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I am going into all my Dell switches to reconfigure them. Is there anything wrong with enabling a mirror port and just leaving it on for all the switches so it is there when I need it?

I am wondering if this might cause instability or high load.

Update: My idea was to have one port monitor all the others. I have done this with 3com and it ended up causing a problem over time. But apparently Dell can't do this anyways (at least on my switch), so the question is moot:

console(config-if)# port monitor g1
console(config-if)# port monitor g2
console(config-if)# port monitor g3
console(config-if)# port monitor g4
console(config-if)# port monitor g5
Too many monitoring sessions.
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3 Answers 3

up vote 1 down vote accepted

I'm not quite sure I understand you. Configuring port mirroring requires you to define both the source and destination ports. You can't set up the destination port by itself and leave it "waiting" for you to attach a source port to it. Am I misunderstanding what you're asking or do you in fact want to set up a specific port mirroring "pair" and leave them in place?

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I just logged into my Netgear to make sure that what I'm about to say is correct - but our switch has the option to forward the traffic from every port, all at once, to a destination port. I wonder what happens if the full bandwidth on the single port is met, do the other requests have to wait for the monitor port to catch up? –  Mark Henderson Feb 25 '10 at 4:16
    
Hmm... good question. In addition, I'm fairly certain that on Dell switches you're limited to monitoring 8 or 12 ports at a time only. The same "pairing" rule applies though, you have to set them up in "pairs" as in setting both the source and destination port(s). You can't only set up "one side" of the pair. –  joeqwerty Feb 25 '10 at 4:45

I might be way out in the left field here but. I think that port mirroring is probably the least "overheaded" feature on a switch. Since it really only reproduce the signal from one port to another one.

The only worry I would have with that is security...

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I can't speak for all PowerConnects, but I've used a 2716, 2824, 3324, 3524, & 5324 & they're all limited to 4 source ports for each mirrored destination port. So if you want to monitor all of the ports on a 5324, you'd have to assign 5 of them as destination ports leaving you only 19 source ports. Sort of an expensive hassle in terms of time & ports, but it's doable.

In one situation I needed to monitor 12 ports regularly so I had 3 destination ports connected to a single monitor machine with 4 NICs (each monitoring NIC can't do anything else, so the 4th was needed for network connection).

I've never noticed any problems with leaving the mirror ports connected all the time. I can't say that I've done substantial testing to confirm that it doesn't create additional load on the switch, but it hasn't been a problem yet.

Also if the combined traffic on all of the source ports exceeds the limits of the destination port, it just gets dropped. Or at least it seemed to in my experience. If it was buffering packets it wasn't very large.

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