RE:[isp-bgp] Multihoming Best Practice

["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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