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New Member

Multiple vlan traffic from cisco 3548 to 3com 3300

Dear friend,

We have our private environment with cisco switches.

A cisco 6006 layer 3 switch is used as vtp server and the vlan configuration are send to 12 cisco 3548 leaf switches which are configured as clients. There is a situation where a switch is to be installed for maintaing three vlans in a building. we had 3com 3300 switch and we are trying to utilize it in this purpose. It is possible to created the required vlans in 3com 3300. But the problem is how can I use a single link from cisco 3548 to 3com 3300 to handle multiple( 3 ) vlan traffic. what are all the configuration to be done in cisco and 3 com part.

Kindly inform me the details necessary to handle this process.

Thanks in advance.


Re: Multiple vlan traffic from cisco 3548 to 3com 3300

On the Cisco side:

Configure the port that will connect to the 3Com switch as a VLAN trunk port, using 802.1Q encapsulation. For example,

interface FastEthernet0/48

switchport trunk encapsulation dot1q

switchport mode trunk

This will send 802.1Q-tagged traffic for each VLAN out through this interface. The native VLAN (the one that is associated with any untagged traffic) is VLAN 1; since this is the default, it won't show up in your configuration.

If you want to optimize what goes out on that link, you can remove/clear/prune unnecessary VLANs from that trunk. That way, only traffic for the VLANs that the 3Com switch has ports for will go out over the switch-to-switch connection.

On the 3Com side:

Configure the port that will connect to the Cisco switch to belong to each of the VLANs you implement on the 3Com switch. If you configure it interactively, it will prompt you for whether you want it to be tagged or untagged; make that port tagged.

NOTE: be careful NOT to tag ports on the 3Com switch that will have computers or printers attached to them. Those devices need untagged ports (unless they have network adapters that can read 802.1Q tags, and are configured to be on multiple VLANs).


Use an Ethernet crossover cable (if using 10/100 ports) to connect the two switches together.

If you're using Gigabit Ethernet fiber ports, you want to patch in so that TX connector from one switch goes into RX connector on the other switch. Easy enough to do: just patch the cable in securely; if you get no link light, on one of the switches swap the connectors between the TX and RX ports of the interface and the light should come on. (Don't bother tracing which strand goes where; you have a 50% chance of getting it right the first time when you patch in; and if you guess wrong, there's only one other way you can connect it, and that's by reversing the connectors on one interface.) If you get NO link light, check the patch cable.

Hope this helps.


Re: Multiple vlan traffic from cisco 3548 to 3com 3300

Forgot to add this in the Cisco part of my previous post, just before you get to the 3Com part.

An example of removing VLANs from a trunk connection:

interface FastEthernet0/48

switchport trunk allowed vlan remove 2,4,7-1001

This will leave VLANs 1, 3, and 5-6 on the trunk, but remove all the others whether they exist on the Cisco switch or not. NOTE: you cannot remove VLAN 1, or VLANs 1002-1005.

Use the "show interface FastEthernet0/48 switchport" command to check your work. (Substitute the actual interface you're using where it says FastEthernet0/48.)

New Member

Re: Multiple vlan traffic from cisco 3548 to 3com 3300

Thanks konigl,

I have tried the configuration as per your information. As far as the cisco part is concerned, no problem in the configuration. But in 3com part, I had a problem with the configuration. we had 3com 3000 superstack II switch. In this switch configuration, 16 vlans can be created locally.

The problem is how can I map the vlans in cisco switch to the vlans in 3com switch(since the vlans in 3com can be numbered only from 1 to 16). This vlan mapping is possible in 3com 1100 switch. Is it possible to do the vlan mapping in this 3com 3300 switch. If possible, how can I go over that.

Thanks in advance.


Re: Multiple vlan traffic from cisco 3548 to 3com 3300

Switches from the 3Com SuperStack II Switch 3000 series -- FX (3C16940A), TX (3C16941A), and 10/100 (3C16942A) -- do not do 802.1Q VLAN tagging; they only work with proprietary 3Com VLT tagging. And as you have noted, those VLAN numbers only run from 1 to 16.

