User manual

36 Publication 1783-UM003D-EN-E - December 2009
Chapter 2
The STP uses a spanning-tree algorithm to select one switch of a redundantly
connected network as the root of the spanning tree. The algorithm calculates
the best loop-free path through a switched Layer 2 network by assigning a role
to each port based on the role of the port in the active topology:
• Root—A forwarding port elected for the spanning-tree topology
• Designated—A forwarding port elected for every switched LAN
segment
• Alternate—A blocked port providing an alternate path to the root
bridge in the spanning tree
• Backup—A blocked port in a loopback configuration
The switch that has all of its ports as the designated role or as the backup role
is the root switch. The switch that has at least one of its ports in the designated
role is called the designated switch.
Spanning tree forces redundant data paths into a standby (blocked) state. If a
network segment in the spanning tree fails and a redundant path exists, the
spanning-tree algorithm recalculates the spanning-tree topology and activates
the standby path. Switches send and receive spanning-tree frames, called
bridge protocol data units (BPDUs), at regular intervals. The switches do not
forward these frames but use them to construct a loop-free path. BPDUs
contain information about the sending switch and its ports, including switch
and MAC addresses, switch priority, port priority, and path cost. Spanning tree
uses this information to elect the root switch and root port for the switched
network and the root port and designated port for each switched segment.
Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) (IEEE 802.1D-2004) uses
point-to-point wiring and provides rapid convergence of the spanning tree.
RSTP is enabled by default.
Storm Control
Storm control prevents traffic on a LAN from being disrupted by a broadcast,
multicast, or unicast storm on one of the physical interfaces. A LAN storm
occurs when packets flood the LAN, creating excessive traffic and degrading
network performance. Errors in the protocol-stack implementation, mistakes
in network configurations, or users issuing a denial-of-service attack can cause
a storm.
TIP
If you are connecting the switch to a Cisco network switch, the
typical default is PVST+, not RSTP. To provide compatibility, one
or the other switch will need to be modified.