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Port Mirror (SPAN) Application Notes

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Beginner

Recently, I came across a port mirror issue of packet loss when mirrored traffic exceeded 5Gbps in a 10Gbps stack system.

 

The topology looks like this:

S1------S2

2 switches are stacked together as a single stack system and stack link speed = 10Gbps.

Traffic in one port of S1 is mirrored to another port in S2.

When test is performed, when traffic >5Gbps, there is packet loss.

After debugging, I discovered that the reason is that the original traffic also travels through the stack link between S1 and S2. Therefore, together with mirrored traffic, the link was saturated. And that's why packet loss was observed.

 

As a lesson learnt, please check other traffic going through the same link that carries mirrored traffic to make sure the total sum does not surpass the link rate limit to avoid packet loss due to congestion.