Cisco and IEEE 802.1q Committee
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The following list includes a brief explanation of each STP implementation:
Per-VLAN Spanning Tree (PVST)
Default for Cisco switches, it runs a
separate instance of spanning tree for each VLAN. Makes smaller STP for
easier convergence.
Common Spanning Tree (CST)
The 802.1q standard, it runs one large
STP on the entire network regardless of the amount of VLANs. Problems
with convergence can occur in large networks.
Per-VLAN Spanning Tree+ (PVST+)
Allows Cisco switches to commu-
nicate with CST switches.
In the following sections, we'll go into more detail about each type of STP
implementation and its use with VLANs.
Per-VLAN Spanning Tree (PVST)
The STP protocol does not scale well with large switched networks. In large
switched networks, there can be delays in receiving Bridge Protocol Data
Units (BPDUs). These delays can cause instability in the STP database.
Delays in larger switched networks can also cause convergence time prob-
lems, which means that the network will not be forwarding frames.
To solve late BPDU and convergence issues, Cisco created a separate
instance of
Per-VLAN Spanning Tree (PVST)
. It basically makes smaller STP
implementations, which is easier for the switches to manage. Also, with
PVST, each VLAN has a unique Spanning Tree Protocol topology for its
root, port cost, path cost, and priority.
By running PVST, you still provide a loop-free network, but it is based
within each VLAN. The benefits of having a PVST are listed here:
It reduces the STP recalculation time when the switched network is
converging.
The spanning tree topology is smaller.
It makes the switched network easier to scale.
Recovery is faster than with a large network with one STP instance.
It allows administrative control of forwarding paths on a subnet basis.
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