Path Cost Values; Root Bridge; Port Roles - Avaya 8600 Engineering

1.4 Path Cost Values

RSTP and MSTP recommend new path cost values that support a wide range of link speeds. Table 2 lists
the recommended path cost values.
Link speed
Less than or equal 100Kb/s
1 Mb/s
10 Mb/s
100 Mb/s
1 Gb/s
10 Gb/s
100 Gb/s
1 Tb/s
10 Tb/s

1.5 Root Bridge

Just as in STP, with RSTP and MSTP instances, the root bridge is always the bridge with the lowest
Bridge ID within the Spanning Tree instance. The Bridge ID is made up by pre-pending the bridge priority
(2 bytes) with the Bridge MAC address (6 bytes) forming an 8 byte long Bridge ID. Hence if multiple
bridges are configured with the same bridge priority, the bridge MAC address will act as a tie breaker and
uniquely determine the Root bridge.
In a Spanning Tree design, always ensure that a Core node acts as the Root Bridge.

1.6 Port Roles

A port receiving the best BPDU on the switch is the root port and is referred to as a Root Forwarding (RF)
port. This is the port that is the closest to the root bridge in terms of path cost. The spanning tree
algorithm elects a single root bridge in a bridged network per spanning tree instance. The root bridge is
the only bridge in a network that does not have root ports; all ports on a root bridge are normally
Designated Forwarding (DF). There can only be one path towards a root bridge on a given segment
otherwise there will be loops.
July 2010
Table 2 Recommended Path Cost Values
Root Forwarding Port
RSTP/MSTP Technical Configuration Guide
Recommended value
200,000,000
20,000,000
2,000,000
200,000
20,000
2,000
200
20
2
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