Spanning-Tree Operation

802.1D Spanning-Tree Protocol (STP)

Terminology

TermDefinition

downlink port (downstream port)

edge switch

A switch port that is linked to a port on another switch (or to an end node) that is sequentially further away from the STP root device. For example, port “C” in figure 5-12, above, is a downlink port.

For the purposes of fast-uplink STP, this is a switch that has no other switches connected to its downlink ports. An edge switch is sequentially further from the root device than other switches to which it is connected. Also termed wiring closet switch or leaf switch. For example, switch “4” in figure 5-13 (page 33) is an edge switch.

interior switch

In an STP environment, a switch that is sequentially closer to the STP root device than one

 

or more other switches to which it is connected. For example, switches “1”, “2”, and “3” in

 

figure 5-13 (page 33) are interior switches.

single-instance spanning

A single spanning-tree ensuring that there are no logical network loops associated with any

tree

of the connections to the switch, regardless of whether there are any VLANs configured on

 

the switch. For more information, see “Spanning Tree Protocol (STP)” in chapter 9,

 

“Configuring Advanced Features”, in the Management and Configuration Guide for your

 

switch.

uplink port (upstream port)

wiring closet switch

A switch port linked to a port on another switch that is sequentially closer to the STP root device. For example, ports “A” and “B” in figure 5-12 on page 32 are uplink ports.

Another term for an “edge” or “leaf” switch.

When single-instance spanning tree (STP) is running in a network and a forwarding port goes down, a blocked port typically requires a period of

(2 x (forward delay) + link down detection)

to transition to forwarding. In a normal spanning tree environment, this transition is usually 30 seconds (with the Forward Delay parameter set to its default of 15 seconds). However, by using the fast-uplink spanning tree feature, a port on a switch used as an edge switch can make this transition in as little as ten seconds. (In an STP environment, an edge switch is a switch that is connected only to switches that are closer to the STP root switch than the edge switch itself, as shown by switch “4” in figure 5-13, below.)

 

 

 

 

 

 

 

 

 

 

 

 

 

Port 3

 

 

Switch 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch 4

 

 

 

 

 

Switch 1

 

 

LAN

 

 

 

 

6

 

(Root)

 

 

 

(2610-Edge)

 

 

 

 

 

 

 

 

 

 

8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch 3

 

 

 

 

 

 

Port 5

 

 

 

 

 

 

 

 

 

 

 

 

1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Link blocked by STP:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 5-13. Example of an Edge Switch in a Topology Configured for STP Fast Uplink

5-33