Creating One VTP Domain - Connected Across a T1 via Routers

[Steven Barrios]

I have an interesting design problem that I have been working on.  We are attempting to build out an infrastructure with multiple VLANs that spans across multiple buildings.  The only connection between the buildings comes from Routers connected via T1.

To test this we have built out a lab with 2 routers (3660s) and 2 switches (3550s) and connected them as such:

[S1]--------[R1]-------------------T1 Link--------------------[R2]--------[S2]

We have had success passing traffic between the two switches (and devices attached to the switches) by using the no ip routing command and assigning bridge-groups to the interfaces.  This allowed S1 to ping S2 without R1 or R2 having to even have IP addresses.

interface FastEthernet0/0
description To Switch
no ip address
no ip route-cache
duplex auto
speed auto
vlan-range dot1q 1 4094 native
  description To Switch
  bridge-group 1
  exit-vlan-config

Unfortunately, we could not get VTP traffic to pass from the VTP Server (S1) to our VTP Client (S2).  Also, S1 saw R1 as its CDP neighbor... and I would like to make R1 and R2 completely transparent to the switches such that S1 sees S2 as a direct connection.

I know that if our routers were switches, dot1q tunneling would allow us to achieve this, but given that we're dealing with routers traversing a T1 that is not an option here.

What technologies could be implemented on the routers that would make them as transparent as possible and allow the VTP Domain to be extended over them.  Basically I'd like this network to function as a single Layer 2 broadcast domain, even though there are routers right smack in the middle.

Thank you,

Steve

[Jon Marshall · Accepted solution]

Steve

What are the routers ?

L2TPv3 will allow you to extend a L2 vlan across a L3 routed network.

Jon

[Giuseppe Larosa · Accepted solution]

Hello Steven,

>> The only connection between the buildings comes from Routers connected via T1

how far are the buldings? on the long term placing fiber cables is the best choice.

A T1 is rather slow in comparison with a true GE link.

IF your devices are 2811 or better you can use L2TPv3 as suggested by Jon.

However, these setups are used generally only for a selected small group of vlans that need to be extended on the two sites.

Passing all vlans and all broadcast traffic is a different matter.

see

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t2/feature/guide/gtl2tpv3.html

OR

use the routers as routers and separate the two VTP domains. Doing so the T1 bandwidth is used only by user traffic that really needs to go from side to side.

Otherwise everytime a device does an ARP request on building A it is propagated to building B even if the destination is in building A!

Hope to help

Giuseppe

[Steven Barrios]

Thanks to both of you.  I will look into L2TPv3 and see how that functions in our lab environment.

In the real environment, some of the buildings are many miles apart, so a fiber implementation though preferable won't be immediately possible.  The other problem is that in the real environment there are more than two sites, and it would not be ideal to manage 7 to 10 seperate VTP domains.

I am hoping that smart VLAN usage will cut down on excessive site to site traffic, but there will certainly be some VLANs that have to span all or nearly all of the sites.

Right now I am testing with 3660s, but the final hardware could be something newer if we determine that that is what we need.  We are still in the planning / design phase right now, so nothing is set in stone.

I'll get back to you guys on this after I do some reading and testing.

[Steven Barrios]

There was a definite period of muddling around but we were successful in implementing this in our test environment.

As desired, the 2 switches were able to see each other across the L2TPv3 tunnel as if they were directly connected to one another.  VTP and CDP both passed cleanly.

Thanks again.