Per−Vlan Spanning Tree; Etherchannel - Cisco RJ-45-to-AUX Brochure

Per−VLAN Spanning Tree
You can have many instances of STP running in your network. By running a different instance of STP on a
per−VLAN basis, you can run some VLANs on ports that are blocked by another instance of STP running on
another VLAN. In this way, you can set the priority of each port on a per−VLAN basis, allowing you to use
the redundant links in your network to run an equal amount of traffic on each link. The VLANs individually
determine which links to forward and which links to block.
Just as with the port priority setting, the port with the lowest priority value for each VLAN gets to forward the
frames. If more than one or all the ports have the same priority value for a particular VLAN, the port with the
lowest port number gets to forward the frames for that VLAN.
PVST and PVST+
Per−VLAN Spanning Tree (PVST) is a Cisco proprietary solution to the scaling and stability problems
associated with Common Spanning Tree (CST) in large−scale spanning tree networks. PVST creates a
separate instance of STP on each VLAN in the switch block. This setup gives each VLAN a unique STP
topology containing its own port cost, path cost, priority, and root switch.
By using separate instances of PVST on each VLAN, you reduce the convergence time for STP recalculation
and increase reliability of the network. By implementing PVST, the overall size of the spanning tree topology
is reduced significantly. PVST improves scalability and decreases convergence time, providing faster
recovery in the event of network faults. It also allows control of forwarding paths on a per−subnet basis while
providing a simple technique for Layer 2 redundancy.
PVST does have some disadvantages in the spanning tree. PVST uses more processing power and consumes
more bandwidth to support spanning tree maintenance and BPDUs for each VLAN. Inter−Switch Link (ISL)
uses one spanning tree per VLAN, using PVST over ISL trunks. PVST implementation requires the use of
Cisco ISL encapsulation in order to function.
Per−VLAN Spanning Tree Plus (PVST+) is not well documented by Cisco. IEEE 802.1Q can use PVST+ to
map multiple spanning trees to the spanning tree of authentic IEEE 802.1Q switches.
PVST+ is available in Catalyst software versions 4.1 or newer. Cisco Catalyst switches configured with
version 4.1 or later are considered Cisco PVST+ switches. PVST+ is compatible and interoperable with
legacy type Mono Spanning Tree (MST) and PVST switches without any user intervention—thus PVST+ has
a type of plug−and−play functionality.
Configuring the port priority by VLAN is useful for distributing data across parallel paths. If a parallel
connection exists between two devices, STP will block one of the links. If the port priority is not changed on
one of the ports for each VLAN, the traffic from all VLANs will travel on one link, and one link will be used
only as a backup.
When you need to have multiple links to a destination be able to transmit data as though they were one link,
you can use EtherChannel. Let's take a look at EtherChannel in the next section.

EtherChannel

Fast EtherChannel can provide scaled bandwidth within the campus using full−duplex bandwidth at wire
speed for up to eight bundled links. A bundle is a series of links acting like a single link between two points in
209