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RouterGod Guest Lecture Series

Gary Coleman Explains Priority Queuing

RouterGod is pleased to bring another in the guest lecture series, Priority Queuing by the former star of Different Strokes and current shopping mall security guard, Gary Coleman.  Mr. Coleman arrived dressed very nicely and was a pleasure to work with, unlike other child celebrities we've interviewed who we won't mention (Danny Bonaduce).  At first we thought someone was playing a prank on us, ringing our doorbell and running off, but after the sixth time we noticed Gary standing there with an angry expression.  We apologize for the confusion. 

Hey!  Whatzup?  Anyway people, "whatcha talkn about"?  Ha Ha , just had to throw that in.  Good golly miss molly, let's get down to business.  I have been asked to come here and tell you folks about Priority Queuing and how to configure it on a Cisco Router.  Ever notice how they call an Ethernet frame smaller that 64 bytes a runt?  I take umbrage with that.  They should just call it a "little frame", we would get the picture.  You know what I'm say'n?  Okey Dokey, priority queuing...yeah, ok.  Nuth'n to it but to do it.

Priority queuing, like all methods of queuing can only be enabled on interfaces with bandwidths less than 2.048 Mbps.  In other words, you aint gonna configure queuing on an ethernet interface.  So it's for a WAN link, like ISDN or frame relay.  Now listen up people!  If your link is already overloaded, queuing won't help.  It's only to prioritize time sensitive traffic over lower priority traffic.  With Priority Queuing, you can have up to 4 queues.  They are called: high, medium, normal and low.  Priority queuing is configured using the priority-list command.  The priority list assigns incoming packets to each of the 4 queues based on arguments you configure.

The high priority queue is always emptied first, then the medium queue gets emptied.  Next the normal queue gets emptied and finally the low queue gets emptied.  If the high queue is always full and never gets emptied, none of the other queues will ever get serviced.  That sucks!  This is why you probably don't want to use priority queuing, you probably want to use custom queuing.  Oh, one more thing, people, if a lower priority queue is getting serviced and traffic arrives into a higher priority queue, you guessed it, the router forgets all about the queue it was servicing and immediately takes care of the higher priority queue.  Now that I think of it, Hollywood runs the same way.

Now check this out, you had better assign a default queue for traffic that you forgot to include in the other queues.  Use the priority-list default command. Your boss swore up and down that the only protocol on the network was IP, but what he doesn't know is there is an IPX server that has been in QA since 1988.  Moral of the story, configure a default queue.  You can prioritize incoming packets based on protocol or interface.  You can even use the keyword list to call an access list.  Hot Damn!  Oh, yeah, one more thing you can use the priority-list queue-limit command to change the maximum number packets allowed in each queue to begin with.  By default from high to low, it's 20, 40, 60 and 80.  Here's a config I worked out on the way over here:

priority-list 1 protocol ip high tcp 23
priority-list 1 protocol ipx medium
priority-list 1 interface ethernet 1 medium
priority-list 1 protocol ip list 112 normal
priority-list 1 default low
interface serial 0
priority-group 1

Another Cute Child Actor.


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