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Service Interruption Due to Improper STP Convergence Mode

Publication Date:  2013-10-26 Views:  40 Downloads:  0
Issue Description
On an IDC network (see the attachment), NE80Es are the STP root bridges and the VRRP gateways for all connected terminals. If terminals connect to and disconnect from the network frequently, the spanning tree topology changes and a large number of TC packets are generated. The TC packets affect NE80Es and interrupt services on the network. 
Handling Process

On a Layer 2 network deployed with STP, a topology change causes generation of TC packets which are flooded on the network. Upon receiving the TC packets, NE80E performs corresponding actions according to the configured STP convergence mode as follows:

1. If STP convergence is in fast mode, NE80E deletes related ARP table entries upon receiving TC packets. ARP entries are re-created based on received data flows.

2. If STP convergence is in normal mode, NE80E sets the TTL for related ARP table entries to 0 upon receiving TC packets. The ARP ages these entries.

The STP convergence on the IDC network is in normal mode. Therefore, NE80E keeps deleting ARP table entries when the network topology changes frequently, resulting in service interruption.
Root Cause
The STP convergence on the IDC network is in normal mode. Therefore, NE80E keeps deleting ARP table entries when the network topology changes frequently, resulting in service interruption.
Solution
Change the STP convergence mode to normal.
Suggestions
In normal convergence mode, convergence is slow but services are slightly affected. In fast convergence mode, convergence is fast but services are affected. It is recommended that small-sized IDC networks use the fast mode and medium- and large-sized IDC networks use the normal mode.

END