At 02:13 PM 8/5/01, Cisco Troubleshooter wrote:
>can any body tell,
>
>why we need spanning tree protocol per vlan
If you have a large, switched network, all the switches are in the same
spanning tree. Converging the spanning tree can take a long time. In
addition, traffic flow may not be optimized. The selection of the root
bridge and which interfaces are blocking might not be optimized for all the
applications and devices in the large, switched network.
With per-VLAN spanning tree, each VLAN becomes a single spanning tree with
its own root bridge and own set of blocked ports. This way you can optimize
traffic flow and reduce the amount of work to converge to a spanning tree.
It's somewhat analogous to dividing a routed network into areas or
autonomous systems.
Also, at least with Catalyst 1900 switches, if you allow all VLANs to
travel across both trunks, you will have a loop. If you don't configure
per-VLAN spanning tree, you will have a broken network. You would think
spanning tree would just work around this problem, but it doesn't seem to
when VLANs are configured.
>and vtp why it is needed what purpose it serves
VTP is a management protocol that allows switches to share information
about VLAN names and IDs. It reduces configuration because you can
configure VLAN names and IDs on just one or two server switches. The rest
of the switches act as clients and pick up the info when they boot.
By default, the switches do not keep track of which switches have which
VLANs configured, however. I disagree with the other responder who said VTP
reduces bandwidth usage on links and switches. It's VTP pruning that does
that.
If you configure VTP pruning, then an added VTP message gets sent. The
added message includes VLAN membership information. With VTP pruning, the
switches become a bit smarter and do not forward traffic for a VLAN across
a link or to a switch that has no ports in that VLAN. This must be
configured. Without pruning, VTP just shares info about VLAN names and IDs.
Priscilla
>thnx in advance
>
>jd
>
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Priscilla Oppenheimer
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