Port monitoring performance issues

Question

We have 3500Xl series switches and some are running port monitoring so we can gather data on what kind of traffic is flowing through our network. The question is if 1 ethernet port is monitoring all the other ethernet ports on it, what kind of performance issues could we expect, if any. Is there a way to show the performance stats of the switch as well. For example, see below.
interface FastEthernet0/47
description ethereal87 port monitor
no logging event link-status
port monitor FastEthernet0/2
port monitor FastEthernet0/1
port monitor FastEthernet0/4
port monitor FastEthernet0/3
port monitor FastEthernet0/6
port monitor FastEthernet0/5
port monitor FastEthernet0/8
port monitor FastEthernet0/10
port monitor FastEthernet0/9
port monitor FastEthernet0/12
port monitor FastEthernet0/11
port monitor FastEthernet0/14
port monitor FastEthernet0/13
port monitor FastEthernet0/16
port monitor FastEthernet0/15
port monitor FastEthernet0/20
port monitor FastEthernet0/19
port monitor FastEthernet0/22
port monitor FastEthernet0/21
port monitor FastEthernet0/24
port monitor FastEthernet0/23
port monitor FastEthernet0/26
port monitor FastEthernet0/25
port monitor FastEthernet0/28
port monitor FastEthernet0/27
port monitor FastEthernet0/30
port monitor FastEthernet0/29
port monitor FastEthernet0/32
port monitor FastEthernet0/31
port monitor FastEthernet0/34
port monitor FastEthernet0/33
port monitor FastEthernet0/36
port monitor FastEthernet0/35
port monitor FastEthernet0/38
port monitor FastEthernet0/37
port monitor FastEthernet0/40
port monitor FastEthernet0/39
port monitor FastEthernet0/42
port monitor FastEthernet0/41
port monitor FastEthernet0/44
port monitor FastEthernet0/43
port monitor FastEthernet0/46
port monitor FastEthernet0/45
switchport access vlan 87
no snmp trap link-status
spanning-tree portfast

Answer 1

Hi,
I noticed you are attempting this on a 3500XL switch.I'll attach a useful reference to some constraints with SPAN on XL
http://www.cisco.com/en/US/partner/products/hw/switches/ps708/products_tech_note09186a008015c612.shtml#topic1
These older switches use a blocking, shared memory architecture and shared buffers for groups of 4 ports. One performance condition that is possible in this type of scenario is intermittent packet loss within the group of ports when traffic is high. This condition becomes probable when utilisation of ports start exceeding about 70% of bandwidth simultaneously on all ports. The symptom is intermittent because of the nature of  traffic is typically bursty
Example:
Monitor port is 48.
Servers are 45,46,47 -100MB
Let's say a backup is run on the servers and all servers start to hit 70% of available bandwidth. If port is monitoring, then this is = 3 x 70MB = 210MB which exceeds monitor port.
Monitor port would see overruns drops etc
http://www.cisco.com/en/US/partner/products/hw/switches/ps607/products_tech_note09186a0080125913.shtml#troubleshoot_interface_errors
but the server ports could intermittently start dropping traffic because of available buffer taken up by monitor port. There's no real way to determine at what rate this will happen but something to keep an eye out for. You may isolate the monitor port to it's own group of 4.
HTH
Eugene