Can a SPAN port output be connected without anything on RX?

[Robalob]

I'd like to use a SPAN port in a similar way to a fiber tap.

When doing something like:

monitor session 1 source interface Gi1/1/1
monitor session 1 destination interface Gi1/1/2

If Te1/1/2 is an SFP, e.g. GLC-LH can I connect only the TX optic on Gi1/1/2 to my analyser?

I have been testing this on an old 3750 prior to trying to implement it on a production Cat9500 and I cannot get the port to come up unless I connect the RX path also. Presumably there is some sort of link negotiation happening. I have tried disabling speed negotiation and udld with no luck. Does anyone know if this is possible and if so what needs to be set?

It seems wasteful to be using fiber strands to carry nothing but link negotiation. Being able to do this would save a lot of fiber runs.

[balaji.bandi]

what is the goal here to capture information only TX traffic,

the way you looking to do with fibre optic may not work.

[Robalob]

Capture source options are not of concern here. I am talking about patching of the monitor destination port to analyser device. Irrespective of whether a source is capturing TX, RX or both, the monitor destination only produces TX traffic from the Cisco to the analyser right?

The goal is to not waste fiber strands when no traffic is being carried in that direction. In some situations, these can be costly.

When using a fiber tap, a tap output TX is produced for RX or TX, or two for both. These would connect to an analyser SFP on RX side, with nothing connected to analyser SFP TX.

My question is if SPAN ports can be used like this?

[balaji.bandi]

the way you looking to do with fibre optic may not work.

I have answered as per my experience - I do not believe that works.

[Giuseppe Larosa]

Hello @Robalob ,

you can try to disable keepalive to see if the port is considered up with RX fiber not connected.

Keepalive can use Loop frames with MAC SA = MAC DA = port MAC address and the device expects the other side to send back these frames to say the link is up.

However, I don't think this is enough on fiber based ports on Cisco switches.

int type x/y

no keepalive

Hope to help

Giuseppe

[Robalob]

Thanks for the suggestion @Giuseppe Larosa ,

I tried it and unfortunately it made no difference. It does seem as if Ciscos require some light incoming on RX before they will bring up a monitor destination port.

Rob

[rasmus.elmholt]

Yes, the port needs to be in the UP/UP state for it to mirror the packets out.

I know there are some single fiber strand Duplex standards, maybe they will solve your problem?