how to solve a problem of hanging alarms in Huawei U2000

Let’s assume that you have U2000 NMS server to monitor Huawei devices. We can manage these devices in 2 ways: outband or inband management. Outband management means that you have a separate DCN network to manage devices. It is commonly used for critical nodes, for example for backbone routers. Unlike to backbone network, it is difficult to implement DCN for mobile backhaul networks, where the number of devices reaches hundreds or even thousands. In such situation inband management is implemented to reduce cost. Then the decision how to send SNMP packets to the NMS server is based on routing protocols. The packets travel through the monitored network and are susceptible to all turbulences, which can appear in the network. This may lead to the fact that some SNMP packets may be lost by the network.

Let’s imagine such case. A link between a router and NMS is “DOWN”. No redundant link is established. The router sends SNMP trap to the NMS server but the server is not available. The SNMP packet is lost. Then the link is going to “UP” state and the router send SNMP trap to U2000. This trap is then dropped by U2000 because there is not related “DOWN” trap, which was lost before.

And what’s next?

U2000 synchronizes alarms with devices every 30 minutes and NMS server receives “DOWN” trap from the router, which was lost earlier. As the clearing trap was dropped, this “DOWN” alarm will not be cleared anymore. Then we have “DOWN” hanging (not cleared) alarm in U2000.

How to cope with this problem?

The first solution is to implement inform mode for SNMP packets:

snmp-agent target-host inform …

Managed devices require an acknowledgement from the NM server, after sending inform packets. If a managed device does not receive the acknowledgement, it will resend the inform packet to the NM station and generate alarm logs. If the managed device does not receive an acknowledgement from the NM station, it will store the inform packets in its memory. Anyway using inform mode may consume lots of system resources.

The next solution is to configure private-netmanager option to trap mode:

snmp-agent target-host trap address udp-domain ip-address [ udp-port port-number | { public-net | vpn-instance vpn-instance-name } ] * params securityname security-string [ { v3 [ authentication | privacy ] | v2c | v1 } | notify-filter-profile profile-name | private-netmanager | ext-vb ] *

When a Huawei NMS is deployed and this parameter is configured, a trap message, sent to the NMS, contains more information, such as the trap type, sequence of the trap, and sending time.
In such situation, even U2000 receives a trap from synchronization, it will compare the sequence of the trap and sending time. Then we avoid problems of not cleared alarms in network management system. We have implemented such solution in our customer’s network and it works really fine.