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An ECC storm occurs after the active and standby SCC boards on an OptiX OSN 3500 NE switch over

Publication Date:  2012-07-25 Views:  51 Downloads:  0
Issue Description
The T2000 cannot reach the NE after the active and standby SCC boards on an OptiX OSN 3500 NE of version 5.21.13.47 switch over. Consequently, all ECCs oscillate and many NEs frequently go beyond the reach of the T2000 throughout the network.  
Alarm Information
Many NEs frequently go beyond the reach of the T2000.  
Handling Process
1. Record the optical interfaces of the gateway NE whose DCCs are enabled and then disable the DCCs for these optical interfaces.
2. Set the maximum ECC distance to 5 hops.
3. Enable the DCC for an optical interface. Wait until the network becomes stable. Enable the DCCs for the other optical interfaces.
4. Increase the maximum ECC distance gradually. Make sure that the network is stable before you set a new ECC distance.
5. Proceed with the operations until all the NEs recover. The ECC storm disappears. 
 
Root Cause
1. ECC storm caused during switchover of active/standby SCC boards
In the case of an NE configured with two SCC boards, the active SCC transmits the management information. In the standby SCC, the network interface and optical interfaces are disabled. When the active and standby SCC boards switch over, the original active SCC disables optical interface access and deletes routing table and MAC connection table. The original standby SCC enables optical interface access and sets up MAC connections and routes through HWECC. The NE goes offline and then online. As a result, the ECC floods and then converges.
2. Root causes of the ECC storm
The ECC network consists of 285 NEs. This number exceeds the recommended value. As a result, a change in the ECC routes is likely to cause an ECC storm.
Like IP RIP, HWECC is a distance vector protocol and cannot prevent route loops. Bulk routing information must be broadcast after a route change. Therefore, HWECC is designed as an IGP routing protocol for a small network. In the case of RIP, an ECC spans up to 16 hops. In the case of HWECC, however, an ECC travels up to 64 hops by default. In the case of a route change, the cyclic invalid data information must run through 64 hops before being discarded. For a large network, this is no doubt a disastrous load to the network bandwidth. Few bandwidth resources are available for transferring ordinary routing data. In addition, the other NEs must use some bandwidth resources to set up new MAC connections and routes. Thus, a vicious circle forms. For a long time, the routes cannot be converged. An ECC storm finally occurs. 
 
Suggestions
An ECC storm can be handled in two ways:
1. Set the maximum ECC distance to a smaller value.
The maximum ECC distance is 64 hops by default. This value is far beyond the ordinary requirements. In addition, this value sets a wide ECC route range for search.
Setting the maximum ECC distance to a smaller value can narrow the ECC route range for refresh and thus reduce the possibility of an ECC storm.
In case of an ECC storm, perform the following steps:
(1) Set the maximum ECC distance to 5 hops.
(2) Wait until the network becomes stable. Increase the maximum ECC distance and stabilize the network gradually.
In most cases, set the maximum ECC distance to 21 hops. When a fiber breaks or a loop is too long, however, the T2000 may fail to log in to NEs due to the maximum ECC distance. In this case, reduce the maximum ECC distance.
2. Disable ECC links around backbone nodes.
The ECC stops oscillating immediately when you disable an ECC link around a backbone node.
This assumes that you well know all optical connections in the network.
To disable an ECC link around a backbone node, perform the following steps:
(1) Disable the loops at the access layer to isolate the equipment from the ECC network.
(2) Wait until the ECC stops oscillating. Disable the other links gradually.
Note: Disable only the remote optical interfaces. Leave the routes to the T2000 functional so that the T2000 can log in to and enable the disabled ECCs. 
 

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