Subject: Re: why doesn't NetBSD support bridge in kernel?
To: Will Waites <firstname.lastname@example.org>
From: Markus A. Boeing <email@example.com>
Date: 07/06/2001 11:26:41
Well, my statement regarding the merits of routing wasn't out of context as
your answer implies. I was talking about network size and network features.
Bridging may have its (limited) value in small networks (and only if you
don't need redundant links with decent convergence time within your
topology etc.). Your example of provider interconnect is a pretty weak one.
The technology to interconnect provider networks is "exterior routing
protocol" (BGP4) and not bridging! Bridging between VLANs is not an
extremely clever thing to do because it breaks the idea behind the
invention of VLANs (control broadcast radiation). If you have to connect
VLANs you shouldn't use bridging because that connects broadcast domains.
If you use routing instead you not only keep the broadcast domains separate
but you gain the possibility to control/police your traffic as well. If
you're connecting service providers over VLANs, I bet that you would have
to implement some sort of policy as well. Which leads us to BGP4 but not to
a bridging solution. I've designed quite some large-scale ISP networks, and
I've used bridging (L2 switching) only to construct POP interconnects.
Building resilient POP interconnects involves redundant bridges/switches,
and the use of IP subnets per switched LAN but not fiddling around with
VLAN tags (BTW you don't mix up MPLS(label) and VLAN(tag)?).
Interconnecting other providers always involves BGP, probably on top of a
switched LAN, but not bridging.
At 20:23 05.07.2001, Will Waites wrote:
>On Thu, Jul 05, 2001 at 09:46:28AM +0200, Markus A. Boeing wrote:
> > Well, probably bridging is the most basic network function. Probably it is
> > the least desirable network function as well. If you're going to build
> > large networks that scale well and provide rich functionality, you're
> > better of using IP routing function instead of bridging technology.
>Not true. Bridging is still usefull in some circumstances. For
>example: two providers interconnect and they each have their own
>802.1Q vlan tags that they use. Maybe one is a transport layer
>provider doing LAN extensions and the other is an ISP. Most likely,
>since there are only 4095 useable tags, there will be collisions
>between the two tag domains. Solution: rewrite the tags. Unfortunately
>none of the router and switch vendors (excluding some high end
>catalyst switches and juniper routers -- very costly, at any rate)
>support tag rewriting. You can do it on a unix such as NetBSD with
>bridging support simply by bridging one vlan interface into another.
>Blanket statements like "routing is better than bridging" are
>nonsense. Granted people sometimes will use one or the other where it
>is not appropriate and cause themselves headaches, but that doesn't
>mean that either is "better".