Destination monitor port loose IP connectivity

[aghomri]

Hi Community,

Happy new year to everyone with all the best wishes.

It seems that when I configured the switch with a monitor session the destination port on which I collect the Ethernet traffic will loose immediately the IP connectivity, I am using below simple configuration :

monitor session 1 source interface Fa1/0/1 both
monitor session 1 destination interface Fa1/0/4

I am able to mirror the traffic from Fa1/0/1 to Fa1/0/4, but IP connection is down on port Fa1/0/4.

Is this an expected behavior ? Been reading the documentation for this but there is no mention about this limitation.

Any help on this will be greatly appreciated, Thanks.

Best Regards,

[Georg Pauwen · Accepted solution]

Hello,

this is normal, 'by design' behavior. A SPAN destination port is not usable for traffic other than the mirrored traffic.

--> 

SPAN Destination Port Up/Down
When ports are spanned for monitoring, the port state shows as UP/DOWN.

When you configure a SPAN session to monitor the port, the destination interface shows the state down (monitoring), by design. The interface shows the port in this state in order to make it evident that the port is currently not usable as a production port. The port as up/down monitoring is normal.

[aghomri]

Thanks For the hint Georg, indeed now I connected another device to check and I found that the protocol is down on the interface brief.

However it's strange to have such a limitation, a basic L2-Manageable switch does support port mirroring and keep the interface alive, do you know if there is any reason Cisco is limiting this on SPAN ?

Thanks again for your help.

Best Regards,

[Georg Pauwen]

Hello,

I don't really know why they designed it this way. The port is down, so no MAC addresses are learned. In the (very old and mostly obsolete) CatOS, at least on the 6500 switches, there used to be the option 'set span <learning> which would actually enable MAC address learning, and hence allow end devices to be reached, but that was dropped in IOS,as far as I recall...

[aghomri]

Thanks again Georg for the insights.

Best Regards,

[andri1]

Hi,

Two years ago I have used this command to be able to access the device connected to the "destination" port (port gig 0/2 in my case).

switch(config)#monitor session 1 destination interface gig 0/2 ingress vlan 1

For some reason, I am trying it on the same switch and the device now is not reachable. Does anyone know what has happened and why it does not work anymore?