The 3Com SuperStack II (and 3) Switch 3300's and 1100's can do both 802.1Q and VLT, however, and have been used to interconnect networks using the two VLAN schemes. It has been a LONG time since I have needed to do something like that, though.

If you have examples of how to do this with the 1100, then the steps should be the same on a 3300 -- they run the same code. 3Com's SuperStack II Switch Management Guide covers how to do this in an advanced example, but they don't go into great depth (for example, what if the VLANs on your 802.1Q network are numbered higher than 16). Here's a link to that Guide (file size is 3.2MB, don't click it unless you really want it):

I guess the worst-case scenario is, you have a bunch of ports on one old 3Com switch set up as access ports for each of the VLT VLANs; and physically cross-connect each one via Cat5 crossover patch cable to a corresponding access port on a new 3Com or Cisco switch, where the VLANs are part of an 802.1Q network. You wind up wasting a lot of switch ports to do cross-connects; but if you can't replace the old 3Com switches and you need to tie them in, then you have no choice.

Most of the customers I support that still have 3000's, 1000's, and the old Desktop Switches, have been replacing them because of the dumb way they handle multicasts. These switches treat them as broadcasts, which causes massive network performance degradation while the customer does multicast desktop drive imaging with products such as Ghost.

If you can give me more details about what you're trying to do with the VLANs, I might be able to simulate it here and give you some more feedback. I have some of the really old 3Com switches (a 3000 FX, a 3000 10/100, and a couple of 1000's and Desktop Switches) as well as a few of the newer ones (3300's and 1100's). Just have to dig them out of storage and dust them off. Let me know.

New Member

Re: Multiple vlan traffic from cisco 3548 to 3com 3300

Thanks for your kind help.

Let me explain you the scenario over here. We have a private campus network extending over 2km diameter with 48 vlans. The backbone network is of optical fiber cables. We had a cisco 6006 switch with routing module which is doing the intervlan routing function in one building. We have two cisco 4006 switches(let us name it as 4006 I and 4006 II) in two other buildings. These cisco 6006 switch is connected with four cisco 3548 switches in two floors of the building and a cisco 4006 switch (4006 I)is connected to three cisco 3548 each. Cisco 3548 connected to these switches(cisco 6006 and a cisco 4006) are connected directly to end stations.

The other cisco 4006(4006 II) is connected to three cisco 3548 and one cisco 2950. cisco 2950 is connected to two 3com 1100 switches and four cisco 1538 Hubs. Two Cisco 3548 in this 4006 switch is connected to endstations. The third cisco 3548 is connected to three cisco 1538 Hubs and one Dlink 916 Hub.

The situation is that there is a restructring work in the building where one of cisco 1538 hub(connected to cisco 4006 II) is placed. In that building there are three different departments are to be placed. Prior to that is only one department and hence the connection is established thro cisco 1538 Hub with uplink connected to cisco 3548 switch port. The port in cisco 3548 connecting the Hub is configured as access port to that particular vlan. It has become essential to place a switch in the building to accomodate the three vlans. We have a 3com 3000(3C16942A), one 3com 1100 switch. So, we have tried to utilize this 3com swithces for this purpose.

As per your suggestion, Connecting 3com with cisco switch using cross cables for each vlan is not possible, since there is only four optical fiber cores between cisco3548 and the 3com 3000( to be placed). We left the third and fourth optical fiber core for redundancy. So, we have to use the single pair of optical fiber core to carry the three vlan traffic.

Is it possible with 3com 3000 or else with 3com 1100. suggestions please.

Scenario II(different from the first one):

I am asking you one more clarification regarding 3com 1100 switches. As I said in cisco 4006 (4006 I), a cisco 2950 is connected. There are two 3com 1100 swithces connected to this cisco 2950. A third link is also connected between these 3com switches, that is a mesh is established between the cisco 2950 and 3 com switches. Each 3com switches are configured to have only one vlan. But each 3com switch have different vlan in it. I disable a link connecting to one 3com from cisco 2950, the third link between the 3com switches is not taking over the ip traffic. When I go thro the BPDU status in cisco 2950, It has not received any BPDU from the 3com side.

Will It is possible to establish the mesh between cisco 2950 and 3com switches.

Kindly revert me for further clarifications.

Thanks in advance

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