- Cisco ONS 15454 Manual

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Cisco ONS 15454 Installation and Operations Guide
November 2001
Chapter 9 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 members of the same VLAN.
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 packet-switched ports including Ethernet and SONET
ports. On Ethernet ports, STP is disabled by default but may be enabled with a check box under the
Provisioning > Port tabs at the card-level view. On SONET interface ports, STP activates by default and
cannot be disabled.
The Ethernet card can enable STP on the Ethernet ports to allow redundant 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 network
hosts, STP blocks ports until only one path exists between any two network hosts (Figure 9-23). The
single path eliminates possible bridge loops. This is crucial for shared packet rings, which naturally
include a loop.
Figure 9-23 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 the 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 maximum of eight STP instances
per ONS 15454.
9.5.1 Multi-Instance Spanning Tree and VLANs
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 TLS
services and one for Internet access). Each circuit runs its own STP to maintain VLAN connectivity in
a multi-ring environment.
Procedure: Enable Spanning Tree on Ethernet Ports
Step 1 Display the CTC card view.
43388
Redundant path (blocked)
Primary path (forwarding)