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New Member

Cat 3750 duplicating packets

I am using "monitor session" to mirror traffic to a monitor port, when I mirror a VLAN to the monitor port the switch duplicates every packet it mirrors with the duplicate being 7 microseconds behind the first.

The monitor session works as expected when I mirror an physical port to the monitor port.

Cisco Internetwork Operating System Software

IOS (tm) C3750 Software (C3750-I9-M), Version 12.1(19)EA1d, RELEASE SOFTWARE (fc


Copyright (c) 1986-2004 by cisco Systems, Inc.

Compiled Mon 05-Apr-04 22:40 by antonino

Image text-base: 0x00003000, data-base: 0x007CBC3C

ROM: Bootstrap program is C3750 boot loader

BOOTLDR: C3750 Boot Loader (C3750-HBOOT-M) Version 12.1(19r)EA1b, RELEASE SOFTWA

RE (fc2)

IHPC3711 uptime is 4 weeks, 1 day, 21 hours, 21 minutes

System returned to ROM by power-on

System restarted at 12:10:14 UTC Wed Mar 8 2006

System image file is "flash:c3750-i9-mz.121-19.EA1d/c3750-i9-mz.121-19.EA1d.bin"

cisco WS-C3750-24P (PowerPC405) processor (revision C0) with 118776K/12288K byte

s of memory.


Re: Cat 3750 duplicating packets

not sure of the issue, may be u can check out the following link on configuring SPAN on Catalyst 3750, hope this helps :

Re: Cat 3750 duplicating packets

Are packets your seeing sourced and destined to the same vlan? If so, that is not duplicate, it's a mirrored packet from the source in the same vlan and the destination packet to the same vlan. So, you have two packets, one coming from the source host gets mirrored and then that same packet gets mirrored again as it now see that it's destined to the device in the same vlan. In other words, the rx and tx of the same packet is being mirrored and of course it's the same packet. Try either rx or tx only and see if you no longer see the duplicate packets. with span in both directions having two copies of the same packet is normal.

New Member

Re: Cat 3750 duplicating packets


This happening on packets that are staying within the VLAN, but also on packets that are being routed to another subnet outside the VLAN, (the 3750 is NOT doing the routing)

New Member

Re: Cat 3750 duplicating packets

This posting is correct if the monitoring port is also in the same VLAN.

For the solution to work properly place monitoring port in another VLAN (native VLAN preferably)