Subject: Re: why doesn't NetBSD support bridge in kernel?
To: Markus A. Boeing <firstname.lastname@example.org>
From: None <email@example.com>
Date: 07/06/2001 13:15:52
On Fri, Jul 06, 2001 at 11:26:41AM +0200, Markus A. Boeing wrote:
> 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!
I agree that BGP is not bridging. For the purposes of ISP
interconnect, again I agree BGP is the correct way to do it. However
my example was of an ISP and some other carrier that they might use to
reach customers (likely within a city). You can lease fibre, you can
buy DS1s, DS3s, you can buy ATM PVCs, or in some places you can buy
tagged VLANs. Although it's not very common yet, you can also get an
MPLS LSP sometimes, which is really the right way to do it.
> 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.
Depends on the topology that you make:
ISP | Transport Provider | customer
+-----------+ +--------+ +-----------+ +----------+
| L3 Switch |----| NetBSD |-----| L2 switch |-- ... --| customer |
+-----------+ +--------+ +-----------+ | router |
There is nothing particularly significant about connecting broadcast
domains in this instance: there are only two devices speaking IP as a
member of a particular VLAN -- the L3 switch and the customer
router. A tagged vlan becomes functionally identical to an ATM PVC in
this instance. (Yes I know that most ethernet switch vendors support a
proprietary mechanism where they can push second tag on packets and
tunnel tagged traffic the same way you would with MPLS label
stacking. Some of the L2 carriers that I deal with won't do this
> 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.
Yes, but the other carrier doesn't do IP at all. They are strictly a
transport layer provider. And end users are on the other side of them.
> 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
Yes. I am trying to make VLAN tags functionally equivalent to MPLS
labels. The problem with MPLS is it is still not widely supported by
the vendors or the free OSs.
And BTW you can do this on Juniper routers using what they call CCC
(Circuit Cross Connect), and we have tested it and it works quite
well. Their implementation is general enough to let you do any of the
vlan - vlan
pvc - pvc
lsp - lsp
vlan - lsp - vlan
pvc - lsp - pvc
> Interconnecting other providers always involves BGP, probably on top of a
> switched LAN, but not bridging.
My point is only that there are still some valid uses for bridging. I
agree with you that it should not be used where it is
inappropriate. But there /are indeed/ instances where it is