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Trunking between 3com 1100 and cisco 2950


Kindly help me in sorting out this problem.

The setup in my work place has a mesh with a cisco 2950T switch and two 3com 1100 switches.

cisco 2950 ---------------------------------2nd 3com 1100

port1 | port 2 port 26 | port25

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port 26 | |

1st 3com 1100 -------------------------------

port 25

Configurations are:

1. port 1 and 2 of cisco is configured as trunk port dot1q encapsulation.

2. port 25 of 1st 3com 1100 is configured as trunk and it is connected to port 25 of 2nd 3com switch.

3. port 1 and 2 of cisco is connected to port 26 of 1st and 2nd 3com switches respectively.

4. ports of 1st 3com switches except 25,26 belongs to vlan 10.

5. ports of 2nd 3com switches except 25,26 belongs to vlan 11.

6. ports 25 and 26 of both 3com switches are configured for all vlans ie for vlan 1, 10 and 11.

7. ports 1 and 2 of cisco 2950 has a native vlan of 10 and 11 respectively.

8. ip address of cisco switch is in vlan 1 where as 1st 3com is in vlan 10 and 2nd 3com switch is in vlan 11.

9. Spanning tree and 802.1q vlan tagging are enabled in 25,26 of 3 com switches.

After this configuration, i have found that port 25 of 2nd 3com 1100 is blocking as per STP. When i maually shutdown the port 1(ie port connecting 1st 3com1100), I have found that the blocking port of 2nd 3com1100 has changed it state to listening and then to forwarding. But the connectivity to the 1st 3com is not established.

The only which i have found is the difference in stp costs.

Stp cost for 3com 1100's 100mbps is 15 and the path cost for cisco 2950 is 19 for 100Mbps port. Kindly give me an idea to solve this issue ie to establish the redundant path between these switches.

Thanz in advance

Community Member

Re: Trunking between 3com 1100 and cisco 2950

I don't see anything wrong here, the 2nd 3com has all access ports in Vlan11, while the 1st 3com has all access ports in Vlan10, you must have a routing action between the Vlans to see each other.

Hope I helped



Re: Trunking between 3com 1100 and cisco 2950

STP port cost of 15 on the 3Com's 10/100TX port indicates that you have configured Port Trunking on that interface. This is not for 802.1Q VLAN trunking as in Cisco switches; rather, it is for 802.3ad link aggregation, a standard inspired by Cisco's proprietary EtherChannel technology. A port trunk composed of only one link is no better than a regular link, and is actually a little more work for the 3Com switch to do; I would suggest disabling this.

On the 3Com, default STP port costs are 19 if the port is 100BASE-TX half duplex; 18 if the port is 100BASE-TX full duplex; and 15 if there's a port trunk (2, 3, or 4 links bundled together to operate as one) regardless of duplex setting. But your network is small enough that the differences aren't a factor.

Spanning Tree is usually per-VLAN on the Cisco switch; on the 3Com 1100, there is one Spanning Tree for the entire switch. I am not sure, but I think the 3Com sends its BPDUs on VLAN 1. It sounds like STP reconverged properly when you manually shut down the Cisco 2950's FastEthernet 0/1 port; double-check that each of the ports on the link between the two 3Com switches is a member of each VLAN defined on the switch, and that they are tagging traffic for every VLAN. There should be no untagged traffic between the two 3Coms.

For that matter, there should be no untagged traffic between the 3Com switches and the Cisco switch. Create a dummy VLAN on the Cisco and let that be the native VLAN for each of the VLAN trunk connections to the 3Coms; this will force all the other Cisco-to-3Com traffic for your regular VLANs to be tagged.

On the 3Coms, you can configure the multi-VLAN switch-to-switch connections to forward tagged traffic if the tags are not associated with VLANs that exist on the switch. This could be useful to you later, if your network grows; but it won't really help you troubleshoot this now, so disable it.

On the 3Coms, there is a setting on ports for 802.1Q Learning; set this to disabled. (It's to allow computers with 802.1Q-enabled NICs to tell the switch what VLAN they want the port to be in; not helpful in this situation.)

Hope this helps.

Community Member

Re: Trunking between 3com 1100 and cisco 2950

Thanz for ur kind help.

The setup I have explained is a part of our network. We have a cisco 6006 layer 3 switch as vtp server and 2 cisco 4006 switches 15 others 3548 swithces with this cisco 2950 . We have nearly 40 vlans in our network.

All this switches are configured as vtp client. After connecting 3coms with the cisco 2950 I have tried to ping the gateways for all the vlans configured in the layer 3 switch and i got the successful replies.

can I remove the ports in 3com from trunk?

I have configured the uplink ports of 3com as trunk ports and I have added all those ports to all the three vlans ie default vlan1, 10 and 11. There is one more configuration ie 3com swithces are configured with address in vlan 10 and 11 respectively. The trunk port connecting 3coms in cisco 2950 are configured to be in native 10 and 11 accordingly.

Kindly tell me what i have to do with trunk ports in 3com 1100 switches?

802.1q vlan learning was enabled in 3com switches shall i proceed to disable it. when i check the health of the 3com switches it is found that both the trunk ports in two 3coms are handling traffic. But the thing is that the mesh is not established when any one link between cisco and any one 3com is failed.

Creating a dummy vlan in cisco 2950 is not possible since it is vtp client mode.

give me tips to overcome this.


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