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New Member

question on TRUNKS and DOT1Q ????

dot1q is a mono spanningtree...what does that mean ?

Does it mean one instance of spanningtree per VLAN, per TRUNK, per switch ?????

So if you had a switch with 10 trunks, trunking 10 VLANS, you would have 100 instances of STP ?

Or is it one per .....

Help would be appreciated.

New Member

Re: question on TRUNKS and DOT1Q ????

It means one instance of spanning tree per VLAN.

So 10 VLAN's equals 10 spanning tree's.

Does not matter how many trunks you have.

New Member

Re: question on TRUNKS and DOT1Q ????

Thanks, bit more though.

On a network with 80 odd VLANs, so 80 STP per switch. Trunking goes everywhere onto upwards of 60 switches. Assuming all VLANS go everywhere, would we be looking at 80 odd stp on each switch ???



New Member

Re: question on TRUNKS and DOT1Q ????

we do support 802.1s/1w which is Multiple Spannign tree which reduces the #s of SPT but at the same time ability to have more than on SPT topology (for differet sets of vlans)

check out this URL

Re: question on TRUNKS and DOT1Q ????

The official standard for 802.1q specified a 'mono-spanning-tree' - a single instance of spanning tree for the entire 802.1q domain, (Common Spanning Tree - CST). Cisco Catalysts (by default) run PVST+ - Per VLAN Spanning-Tree+, this is an instance of spanning tree per VLAN. When connected to third party switches on dot1q trunks that support the mono-STP, the Native VLAN will run CST and Cisco will interoperate with it. Cisco uses a different BPDU Multicast MAC address for its PVST+ so it doesn't interfere with the dot1q CST.

New Member

Re: question on TRUNKS and DOT1Q ????

I've concrete case where I will have 2 Catalyst 6500 as backbone switches and standard switches (HP Procurve) as access switches and they're connected with 801.q tagged ports/link.

What I understand: the PVST+ BPDUs of the 6500 will be switched by the access sw because destination Mac @ is not one of the access sw. Then the 2 6500 will do their STP/VLAN as if they were directly connected through a cable. Right? If STP is not activated on Access sw not a big deal because the 2 6500 will manage loop plus I get the stp per Vlan feature. Except that my access switch will not resolve local loop.

If I activate STP on the access switches then they receive the untagged BPDU of the Native VLAN of the 6500 and then they will block one the uplink (if 6500 configured as root and secondary root). Right? Then the PVST+BPDU will be no more switched. So in that case CST will interfere with PVST. No?

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