Posted on August 19, 2015 2:49 pm
 |  Asked by Praveen Rai
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Hi Expert,

I wanna do SPAN on Arista and also wanna vlan information intact when i send monitor session on desitnation port.but i found encapulation dot1q feature missing as we have on cisco switching.

Could you please tell me how to achieve this.for reference i want command like this

monitor session 1 source {Interface} both

monitor session 1 destination{interface} encapsulation dot1q


can we do same on Arista vEOS Switches


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Posted by Dave Thelen
Answered on August 19, 2015 4:33 pm



Generally, Arista platforms will maintain the dot1q state that the original packet has.  That is, if the original packet has a vlan tag, then when it reaches your monitor destination port it should maintain that same tag.   There are couple of caveats on our older platforms that you might actually see a vlan tag on a mirrored frame when the original one did not have one.  If you can send along the platform you are configure the mirror I can let you know if you might see this oddity or not.



Posted by Alexis Dacquay
Answered on August 20, 2015 9:44 am


vEOS natively preserves 802.1Q that it receives on ingress.
For example, in the attached PCAP file (, I have captured traffic on the MLAG-peer link between 2 Arista devices. You could see some MLAG-peering exchanges seen as traffic with 802.1Q tag 4094 (MLAG exchange VLAN).
You could also see some test traffic I generated (switches pinging each others on and .3), using SVI 10 (Layer3 VLAN interface 10): the traffic is tagged with 802.1Q ID 10.

All that traffic was mirrored from the peer-link port-channel 10 and mirrored towards a Linux VM attached on Eth5.

My mirroring config is:

monitor session MIRROR1 source Port-Channel1
monitor session MIRROR1 destination Ethernet5
Arista#show monitor session
Session MIRROR1
Source Ports:
Both: Po1
Destination Ports:
Et5 : active


Note that I have tested with: – Virtual Box 5 – vEOS-lab 4.15.1F – Aboot 2.1.0 – Virtio virtual NICs.

Regards, Alexis


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