RE: cisco Router fundamentals

From: Robin Johnson (Rob.Johnson@dxi.net)
Date: Tue Jun 15 2004 - 14:26:17 GMT-3


Hello,
Im trying to run llq qos outbound across a serial 2Mb circuit and was trying
to work out in what order the router actually processes the policy map
I understand the router determines if the packets destination is reachable
and that it works out the next hop toward the destination and what interface
to send it but as the service policy is applied to the interface itself does
the router re-write the mac and forward it to this interface and then look
at the policy map or does it somehow do it in software before or in between
the routing process?

I guess what im trying to work out is how does the packet that enters the
router know anything about the service policy that is applied to the exiting
interface before actually exiting the router? Is it purely doing a lookup to
an acl whilst in the queue of the exiting interface and then prioritised
ahead of other things in the queue or is it using some sort of scheduler
before hand

Any pointers or links in the right direction would be much appreciated

Example

interface Serial3/0
 bandwidth 2048000
 ip address x.x.x.x x.x.x.x.x
 service-policy output TESTING

class-map match-all SMTP_Class
  match access-group 122
class-map match-any VOIP-signalling
  match access-group 126
class-map match-any Voip_Media
  match access-group 125
class-map match-any Critical-systems
  match access-group 121
  match access-group 124
  match access-group 127
  match access-group 128
  match access-group 129
!
!
policy-map TESTING
  class Voip_Media
   priority percent 33
  class Voip_Sig
   bandwidth percent 5
  class Bus_Systems1
   bandwidth percent 7
  class class-default
   fair-queue 4096



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