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connecting 2 VLAN's on 2 Switches

Posted on 2010-09-08
Medium Priority
Last Modified: 2012-05-10
Dear All,

I have a 3COM 4200G-48pt and a 3COM 4210G-48pt switch.

The 4200G has two VLANs :

* LAN (32 ports)
* SAN (16 ports)

both with untagged member ports. I assume that basically this is a "port VLAN" where I simply split up the switch. For the computers connected to the switch, it is as if I had 2 seperate switches.

There's no need for any traffic going between the two VLAN's.

Now I want to add the new 4210G to get redundancy.

I made the same configuration on the 4210G, with 2 VLAN's with untagged ports.

There are 2 links between each lan on each switch, so :

- Switch 1 - pt 48 --> Switch 2 - pt 48
- Switch 1 - pt 47 --> Switch 2 - pt 47
- Switch 1 - pt 1 --> Switch 2 - pt 1
- Switch 1 - pt 2 --> Switch 2 - pt 2

for some reason, the system works fine for the "SAN - VLAN" but not for the "LAN - VLAN"...


1° is this configuration OK?
2° any clues why it doesn't work with LAN-VLAN ?

3° we have 5 desks with 4 workstations each. Currently, each desk has a small 8 pt switch. Is it a good idea to connect those switches to each 3COM switch?

thanks in advance!


Question by:nd2u
  • 2
LVL 21

Accepted Solution

from_exp earned 1000 total points
ID: 33625680
thing you are going to will never work.

there are "redundant paths" definitions. To deal with redundant ports switches use spanning tree, because redundant paths should be working only when primary are down.
If you have 8port switches at every desk, then most likely they don't support spanning tree, so when you connect it to two switches (which are interconnected) you'll get a loop and your network will stop working.

If you want to interconnect two switches, then you shouldn't do it in way you do, but you have to take single port (or several ports joined in a link aggregation group) in every switch and configure it as tagged for both vlans.

Author Comment

ID: 33625715
maybe it's best we forget about the desktop switches for the moment.

As for the inter switch connection... I don't see what's the problem actually...

So your solution is to interconnect via eg 4 ports of each switch and configure those ports as TAGGED members for each VLAN ?
LVL 21

Expert Comment

ID: 33625753
and to configure not only tagged for each vlan, but also as a member or link aggregation group, otherwise only one of them will work, but 3 other will be disabled by spanning tree.

Assisted Solution

BooSTid earned 1000 total points
ID: 33632322
You do not need to use multiple ports to connect them; although you can if you want to set up an etherchannel.

Ports that are going to pass traffic of multiple VLANS need to be tagged with that VLAN number. This is equivalent to trunking in the Cisco world.

If the port only has one vlan, or an endpoint, it is untagged for that vlan.

If the port is part of an etherchannel, it needs to be part of a trunk group.

If you have ports 5 ports, 1-4 with vlans 1-4, and then a fifth port to carry them all to another switch, you would essentially do this:

Port 1 untagged vlan 1
Port 2 untagged vlan 2
Port 3 untagged vlan 3
Port 4 untagged vlan 4
port 5 tagged vlan 1-4

This is simplified of course.

If you had 6 ports with the same setup, but with ports 5 and 6 carrying all the traffic as a group (etherchannel for cisco guys), you would assign 5 and 6 to trunk group 1, and then tag trunk group 1 with vlans 1-4.

Now that the basics are out of the way, to answer your questions:

1. You need to explain why adding a second switch is bringing you redundancy. For clusters and NIC teams, this is perfectly acceptable. For backbone layers, this is also acceptable. But if this is for endpoint devices with single NICs, what's the point? Are you giving redundancy to the desktop switches? If this is the case, it probably won't work.

2. Need the above info. It will probably be helpful to post configs, as it may not be a topological issue, but one of configuration.

3. Desktop switches to individual ports should be fine, just be careful not to introduce any kind of loop. Spanning tree may or may not take down your whole network heh.

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