Subject: Re: why doesn't NetBSD support bridge in kernel?
To: Will Waites <>
From: Markus A. Boeing <>
List: tech-net
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".