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As discussed earlier, 802.1Q has defined
standards-based technologies for handling VLANs. To reduce the
complexity of this standard, the 802.1 committee specified only a
single instance of Spanning Tree for all VLANs. Not only does this
provide a considerably less flexible approach than the PVST adopted
by Cisco, it also creates an interoperability problem. To address
both of these issues, Cisco introduced the Per-VLAN Spanning-Tree
Plus Protocol (PVST+) in 4.1 code on the Catalyst 5000s (all
Catalyst 4000s and 6000s support PVST+). This feature allows the two
schemes to interoperate in a seamless and transparent manner in
almost all topologies and configurations.
There are both advantages and
disadvantages to using a single Spanning Tree. On the upside, it
allows switches to be simpler in design and place a lighter load on
the CPU. On the downside, a single Spanning Tree precludes load
balancing and can lead to incomplete connectivity in certain VLANs
(the single STP VLAN might select a link that is not included in
other VLANs). Given these trade-offs, most network designers have
concluded that the downside of having one Spanning Tree outweigh
the benefits. Although the initial release of 802.1Q specified only
a single instance of STP, the IEEE is currently working on multiple
instances of STP in the 802.1S group.
PVST+ allows for three types of regions in the network:
- A group of traditional (pre-4.1)
Catalyst switches form a PVST region with each VLAN using a
separate instance of STP.
- Pure 802.1Q switches use a single
instance of STP, the Mono Spanning Tree (MST). A group of these
switches forms an MST region.
- Catalyst Switches running 4.1 and
later code form a PVST+ region.
Given that pure 802.1Q switches
support 802.1Q-style trunks and PVST switches support only ISL
trunks, these regions can be connected in only a limited set of
combinations:
- PVST and PVST+ regions can connect
over ISL trunk links.
- MST and PVST+ regions can connect
over an 802.1Q trunk.
Notice that an MST and PVST region
cannot be connected via a trunk link. Although it is possible to
provide a nontrunk connection between the two regions by using an
access (nontrunk) link, this is of limited use in real-world
networks. The Figure illustrates the three types of STP regions and
potential linkages.
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