[c-nsp] Mpls on fastethernet <-> atm route-bridge ip link?

Code Monkey have.an.email at gmail.com
Tue Feb 28 19:21:26 EST 2006

On 2/10/06, Sean Watkins <sean at northrock.bm> wrote:
> >>  ip pim sparse-dense-mode
> >>  atm route-bridged ip
> >>  tag-switching mtu 1508
> >>  pvc 80/50
> >>   tx-ring-limit 3
> >
> > Yes, I have this kind of setup. It's been happily running for
> > something like two years, with no MTU problems, both sides acting both
> > as P routers and PE routers (so I'm quite certain the labels go
> > through OK).
> >
> I wonder if the atm route-bridge ip command causes it to fail?
> What exactly is the difference from atm route-bridged ip and the
> aal5snap command? Both are RFC1483 right?

I may not be completely correct here, but I think that aal5snap is the
manner in which packets are transmitted over ATM, and that
route-bridge means that MAC addresses should be encoded because it's
ethernet on the other side.

In practice, put atm route-bridged ip on the ATM interface (let's call
it PE) when the other end of the ATM link (let's call it CE) is a box
you don't [want to] control that has an Ethernet interface and no IP
address that you [want to] know about, and you want to pretend that
you're plugging your CE ethernet cable directly into the PE ATM

Example: when your provider gives you ATM on one side and Ethernet on
the other, or when you put a CE DSL modem in bridged mode for some
reason (like 4-wire SDSL before 878s to Cisco sites with Cisco
ethernet ports free wanting to run EIGRP but not pay for a 4-wire
SHDSL Cisco module, before 878s existed)

Don't put atm route-bridged ip when you control the CE ATM interface
and put an IP address on it.

Example: when CE is a Cisco 8x7 or 8x8.

On a related note, *my* current problem is that I want to use MPLS
over a SHDSL interface, and I have an 878. 878 doesn't support MPLS in
any IOS I can find (Cisco software tool *still* doesn't support 87x),
I can make the 878 into a dumb bridge but not push the MTU beyond
1500, so I think I have spring for an SHDSL WIC.

> >  tag-switching mtu 1530
> >  tag-switching ip
> >  pvc xxx/xxx
> >
> I attemped to change my config, to be the same as yours, and I
> couldn't get data working. I stuck an access list on the port, and I
> could see frames coming in, from the fastethernet side to the atm
> side, but I couldn't ever get data from the ATM side going back to
> the ethernet port.

Sounds like you didn't have the atm route-bridged ip command? Looking
back I see I filtered it out in my cut-n-paste, which kind of obviates
the whole point, since Oliver Boehmer suspects a problem there. Just
to be complete, my config is

interface ATM4/0.1 point-to-point
 description xxxxxxxxx
 ip address xxxxxxxxx
 ip mtu 1500
 ip ospf network broadcast
 ip ospf cost 10
 atm route-bridged ip
 pvc xxx/xxx
  vbr-rt 10240 10240 95
  encapsulation aal5snap
  service-policy output xxxxx
 tag-switching mtu 1530
 tag-switching ip

> > The encapsulation has to be OK between your ATM interface and the
> > converter, so you can't just change them, but I think that the choice
> > of encapsulation might impact the labels. If the labels weren't going
> > through I'd have advised you to test aal5snap since it works for me.
> >
> I'm asking the service provider what they are using; according to a
> quick conversation I had this afternoon he can't change the aal5snap
> on the fastethernet port only on the atm side? if that makes any sense?

Yes, aal5snap is the ATM encapsulation, the ATM encapsulation has to
be the same between both ATM interfaces. Some things are possible
using some encapsulations and not others.

> > It dies because your ip mtu is 1500.
> >
> > To see if jumbo frames go through, don't worry about MPLS (turn it
> > off!) and try setting the ip mtu to 1508 on both interfaces.
> Yah I can't change it.. I get this message:
> Rtr(config-if)#mtu 1503
> % Interface FastEthernet5/0 does not support user settable mtu.

Oh. You won't be able to test that the way I thought, then, but since
it doesn't complain about tag-switching mtu 1508 there's still a
chance it might work.


More information about the cisco-nsp mailing list