Connecting Cisco & Non-Cisco Switches

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Feb 12th, 2008
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I have a setup like: - Dual Cisco distribution switches working on PVST+ and connected on dot1q trunk.

A non Cisco access switch working on CST and connected to dual Cisco switches on access port making a triangle topology for redundancy.

Since I don't have access to non cisco switches for making trunk and VLAN's, so connectivity is like: port at the Cisco end switches are in VLAN 100 and ports at access switches are in default VLAN 1.

Do my Cisco PVST+ will operate with non cisco CST without creating loop. (Since I have a triangle topology)

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Hi Rohit,

There is no logical loop in your topology as non-cisco switch is having only VLAN-1 and cisco switches are having vlan-100 at the access (non-trunk) port at which non-cisco switch is connected. So they may appear to be in loop but in logical sense there is no loop for both vlan1 as well as for vlan 100. I believe non-cisco switch won't be able to comunicate with the gateway i.e. hosts connected to non-cisco switch (under vlan 1) won't be able to ping the default gateway until you configure your cisco switches for vlan1 at switchports which connect to non-cisco switch. Now you will see the STP loop in your setup.


rohit_s Tue, 02/12/2008 - 02:49
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Hi Gaurav,

Thanks for the update...

In my case, though all the ports of non-cisco switch will be in VLAN 1 but they will have the IP Subnet of VLAN 100 range. The moment they hit the cisco switch they will be tagged VLAN 100 and will work normally. This scenario is good for situation wherein you don't touch non-cisco switches but still put them in specific VLAN.

But for the looping scenario, I am still not sure as there might be situation of "port inconsistent" at Cisco switches as per Cisco documents.


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