Creating VLAN Standards

Creating VLAN Standards
The history of using STP with VLANs is interesting, because it acts as a macro for how many standards
have been developed. In the past, Cisco and the IEEE have differed in their approaches to
the use of these two protocols together.
As you discovered in Chapter 15, “Layer 2 Switching and the Spanning Tree Protocol
(STP),” STP has some well-understood problems. First, convergence will be relatively slow
because of the forwarding delays. This is unacceptable in modern networks where users and
applications expect immediate recovery from equipment failures. Additionally, it is likely that
a general spanning tree topology applied to all VLANs will result in suboptimal paths for some
users. The result has been a spate of developments, some proprietary and some standards-based,
to overcome these problems.
Per-VLAN Spanning Tree (PVST) is a Cisco proprietary implementation of STP. PVST uses
Inter-Switch Link (ISL) routing and runs a separate instance of STP for each and every VLAN.
The IEEE uses Common Spanning Tree (CST), which is defined with IEEE 802.1Q. The
IEEE 802.1Q defines one spanning tree instance for all VLANs. A new mechanism, recently
standardized as 802.1s, allows multiple spanning tree instances but in a more complex fashion;
Creating VLAN Standards
503
it runs multiple instances of STP on a one-to-one basis with VLANs. There is one more implementation
of STP, and that is called PVST
+
. Because it ends with a plus sign, it must be better,
right? Well, maybe. What it does is allow CST information to be passed into PVST. Cisco thinks
it would be easier if you simply had all Cisco switches; then you wouldn’t even have to think
about this issue.
This chapter covers the current protocols supported by Cisco and compares the options. The
following list includes a brief explanation of each STP implementation:
Per-VLAN Spanning Tree (PVST)
Default for Cisco switches; runs a separate instance of
spanning tree for each VLAN. Makes smaller STP implementations for easier convergence.
Common Spanning Tree (CST)
The 802.1Q standard; runs one large STP on the entire network
regardless of the number of VLANs. Problems with convergence can occur in large networks.
Per-VLAN Spanning Tree
+
(PVST
+
)
Allows Cisco switches to communicate with CST switches.
Multiple Spanning Tree (MST)
The 802.1s standard, supported by Cisco on IOS-based
switches since versions of 12.1. Allows multiple instances of STP and group VLAN mapping.
In the rest of this section, we’ll go into more detail about each type of STP implementation
and its use with VLANs