ZyXEL Communications MSAP2000 manual Spanning Tree Protocol, STP Path Costs

MSAP2000 AAMS User’s Guide

CHAPTER 13

Spanning Tree

Protocol

This chapter introduces the Rapid Spanning Tree Protocol (RSTP).

13.1RSTP (Rapid Spanning Tree Protocol) and STP (Spanning Tree Protocol)

RSTP adds rapid reconfiguration capability to STP. The switch supports RSTP and the earlier STP. RSTP and STP detect and break network loops and provide backup links between switches, bridges or routers. They allow a device to interact with other RSTP or STP-aware devices in your network to ensure that only one path exists between any two stations on the network. The switch uses RSTP by default but can still interoperate with STP switches

(although without RSTP's benefits).

The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value

(MAC address). Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost - see the following table.

Table 33 STP Path Costs

	LINK SPEED	RECOMMENDED VALUE	RECOMMENDED RANGE	ALLOWED
	RANGE
Path Cost	4Mbps	250	100 to 1000	1 to 65535
Path Cost	10Mbps	100	50 to 600	1 to 65535
Path Cost	16Mbps	62	40 to 400	1 to 65535
Path Cost	100Mbps	19	10 to 60	1 to 65535
Path Cost	1Gbps	4	3 to 10	1 to 65535
Path Cost	10Gbps	2	1 to 5	1 to 65535

On each bridge, the root port is the port through which this bridge communicates with the root. It is the port on this switch with the lowest path cost to the root (the root path cost). If there is no root port, then this switch has been accepted as the root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root among the bridges connected to the LAN.

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