Subnet Size for BGP peers.

Barton F Bruce barton at
Thu Jul 30 04:22:27 UTC 2009

----- Original Message ----- 
From: "Jim Wininger" <jbotctc at>
To: <nanog at>
Sent: Wednesday, July 29, 2009 3:59 PM
Subject: Subnet Size for BGP peers.

> I have a question about the subnet size for BGP peers. Typically when we
> turn up a new BGP customer we turn them up on a /29 or a /30. That seems 
> to
> be the "norm".

> We connect to many of our BGP peers with ethernet. It would be a simple

So what is wrong with a /31? We use /30s but if you are short on IP space, 
look at using /31 rather than /30 links. Cuts your space usage in half.

If I remember correctly, the BIG problem with using /31s when they first 
became "legal" was to decide if the customer still gets the higher numbered 
IP address (or you the lower one), or if you still get the ODD number. No 
kidding, it is a problem for some!

Where you are on ethernet, use a seperate 802.1q vlan per customer and have 
your switch give the customer untagged packets. If you have downstreams in 
your COLO, and either free or as a paid service, offer to setup private 
vlans in your switch for any pair or group of customers that need to also 
connect to each other privately for whatever they are doing. In that latter 
case, they will be getting tagged packets but their routers or switches 
should have no problem dealing with them.

We don't charge for physical crossconnects, so this has saved us having to 
do physical crossconnects between customers, and has saved customers router 

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