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VLAN trunks, spanning tree and a port blocked.

We are upgrading out internal office network.

I have setup 4 switches and 1 router using the following config.

2 SGE2010p's are stacked together forming vlan 30 (to be used for phones)

2 SGE2010's stacked for workstation's under vlan 20

Both switches are apart of vlan 1 (the default)

IP Assignments are

vlan 1 :

vlan 20:

vlan 30 :

Ports 1 on each stack are connected to a Cisco 890 (port fe2 and fe3) in trunk mode.

Ports 2 and 3 are in an LAG group and connected to the neighboring stack (I have attached a diagram), and are also in trunk mode).

At this point one port (port 1 on either stack) goes into blocking mode, and I get why ,it's a physical loop.

Here is my problem. I need more than a 100mb link between switches (which is what pulling the LAG between switches gets me), but I don't want to route phone traffic through my workstation switches to get out of the LAN.

Is it possible using these switches and router to have a LAG between stacks and a link to the router for traffic to ?

I have attached a diagram of the config.

Everyone's tags (5)

VLAN trunks, spanning tree and a port blocked.

Hello John, I think it may be a best case scenario to drop the lag and use a gbic from each switch to the router. I don't see a prudent way to accomplish what your information depicts.

With the LAG in the middle, spanning tree is going to make a mess on about every scenario I can conceive. Even if you layer 3 the switch and add an ip route for the respective subnet traffic, that can work but I'm not sure it is worth the adminstrative headache. Also, regardless of what media you choose, if you need a connection between the switches, there will always exist the network loop OR the traffic flow will go through one switch or the other.  So the only logical conclusion seems to be to drop the link between switches and see if there is a compatible fibre connection from the switch to the router.

-Tom Please mark answered for helpful posts

VLAN trunks, spanning tree and a port blocked.

Our router doesn't support GBIC connects, so the fiber route is out.

It looks like the switch ports on the router do support LAG's, so I could bond a few interfaces into the router and drop the interswitch link.

Our problem is we will have 2 servers on the phone vlan switches, one obviously to control the phones and the other for call center related programming, and a single 100mb link between work stations and that server is not going to be enough.

I think I would opt to route one stack of switches through the other before I yanked the LAG between the two.

Anyway, I appreciate the advice, even if its a bit disappointing.


VLAN trunks, spanning tree and a port blocked.

John, one thing I have overlooked, these switches are SGE2010. This means they are 10/100/1000 switches.

Cat 5 is 10/100

Cat 5e is 10/100/350

Cat 6 is 10/100/1000

If you're only negotiating 10/100, there is a problem.


-Tom Please mark answered for helpful posts

VLAN trunks, spanning tree and a port blocked.


The router only has 1GE port on it.

So while the LAGs connect at GigE, the uplink to the router obviously does not.


VLAN trunks, spanning tree and a port blocked.

John, just a thought, may be MSTP is your solution.

An example of MSTP would be;

Port #1 is blocked in one spanning tree instance while the same port is forwarding in a different spanning tree instance. The MSTP is an expansion of PVST and 802.1q.

So perhaps spanning tree can block a vlan on the port but forward the other vlan?


-Tom Please mark answered for helpful posts

VLAN trunks, spanning tree and a port blocked.

That's a solution I can work with. I have never heard of MSTP (my spanning tree experience is pretty limited, I am much better in other areas). I am off to find a MSTP reference, I really appreciate the advice Thomas, very much!