smp606
join:2002-01-16
PA | reply to Covenant
Re: receive discards
Excellent information!
Quick question, i am getting a good deal of output drops on my t1 serial interface. Is there a way to tell which device on the network is causing this? |
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Covenant
Premium,MVM
join:2003-07-01
England
| It may be a number of devices, if you are getting loads of output drops, you can configure "ip accounting" under the serial interface but that just shows you what hosts were the biggest talkers and may not always pin point the culprit as it may also be due to "bursts" in traffic from the least talkative hosts. IP accounting looks at traffic going OUT from an interface.
You can also configure ip route-cache flow under the LAN interface which shows traffic coming IN to the router. That will tell you the hosts, ports in hexadecimal and the number of packets in each flow but it is not cumulative like ip accounting unless the flow is consistent and still active. Once the flow is over, it will disappear from the output.
Depending on how time sensitive your traffic is or how tolerant it is to drops, you may want to implement some type of QoS/traffic-shaping under the serial interface with WRED to not tail drop traffic but randomly drop packets depending on precedence/DSCP but that requires you to mark packets coming in on the LAN interface using a policy-map to mark with high precedence/DSCP packets that are important to the function of the business and then low mark the packets such as internet/ftp/email, and anything else you think should be classified as that.
If your traffic is not latency sensitive, then you can just increase the hold queue but that just increases the delay for the packets at the end of the queue to be sent out (serialisation) so to work out what the middle ground is, is going to require a lot of effort and trial and error unless you get a traffic analyser SPANing the router's LAN interface to see what is actually going through the router and then configuring CBWFQ with WRED combined with traffic-shaping and packet marking on the LAN interface to intentionally drop non-important packets and not drop others unless it has to.
Sorry to go on, but this is a big topic which can't be fully justified in the short paragraphs written above.
Hope it's useful anyhow.
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If only my employers can see how much effort I put into the Cisco forum. They would then understand why I sleep at my desk.  |
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smp606
join:2002-01-16
PA
| Great!! Thanks for the info. It all makes sense, as i was seeing a few output drops every once and a while, then when i implemented some QOS for my VoIP traffic, i saw the output drops go way up (because of the service policy out??).
I am sure my QOS setup needs some help as well. Sorry to tie up this thread i will start anothoer more specific one. Thanks for all the help!! |
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