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My goal is to have two ERSPAN sessions between two routers, R1 and R2.

Currently, one ERSPAN session is already configured from R1 to R2, copying traffic of vlan 35 to a physical port on R2.

I need to create the same ERSPAN the other way around : copy traffic of vlan 35 from R2 to a physical port on R1.

What I am worried about is that this would create a loop. Would my vlan 35 traffic be copied back and forth between each router ? Or once it is copied from R1 to R2, it won't get copied back by the new ERSPAN session ?

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    That really does not make sense. The goal of any type of SPAN session is to send traffic to a monitor., and the should be on one side or the other. The VLAN will only exist on one side, and you are trying to send it to a monitor on the other side. SPAN is not meant to be bidirectional. I think you have an X-Y problem, and you are focusing on yor proposed solution. You need to explain the problem you want to solve instead of the proposed solution.
    – Ron Maupin
    Jul 12 at 12:28
  • We have vlan 35 spanned between two sites. So this vlan is known on both sides. Currently, our monitor is on site B. But we are about to install a new monitor, this time on site A, hence the need to 'reverse' the configuration. But both monitors will exist at the same time.
    – pHeoz
    Jul 12 at 14:14
  • First, bridging a VLAN across separate sites is really not a good idea. We live in a layer-3 world, and really nothing is dependent to be on the same layer-2 LAN anymore. You risk killing both sites with layer-2 problems when they should be isolated at layer-2.
    – Ron Maupin
    Jul 12 at 15:47

1 Answer 1

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If you are trying to monitor using RSPAN or ERSPAN, the idea is that monitoring a VLAN would be sent to a separate monitoring VLAN.

You can set up separate monitoring VLANs on each site where you have your monitors. Use RSPAN and ERSPAN to send to the monitor VLAN on the other site, not on the VLAN you are trying to monitor. Do not bridge the monitor VLAN between the sites, have two different monitor VLANs.

On site A, use RSPAN to send to the monitor VLAN on site A, and use ERSPAN to send to the monitor VLAN on site B.

On site B, use RSPAN to send to the monitor VLAN on site B, and use ERSPAN to send to the monitor VLAN on site A.

Doing that will avoid the possibility of layer-2 loops.

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