Problem Description

When the inspection tool conducts inspection on NG SDH equipment, the inspection result shows " the type of SSN1 line board in cycle will lead to ECC cycle ", the first-line engineers are not clear about the meaning expressed by the inspection result

Warning message

None

Processing

It is enough to understand that the first-line engineers need to pay attention to this specification restriction when using the inner loopback function of N1 series line boards.

Root cause

SSN1 series line veneer (veneer total capacity of STM-16 and below) inner loopback can cause ECC loopback. In particular, a large number of inner loopback alarm messages appear after an inner loopback is issued for this type of veneer in the following configuration: the ECC communication uses the IP protocol stack, and the network element on which the veneer is located is a non-gateway network element, and its communication with the gateway network element is carried out through the optical port on which the inner loopback is issued.

When the SSN1 series line veneer is set up with an inner loopback for the optical port or an inner loopback for the first VC4 channel, it will cause the ECC to loop back, looping back all the D1-D12 overhead bytes.

In the following application: the ECC communication uses the IP protocol stack and the network element on which the single board is located is a non-gateway network element, and its communication with the gateway network element is through the optical port that sends down the inner loopback.

Then, after the downgoing inner loopback, the packets sent out from this optical port will continue to be sent out from this optical port, and also loop back to itself to receive. An alarm to be reported, the first need to send to the gateway network element, according to the route to go out from the optical port, the result is that a copy of the packet from the optical port sent to the gateway network element, they received a copy of their own, and they received a copy of this will be sent from the optical port (because the destination address of the packet remains unchanged, in the IP layer forwarding is still going out from the optical port); and so on and so forth, resulting in a lot of packets sent to the other end. This results in a lot of packets being sent to the other end. The IP packet has a TTL item (survival period), initially set to 255 when the data is sent, every time after a route forwarding TTL value will be reduced by 1 (to prevent the packet from being forwarded endlessly), when the TTL value is 0, the packet will be discarded. So eventually the packet will be sent 255 times, thus you will see that LOOP_ALM will be transiently and frequently reported 255 times to start with.

The N1 series 10G rate boards and the N2 series boards, due to the use of different chips, will not loop back the D1~D12 inside the first VC4 when performing the optical port loopback and the channel loopback of the first VC4, so there is no such specification limitation.

Recommendations and Summary

In addition, most of the Metro SDH circuit boards have this similar problem.