I am just wondering what my expecations should be in a bgp  
> peering scenario where I am multihomed with my own ASN and arin  
> assigned ip space. At issue is the fact that my backup isp forced me  
> to use ebgp multihop to peer with a router internal to their network  
> and not the border router I am directly attached to, and secondly,  
> that they say I am not allowed to prepend at all - they will do it  
> for me, and from the looks of things they have established a route- 
> map that just prepends their AS 6 times to my announcement.
>   This smells of bad engineering. I have looked up the bgp report  
> for my provider and they have 0 downstream AS's, and the week that  
> this project has taken (and it's still not up and working) has left  
> me with less than absolute confidence in the provider. I want to  
> know if anyone has an opinion on ebgp multihop for external  
> customers, and wether I should really have an expectation to be able  
> to assign my prepends as suits my needs? Are there any conditions  
> that could make this fail that I should be aware of?

First, Rule Number One: Your network, your decision.  (Okay, rule  
number one not about spam. :)  Your transit provider owns your transit  
provider's network, he can run it as he pleases.  There is nothing  
"wrong" with multi-hop or not allowing you to prepend.  Bits are  
flowing, and you are connected.

Now that we know he _can_ run his network like that, I would never buy  
transit from someone who _does_ run a network like that.  I've seen  
multi-hop before, but it is usually when something unusual is  
happening.  For instance, if you are in a strange location and the  
edge router near you does not speak BGP.  It is not horrible, but  
certainly not something I personally would prefer.  Multi-hop can  
present more or at least different failure modes than direct BGP, but  
those can be managed with clue and effort.

The non-prepending thing though, that's Just Plain Silly.  And hints  
at a general lack of clue.  Which means that multi-hop is dangerous as  
it requires more clue, not less, to manage properly.


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