RE:[isp-bgp] Multihoming Best Practice

[vdadlaney@...]

Hi Ejay,

Could you please elaborate on option 1. I apologize but if I understood the scenario correctly than option 1 would work and will be redundant as well because in order for the provider to advertise the address space I assume it would be done either via redistribution or via the network statement. If using the redistribute statment if the static route is pointed to the interface on the provider router than it won't advertise the route if the interface goes down and if using the network statement than it won't advertise the route if the link goes down because the route has to be in the routing table in order for BGP to advertise it. It will cause the inconsistent AS in BGP however from a redundancy prespective I think it should work. Please let me know if I have misunderstood. Thanks

Regards,

Vikram

 -------------- Original message ----------------------
From: "Ejay Hire" <ejayhire@...>
> >what do you think, which ist the best method to announce PI space over 2 or 
> >more provider?  Do you use private-as numbers for the customers or is it 
> >better
> >to use an registered AS?
> 
> Hello.
> 
> As you've seen, there are a lot of opinions on this.
> 
> Here are my thoughts.
> 
> For a customer that IS multihomed to you and another provider, they should 
> acquire an ASN and use it to advertise their IP space.  Some people, for 
> whatever reason, (they are cheap, don't know any better, or don't have the 
> proper equipment), use creative ways around this.
> 
> Ways around this.
> 1.  Have each provider advertise the IP space and static route it to the 
> customer.
> Problem:  Not redundant in provider link failures.  Causes Inconsistent 
> Origin AS in BGP.
> 
> 2.  (Slightly better than #1) Have each provider advertise the IP space and 
> route it to the customer.  Use an IGP and redistribution or some other 
> technique to advertise the route dynamically based on the customer's link 
> up/down state.
> Problem:  Not redundant in provider internal routing failures, Causes 
> Inconsistent Origin AS in BGP.
> 
> 3.  Have the customer run BGP with a private ASN, advertising the route to 
> both providers.  Both providers "leak" the private ASN to the internet.
> Problem:  The Private ASN will be blocked by several large AS, and will 
> cause reachability problems.  This is really bad, don't do this.
> 
> 4.  (Slightly better than all of the above, but still bad.)  Have the 
> customer run BGP with a private ASN, advertising the route to both 
> providers.  Both providers strip the private ASN, and advertise the route 
> with their ASN to the internet.
> Problem:  Inconsistent Origin AS in BGP.
> 
> #4 is the most workable solution, if the customer won't get an ASN.
> 
> An Inconsistent Origin AS means that the route is in the global BGP tables, 
> but it appears to be advertised from more than one advertiser, indicating a 
> possible misconfiguration.  This used to cause router issues and issues with 
> other tools, (netflow collection uses origin AS data), but is in production 
> use on the internet today.
> 
> You can see some Inconsistent origin AS blocks by going to 
> route-views.oregon-ix.net and do a show ip bgp inconsistent-as.
> 
> Finally, One other case.  If a customer owns some PI IP space, but is NOT 
> multihoming, then you should advertise the IP space for them, and NOT 
> require them to run BGP or any of the above kludgeophiles.
> 
> -ejay
> 
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