One reason that may force others down this track comes from exceeding the # of configurable BGP sessions on a box (think chassis switches). It does add a good bit of complexity in the initial roll-out but it's really not all that bad once you get used to it. The one piece that seems to make it a little easier is that you get a consolidated view on some devices, where the prefix counts are shown for both address families under the same "show ip bgp neighbor" display. david. On Fri, Dec 21, 2012 at 3:04 PM, Leo Bicknell <bicknell@ufp.org> wrote:
In a message written on Fri, Dec 21, 2012 at 11:45:24AM -0700, Pete Ashdown wrote:
I've got a peer who wishes me to send my IPv6 announcements over IPv4 BGP. I'm running around in circles with JTAC trying to find out how to do this in JunOS. Does anyone have a snippet they can send me?
A believe you got the snippet, but I wanted to expand on why this is a bad idea. From a protocol perspective, BGP can create one session over a particular transport (IPv4, or IPv6 typically) and then exchange routes for multiple address families (IPv4 unicast, IPv4 multicast, IPv6 unicast, IPv6 multicast, or even all sorts of fun MPLS stuff). From a network management perspective doing so can complicate things immensely.
Today networks want to deploy IPv6 without impacting their IPv4 network. Adding IPv6 AFI to an IPv4 transport session will tear it down, impacting IPv4 customers.
Tomorrow, when IPv4 transport fails, IPv6 customers are also impacted by the failure of the transport, even though there may be no IPv6 routing issues. There is also a chance that IPv6 forwarding fails, but the routing information lives on running the traffic into a black hole since the routing information isn't sharing the failed transport.
In the future, IPv4 will be removed from the network. If all of the transport is IPv4, those sessions will have to be torn down and new ones built with IPv6 transport before the IPv6 only network can live on.
I believe the vast majority, approaching 100% of larger ISP's move IPv4 routes over IPv4 transport, and IPv6 routes over IPv6 transport, treating the two protocols as ships in the night. It elminates all three problems I've listed above at the grand expense of your router having to open/track 2 TCP connections rather than one; a trivial amount of overhead compared to the routes being exchanged.
Of course, there are people who like to be different, sometimes for good reasons, often not... :)
-- Leo Bicknell - bicknell@ufp.org - CCIE 3440 PGP keys at http://www.ufp.org/~bicknell/