"Your packets are fragmented
and their time to live has
exceeded"

"The American packets will die at the
hands of our brave access-lists"

"We will beat your packets
with our shoes and roast
your configs in hell!"
 

Oh, Hello!  Hey, what's that you're loading into
your Cadillac Escalade?

MSS

It is a machine that makes a drink the infidels call a "Slurpie", actually a very tasty beverage.  The fellow who owns this store is named Apu Nahasapeemapetilon, he said that I may have it.

Johna Barson

OK, great, well I'm here about Policy Based Routing, can you tell us what it is and how it works?

MSS

I triple guarantee you, there are no American soldiers in Baghdad

Johna Barson (smiles at her escort, Sgt. Ahmet Candan, USMC)

Right.... Anyway, about Policy Based Routing?

MSS

OK, It's like this, routers move packets from one interface to another, they determine which interface to send the packet out of by looking at their routing table.  This is the natural manner in which a Cisco router works.  If you want the router to handle a packet in a different manner, to send it out an interface it might not normally or perhaps you want to change the precedence value of the packet, that is PBR.  You do it with Route Maps

Johna Barson

Why would you change the precedence of a packet?

MSS

Infidel!  So that other routers downstream can provide different service levels, or prioritize the packets based on what the precedence field is set to.  The route map uses and access list to identify the traffic, it alters the precedence, routers downstream use access-lists to recognize these packets and Priority or Custom Queuing to put the packets in special queues.  This is but one way to do Policy Based Routing, there are many!

Johna Barson

What would the config look like on the ingress, or classifying router?  Say you wanted to prioritize traffic going to a webserver at 172.16.1.1!

MSS

!
access-list 100 permit tcp any host 172.16.1.1 eq 80
!
route-map iraq 10
 match ip address 100
 set ip precedence 7
!
interface serial 0
 ip policy route-map iraq
!

Johna Barson

Oh, I see!  Any traffic that enters the serial interface that matches the access-list will have the precedence set to 7.  Awesome!  What happens to traffic that is not permitted by the access-list, is it denied?

MSS

No!  It is routed normally

Johna Barson

OK, what would the config look like on the routers between the classifying router and the web server?  We want to guarantee 50% of the bandwidth to this traffic.

MSS
!
access-list 100 permit ip any any precedence 7
!
queue-list 1 protocol ip 1 list 100
queue-list 1 default 2
queue-list 1 queue 1 byte-count 2000
queue-list 1 queue 2 byte-count 2000
!
interface serial 0
 custom-queue-list 1
!

Johna Barson

I have to admit, that's pretty cool.  So you can tag certain traffic as it enters your routing domain, and then elsewhere in your domain, identify and handle that traffic differently, so that's PBR?

MSS

That's just one example, there are as many as there are grains of sand in the desert!  Death to the Infidels! 

Johna Barson

Yeah, whatever.  Where are you going to now?

MSS

I am the new spokesman for Isuzu, I am replacing an infidel named Joe.
Praise Allah!

Back to main page

Copyright 1999 - 2005 RouterGod Online Magazine