Spanning Tree + VLANS [7:10724]

[Ash Aslam]

How many STP are possible to have on one Catalyst Switch?

Can anyone confirm if each VLan has it's own Spanning Tree priority number?

The above (second) question is in reference to Bruce Caslow's explanation in
BRS Vol. 1, Page 821, last sentence in Paragraph 2.

Any thoughts!

Thanks & Regards,
Ash




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Re: Spanning Tree + VLANS [7:10724]

[Michael L. Williams]

You can choose to either run common (or mono) spanning tree where there is
only 1 spanning tree for all VLANs.  Advantage:  Less overhead.
Disadvantage:  Suboptimal trees for some VLANs and a change causes one big
STP recalc.  You could also choose to run PVST (Per VLAN Spanning Tree) in
which case each VLAN gets it's own spanning tree.  Advantage:  Optimal trees
for each VLAN and changes in one VLAN won't always cause STP recalcs for all
other VLANs.  Disadvantage:  More overhead.

This may help some.. it's about VTP and pruning, but it gives limits to
VLANS, etc



Platform: Core switches are Catalyst 6500s with Sup1A and PFC1 running
software 5.5(7). According to the release notes, this platform cannot have
more than 4000 STP logical ports.

Access switches are either: Catalyst 5000s with Sup2 who do not support more
than 1500 STP logical ports, or Catalyst 5000s with Sup1 and 20 M of DRAM,
which do not support more than 400 STP logical ports.

Number of VLANs: Remember that as we use VTP, a VLAN on the VTP server will
be created on all switches in the network. If we have 100 VLANs, the core
will handle roughly (100 VLANs x 81 trunks )= 8100 logical ports (above the
limit), and the access switch will handle (100 VLANs x 5 trunks)= 500
logical ports. Catalysts in the core will be above their supported number of
logical ports, and access switches with Sup1 will also be above the limit.



Here's a link to the whole page.. (careful of wrap)

http://www.cisco.com/warp/public/473/21.html

Hope this helps!
Mike W.

"Ash Aslam"  wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> How many STP are possible to have on one Catalyst Switch?
>
> Can anyone confirm if each VLan has it's own Spanning Tree priority
number?
>
> The above (second) question is in reference to Bruce Caslow's explanation
in
> BRS Vol. 1, Page 821, last sentence in Paragraph 2.
>
> Any thoughts!
>
> Thanks & Regards,
> Ash




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Re: Spanning Tree + VLANS [7:10724]

[MikeN]

The answer to this question depends on which Catalyst switch you are talking
about. The 2900XL's can have up to 68 VLANS and therefore you would be able
to have 64 instances of spanning tree running. Show vtp stat will show you
the possible number of vlans on your hardware.

You your second question regarding how many priority numbers: each instance
of spanning tree will have a root bridge chosen by the priority and mac
address. Therefor, whichever switch wins the root war, will be root for all
instances of spanning tree. You can force this per instance with a
spanning-tree priority command and set the cost and the assigned vlan #.
Clark & Hamilton in the Cisco Lan Switching book recommend 100 for your root
bridge and 200 for your backup root bridge.

HTH
MikeN
Network Engineer



""Ash Aslam""  wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> How many STP are possible to have on one Catalyst Switch?
>
> Can anyone confirm if each VLan has it's own Spanning Tree priority
number?
>
> The above (second) question is in reference to Bruce Caslow's explanation
in
> BRS Vol. 1, Page 821, last sentence in Paragraph 2.
>
> Any thoughts!
>
> Thanks & Regards,
> Ash




Message Posted at:
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