QoS Question

[rtjensen4]

Hello,

I have what is hopefully an easy question for someone to answer regarding QoS.

Firstly, If I understand it correctly, QoS only kicks in when congestion is experienced on a link. How does the switch / router determine that a link is congested? Do you have any good links that could explain this better for me?

Secondly, What I'm trying to accomplish is to have some replication traffic across the WAN be allowed to use as much bandwidth as it wants up until there's congestion, then i want that traffic to be the first traffic to be dropped to allow other business traffic / VoIP to pass. How would I go about this?

Platform is 4507R with SupIV

QoS is honestly a very weak point with me. Thanks in advance for any help.

[Collin Clark · Accepted solution]

1. It is determined by the Tx ring.

2. It's actually pretty easy, you could create a class with the apps and voice, then when congested they get priority and the rest of the bandwidth gets shared. Or if you want, you can create a scavenger class and put the replication in that class and restrict to something like 56K. There is a good book on QoS-

http://www.amazon.com/Cisco-Certification-Guide-Telephony-Self-Study/dp/1587201240/ref=sr_1_1?ie=UTF8&s=books&qid=1248978799&sr=8-1

And the Cisco SRND is excellent-

http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book.html

Hope that helps.