Medianet Reference Guide - Cisco

Chapter 3
Medianet Availability Design Considerations
Network Availability Technologies
Figure 3-10
STP Port States
Power-on
initialization
Blocking
state
Listening
state
Disabled
state
Learning
state
Forwarding
state
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You can enable STP globally on a per-VLAN basis, using Per-VLAN Spanning-Tree (PVST), by
entering the following command:
Switch(config)# spanning-tree vlan 100
The two main availability limitations for STP are as follows:
• To prevent loops, redundant ports are placed in a blocking state and as such are not used to forward
frames/packets. This significantly reduces the advantages of redundant network design, especially
with respect to network capacity and load sharing.
• Adding up all the times required for STP port-state transitions shows that STP can take up to
50 seconds to converge on a loop-free topology. Although this may have been acceptable when the
protocol was first designed, it is certainly unacceptable today.
Both limitations are addressable using additional technologies. The first limitation can be addressed by
using the Cisco Virtual Switching System (VSS), discussed later in this section; and the second
limitation can be addressed by various enhancements that Cisco developed for STP, as is discussed next.
Cisco Spanning Tree Enhancements
To improve STP convergence times, Cisco has made a number of enhancements to 802.1D STP,
including the following:
• PortFast (with BPDU Guard)
• UplinkFast
• BackboneFast
STP PortFast causes a Layer 2 LAN port configured as an access port to enter the forwarding state
immediately, bypassing the listening and learning states. PortFast can be used on Layer 2 access ports
connected to a single workstation or server to allow those devices to connect to the network immediately,
instead of waiting for STP to converge, because interfaces connected to a single workstation or server
should not receive BPDUs. Because the purpose of PortFast is to minimize the time that access ports
must wait for STP to converge, it should only be used on access ports. Optionally, for an additional level
of security, PortFast may be enabled with BPDU Guard, which immediately shuts down a port that has
received a BPDU.
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Medianet Reference Guide
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