user manual

REVIEW DRAFT—CISCO CONFIDENTIAL
14-4
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter 14 Ethernet Operation
14.1.4 Ethernet Link Integrity Support
14.1.4 Ethernet Link Integrity Support
The G-Series supports end-to-end Ethernet link integrity (Figure 14-2). This capability is integral to
providing an Ethernet private line service and correct operation of Layer 2 and Layer 3 protocols on the
attached Ethernet devices. End-to-end Ethernet link integrity essentially means that if any part of the
end-to-end path fails, the entire path fails. Failure of the entire path is ensured by turning off the transmit
lasers at each end of the path. The attached Ethernet devices recognize the disabled transmit laser as a
loss of carrier and consequently as an inactive link.
Figure 14-2 End-to-End Ethernet Link Integrity Support
Note Some network devices can be configured to ignore a loss- of-carrier condition. If a device configured to
ignore a loss-of-carrier condition attaches to a G-Series card at one end, alternative techniques (such as
use of Layer 2 or Layer 3 keep-alive messages) are required to route traffic around failures. The response
time of such alternate techniques is typically much longer than techniques that use link state as
indications of an error condition.
As shown in Figure 14-2, a failure at any point of the path causes the G-Series card at each end to disable
its Tx transmit laser, which causes the devices at both ends to detect a link down. If one of the Ethernet
ports is administratively disabled or set in loopback mode, the port is considered a “failure” for the
purposes of end-to-end link integrity because the end-to-end Ethernet path is unavailable. The port
“failure” also disables both ends of the path.
14.1.5 Gigabit EtherChannel/IEEE 802.3ad Link Aggregation
The end-to-end Ethernet link integrity feature can be used in combination with Gigabit EtherChannel
capability on attached devices. The combination provides an Ethernet traffic restoration scheme that has
a faster response time than alternate techniques such as spanning tree rerouting, yet is more bandwidth
efficient because spare bandwidth does not need to be reserved.
The G-Series supports all forms of link aggregation technologies including Gigabit EtherChannel
(GEC), which is a Cisco proprietary standard, and the IEEE 802.3ad standard. The end-to-end link
integrity feature of the G-Series allows a circuit to emulate an Ethernet link. This allows all types of
Layer 2 and Layer 3 rerouting to work correctly with the G-Series. Figure 14-3 illustrates G-Series GEC
support.
ONS 15454
SDH
G1000-4 port
AB C D E
71324
Tx
Rx
ONS 15454
SDH
G1000-4 port
Tx
Rx
VC4-N
SDH