7-2
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 7 Configuring STP and RSTP
STP Overview
STP Overview
STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the
network. For a Layer 2 Ethernet network to function properly, only one active path can exist between
any two stations. Spanning-tree operation is transparent to end stations, which cannot detect whe ther
they are connected to a single LAN segment or a switched LAN of multiple segments.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The spanning-tree algorithm calculates the best loop-free pa th throughout a switched Layer 2
network. 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 the frames to construct a loop-free
path.
Multiple active paths among end stations cause loops in the network. If a loop exists in the net work, end
stations might receive duplicate messages. Switches might also learn end-station MAC addresses on
multiple Layer 2 interfaces. These conditions result in an unstable network.
Spanning tree defines a tree with a root switch and a loop-free path from the root to all switches in the
Layer 2 network. 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.
When two interfaces on a switch are part of a loop, the spanning-tree port priority and path cost settings
determine which interface is put in the forwarding state and which is put in the blocking state. The port
priority value represents the location of an interface in the network topology and h ow well it is located
to pass traffic. The path cost value represents media speed.
Supported STP Instances
The ML-Series card supports the per-VLAN spanning tree ( PVST+) and a maximum of
255 spanning-tree instances.
Bridge Protocol Data Units
The stable, active, spanning-tree topology of a switched network is determined by these elements:
• Unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch
• Spanning-tree path cost to the root switch
• Port identifier (port priority and MAC address) associated with each Layer 2 interface
When the switches in a network are powered up, each functions as the root switch. Each switch sends a
configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree
topology. Each configuration BPDU contains this information:
• Unique bridge ID of the switch that the sending switch identifies as the root switch
• Spanning-tree path cost to the root
• Bridge ID of the sending switch
• Message age
• Identifier of the sending interface
• Values for the hello, forward delay, and max-age protocol timers