In the 1+1 architecture, there is one working interface (circuit) and one protection interface, and the same payload from the transmitting end is sent to both the receiving ends. The receiving end decides which interface to use. The line overhead (LOH) bytes (K1 and K2) in the SDH frame indicate both status and action.

The protection interfaces need to be configured with an IP address of the chassis that has the working interface, using MSP commands. The MSP Protect Group Protocol, which runs on top of UDP, provides communication between the process controlling the working interface and the process controlling the protection interface. Using this protocol, interfaces can be switched because of a chassis failure, degradation or loss of channel signal, or manual intervention. In bidirectional mode, the receive and transmit channels are switched as a pair.

Two SDH connections are required to support MSP.

The following lists the benefits of MSP:

• MSP is supported on all the ports except on the 8th port with STM-64.

• Loopback, BERT, ACR and DCR, and Clocking should be configured on physical member controllers.

SDH linear MSP 1+1 provides protection against both fiber cuts and front card or back card failures. MSP 1+1 requires a redundant protection line for every working line. The traffic is simultaneously carried by the working and the protection lines. Hence, the receiver that terminates the MSP 1+1 should select the traffic from one of the line and continue to forward the traffic. MSP 1+1 provides protection in unidirectional and bi-directional modes:

In the above figure, two devices are connected to provide MSP 1+1 bi-directional protection. The highlighted one is the working line and the other is the protection line. The traffic is transmitted on both working and protection lines and received only on one line.

In a scenario where you encounter a fiber cut,

1. There is a cable cut in the working line. So, the device 2 receives a Loss of Signal (LOS) on working line.

2. Device 2 starts generating K2 byte and sends it to the Device 1 over the protection line.

3. Device 1 receives the K2 byte and reacts on the receiving K2 byte.

4. Device 1 starts sending K1 byte to the Device 2 on the protection line.

5. Device 1 starts sending K2 byte to Device 2 on the protection line.

6. Device 2 receives the K1/K2 byte and starts receiving the data from protection line. The protection line now acts as the active line.

7. Device 2 sends K2 byte over the new active line to Device 1 . Device 1 receives this signal and starts accepting the data from this new active line.

In the above figure, two devices are connected to provide MSP 1+1 unidirectional protection. The figure shows a working line and a protection line. The traffic is transmitted on both working and protection line and received only on one line.

In a scenario where you encounter a fiber cut,

1. Device 1 receives a LOS on RX working line.

2. Device 2 detects LOS and starts receiving the data from the protection line. The protection line now becomes the active line.

3. Device 1 receives the K2 byte and knows about switching event on device 2.

This section describes the configuration of MSP.

Configuring Bi-directional MSP (SDH Framing)

To configure bi-directional MSP (SDH Framing):

enable
configure terminal
controller MediaType 0/5/0
mode sdh
controller sdh 0/4/0
clock source internal
aps group acr 1
aps working 1
exit
controller sdh 0/5/0
aps group acr 1
aps protect 1 10.7.7.7
end

Note	To restore the system to its default condition, use the no form of the command. Note '10.7.7.7' is the loopback interface IP address on the same router.

Configuring Revertive Mode

To configure revertive mode:

enable
configure terminal
controller MediaType 0/5/0
mode sdh
controller sdh 0/5/0
clock source internal
aps group acr 1
aps working 1
exit
controller sdh 0/5/0
aps group acr 1
aps protect 1 10.7.7.7
aps revert 3
aps adm
end

Note	3 is the time in minutes for reverting back to the working port after it becomes available.

Configuring Unidirectional MSP (SDH Framing)

To configure unidirectional MSP (SDH Framing):

enable
configure terminal
controller MediaType 0/5/0
mode sdh
controller sdh 0/5/0
clock source internal
aps group acr 1
aps working 1
exit
controller sdh 0/5/0
aps group acr 1
aps protect 1 10.7.7.7
aps unidirectional 
aps adm
end

Note	To restore the system to its default condition, use the no form of the command.

This section describes the configuration of MSP for CESoPSN.