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Basic QoS at Layer 3



When setting up a basic CB marking, we need to do the following steps:
  1. We first need to define our access lists (lines 28-32) to find the traffic that we want to classify.
  2. Next, we need to create a class map that links to an access list (lines 4-7).  You could think of this step as naming the access list.
  3. Now we create the policy map with whatever class maps should be included (lines 10-17), and define how we should mark the traffic
  4. Finally, we need to bind a policy map to an interface (line 26).  Note that you can bind two policy maps to an interface, one inbound, and one out.

Example of Basic CB Marking
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!
ip cef
!
class-map match-all HI
   match access-group 101
class-map match-all LO
   match access-group 100
!
!
policy-map IPFRCOS
   class HI
      set ip dscp af31
      bandwidth percent 35
   class LO
      set ip dscp af21
   class class-default
      set ip dscp default
!
!
interface Serial0/0/0
   description description IPFR AT&T Ckt ID:
   bandwidth 1536
   ip address 10.109.2.81 255.255.255.252
   encapsulation frame-relay IETF
   frame-relay lmi-type cisco
   service-policy output IPFRCOS
!
access-list 101 permit tcp any eq telnet any
access-list 101 permit tcp any any eq telnet
access-list 101 permit icmp any any
access-list 101 permit tcp any any eq 1494
access-list 101 permit udp any any eq 1604
!



Cisco's Recommended Values for Marking:

Type of Service CoS Precedence DSCP
Voice Payload 5 5 EF
Video Paylod 4 4 AF41
Voice/Video Signaling 3 3 AF31
High-priority or gold data classes 2 2 AF2x (x=1-3)
Medium-priority or silver data 1 1 AF1x (x=1-3)
All else 0 0 Default


IETF RFCs have defined special keywords, called Per-Hop Behaviors, for specific DSCP markings

  • EF: Expedited Forwarding (RFC3246)  
    • (DSCP 46)
  • CSx: Class Selector (RFC2474)
    • Where x corresponds to the IP Precedence value (1–7)
    • (DSCP 8, 16, 24, 32, 40, 48, 56)
  • AFxy: Assured Forwarding (RFC2597)
    • Where x corresponds to the IP Precedence value (only 1–4 are used for AF Classes)
    • And y corresponds to the Drop Preference value (either 1 or 2 or 3)
    • With the higher values denoting higher likelihood of dropping
    • (DSCP 10/12/14, 18/20/22, 26/28/30, 34/36/38)
  • BE: Best Effort or Default Marking Value (RFC2474)
    • (DSCP 0)

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