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REVIEW DRAFT—CISCO CONFIDENTIAL
14-16
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter 14 Ethernet Operation
14.3.6 E-Series Spanning Tree (IEEE 802.1D)
Note IEEE 802.1Q was formerly IEEE 802.1P.
Note E-Series cards in port-mapped mode and G-Series cards do not support priority queing (IEEE 8021.Q).
14.3.6 E-Series Spanning Tree (IEEE 802.1D)
The Cisco ONS 15454 SDH operates Spanning Tree Protocol (STP) according to IEEE 802.1D when an
Ethernet card is installed. The E-Series card supports common STPs on a per circuit basis up to a total
of eight STP instances. It does not support per-VLAN STP. In single-card mode, STP can be disabled or
enabled on a per circuit basis during circuit creation. Disabling STP will preserve the number of
available STP instances.
STP operates over all packet-switched ports including Ethernet and STM-N ports. On Ethernet ports,
STP is enabled by default but can be disabled. A user can also disable or enable STP on a
circuit-by-circuit basis on unstitched Ethernet cards in a point-to-point configuration. However, turning
off STP protection on a circuit-by-circuit basis means that the ONS 15454 SDH system is not protecting
the Ethernet traffic on this circuit, and the Ethernet traffic must be protected by another mechanism in
the Ethernet network. On STM-N interface ports, the ONS 15454 SDH activates STP by default, and STP
cannot be disabled.
The Ethernet card can enable STP on the Ethernet ports to create redundant paths to the attached Ethernet
equipment. STP connects 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 14-14). The
single path eliminates possible bridge loops. This is crucial for shared packet rings, which naturally
include a loop.
Figure 14-14 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 STP algorithm reconfigures the STP 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 SDH supports one STP instance per circuit and a maximum of eight STP instances per
ONS 15454 SDH.
The Circuit window shows forwarding spans and blocked spans on the spanning tree map (Figure 14-15).
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Redundant path (blocked)
Primary path (forwarding)