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Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 23 E-Series and G-Series Ethernet Operation
E-Series Spanning Tree (IEEE 802.1D)
Note IEEE 802.1Q was formerly known as IEEE 802.1P.
Note E-Series cards in port-mapped mode and G-Series ca rds do not support priority queing (IEEE 802.1Q).
E-Series Spanning Tree (IEEE 802.1D)
The E-Series operates IEEE 802.1D Spanning Tree Protocol (STP). The E-Series card supports co mmon
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 OC-N/STM-N ports. On Ethernet
ports, STP is enabled by default but can be disabled. A user can al so disable or enable STP on a
circuit-by-circuit basis on Ethernet cards configured as single-card EtherSwitch (unstitched) in a
point-to-point configuration. However, turning off STP protection on a circuit-by-circuit basis means
that the SONET/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. O n OC-N/STM-N interface ports, the
ONS node activates STP by default, and STP cannot be disabled.
The Ethernet card can enable STP on the Ethernet ports to create redu ndant 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 23-17). The
single path eliminates possible bridge loops. This is crucial for shared packet rings, which naturally
include a loop.
Figure 23-17 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 th e 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 node
supports one STP instance per circuit and a maximum of eight STP instances per ONS node.
The Circuit window shows forwarding spans and blocked spans on the spanning tree map (Figure 23-18).
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Redundant path (blocked)
Primary path (forwarding)