Cisco Systems 15454M6DC 9.5 Spanning Tree (IEEE 802.1D)

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Cisco ONS 15454 Installation and Operations Guide

November 2001

Chapter9 Ethernet Operation

Spanning Tree (IEEE 802.1D)

Note If Tagged is chosen, the attached external devices must recognize IEEE 802.1Q VLANs.

Note Both ports on individual E1000-2/E1000-2-G cards cannot be me mbers of the s ame VLA N.

9.5 Spanning Tree (IEEE 802.1D)

The Cisco ONS 15454 operates spanning tree protocol (STP) according to IEEE 802.1D when an

Ethernet card is installed. STP operates over all packe t-sw itche d port s inc lud ing E t herne t a nd SON ET

ports. On Ethernet ports, STP is disabled by default but m ay b e enab l ed w ith a c hec k b ox und er t he

Provisioning > Port tabs at the card-level view. On SONET interface ports, STP activ ates by default and

cannot be disabled.

The Ethernet card can enable STP on the Ethernet ports to allow redu ndant paths to the attached Ethernet

equipment. STP spans cards so that both equipment and facilities are protected against failure.

STP detects and eliminates network loops. When STP detects multiple paths between any two networ k

hosts, STP blocks ports until only one path exists between any two network hosts (Figure9-23). The

single path eliminates possible bridge loops. This is crucial for shared packet rings, which naturally

include a loop.

Figure9-23 An STP blocked path

To remove loops, STP defines a tree that spans all the switches in an extended network. STP forces

certain redundant data paths into a standby (blocked) state. If one network segment in t he STP becomes

unreachable, the spanning-tree algorithm reconfigures the spanning-tree topology and reactivates the

blocked path to reestablish the link. STP operation is transparent to end stations, which do not

discriminate between connections to a single LAN segment or to a switched LAN with multiple

segments. The ONS 15454 supports one STP instance per circuit and a ma ximum of eight STP instances

per ONS 15454.

The ONS 15454 can operate multiple instances of STP to support VLANs in a looped topology. You can

dedicate separate circuits across the SONET ring for different VLAN groups (i.e., one for private TL S

services and one for Internet access). Each circuit runs its own STP to maintain VLAN connectivity in

a multi-ring environment.

Step 1 Display the CTC card view.

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Redundant path (blocked)

Primary path (forwarding)