Don't span high volume traffic to WS-X6548-GE-TX or WS-X6148-GE-TX oversubscribed line cards. I was scratching my head for the last two hours after a span session I created on 6500 caused other network users loose network connectivity/slowness. Later I find out that the line card I used to sniff traffic is not fit for what I was trying to achieve.
If you trying to span high volume traffic or thinking to use these (WS-X6548-GE-TX or WS-X6148-GE-TX) line cards for server farms, you must think again. I learnt my lesson when I tried to span a large of volume of traffic from another port to sniffer pc connected on to WS-X6148-GE-TX. This caused issues to other hosts connected on this line card. They response time increased and they were getting very slow network response.
These cards are over subscribed cards which means if one ports is being utilized heavily its going to effect other adjacent ports in the row of 8. This Cisco document goes into detail of how 8 ports share of a biffer of 1MB. if you see the following counter increasing, it would mean you are loosing packets. Following are output for two ports are in two blocks and you can see for one block the traffic is being dropped while the other one is OK on the same line card.
6500#sh counters interface gigabitEthernet 2/12 | i qos3Outlost