Linking two sites together with layer 2 ethernet private line

[oysterman]

Hi all,

Hope you can help.

We have 2 sites (number of switches in each site working fine) which we want to link together.

We have purchased a 100mb layer 2 ethernet circuit from a carrier (similar to BT's LES 100) which we will use to link the sites.

As we have only purchased one circuit, there will only be one interconnect between sites.

I was planning on creating a new VLAN on the switches that we will be linking across sites, and connecting this new circuit to the ports within this VLAN. I will also have a router at each site in the same VLAN to move traffic beween subnets at each site.

My question relates to spanning tree - MSTP.

Basically as I have no direct control over this new circuit (which can go up and down if the carrier does some maintenance etc), I think it would be good to disable spanning tree on the ports we connect this circuit to (on each switch) as I do not want any disconnects on this circuit to force spanning tree to recalculate / re-converge etc to the rest of the switches / vlans at each site (which will cause brief outages whilst doing so).

Is this the correct approach?

Thanks

John.

[acampbell]

Hi,

If you are worried about convergence etc.

May be you would be better to run IP routing between the sites.

Turn an interface into layer 3 at each end of the new link.

At site A

!

int fas 0/1

desc SITE A

no switchport

ip address 192.168.255.1 255.255.255.252

!

At site B

!

int fas 0/1

desc SITE B

no switchport

ip address 192.168.255.2 255.255.255.252

!

Add some Ip routes at each site etc etc

Now the sites are independant of each other as regards to any layer 2 issues , spanning tree etc

Regards

Alex

[oysterman]

Hi Alex,

Many thanks for the suggestion, however the switches at each site are L2 devices.

I like your thinking though...

Thanks

John

[acampbell · Accepted solution]

Ok John

Site A

!

vlan 555

name INTERSITE

!

int fas 0/1

desc to SITE B

switchport host

switchport access vlan 555

!

On router or L3 switch

int vlan 555

ip address 192.168.255.1 255.255.255.252

!

Site B

!

vlan 555

name INTERSITE

!

int fas 0/1

desc to SITE A

switchport host

switchport access vlan 555

!

On router or L3 switch

int vlan 555

ip address 192.168.255.2 255.255.255.252

!

Again add routes to allow traffic between site

As VLAN 555 will be point to point there is no sanning tree loops etc

Thus the site are still basically L2 independant

HTH

Alex

[oysterman]

Thanks Alex.