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How to set up MLPPP and LFI in a VoIP environment?

Core Issue

You can turn on fragmentation for low speed links (less than 768kbps). Set the fragment size so that voice packets are not fragmented and do not experience a serialization delay greater than 20 ms. Set the fragmentation size based on the lowest port speed between the routers. For example, if there is a hub and spoke Frame Relay topology where the hub has a T1 speed and the remote routers have 64 K port speeds, the fragmentation size should be set for the 64 K speed on both routers. Any other Permanent Virtual Circuits (PVCs) that share the same physical interface should configure the fragmentation to the size used by the voice PVC.

To determine the fragmentation size values, refer to this chart:

Lowest link speed in path

Recommended fragmentation size (for 10 ms serialization)

56 Kbps

70 bytes

64 Kbps

80 bytes

128 Kbps

160 bytes

256 Kbps

320 bytes

512 Kbps

640 bytes

768 Kbps

1000 bytes

1536 Kbps

1600 bytes

Resolution

This is a sample configuration:

access-list 101 permit udp any any range 16384 32767
access-list 101 permit tcp any any eq 1720
!
access-list 102 permit tcp any any eq 23
!
class-map voice
    match access-group 101

!--- Traffic that matches ACL 101 is classified as class voice.

class-map data

   match access-group 102

!--- Traffic that matches ACL 102 is classified as class data.

policy-map mlppp

   class voice

   priority percent 50

   class data

   bandwidth percent 25

   class class-default

   fair-queue

   !

interface BRI2/1  
  no ip address 
  encapsulation ppp 
  dialer pool-member 1 

  !--- This is a dialer pool 1 member.

  isdn switch-type basic-net3 
  no cdp enable 
  ppp authentication chap 
!
interface BRI2/2
  no ip address 
  encapsulation ppp 
  dialer pool-member 1 

  !--- This is a dialer pool 1 member.

  isdn switch-type basic-net3 
  no cdp enable 
  ppp authentication chap 

interface Dialer2 
  ip unnumbered Loopback0 
  encapsulation ppp 
  dialer pool 1 
  dialer load-threshold 1 either 
 
! -- Load level (in either direction) for 
  ! -- traffic at which additional connections 
  ! -- are be added to the MPPP bundle.
  ! -- Load level values range from 1 (unloaded) 
  ! -- to 255 (fully loaded).

  dialer string 6113
  dialer string 6114 
  dialer-group 1 
  ppp authentication chap 
  ppp multilink 

  ! -- Allow Multilink Point-To-Point Protocol (MLPPP) for the four BRI channels.

  service-policy output mlppp
 
! -- Apply the service policy to the dialer interface.

For information on the caveats and limitations involved with VoIP related QoS features (including Link Fragmentation and Interleaving (LFI)) with ISDN and tested sample configurations, refer to Designing and Deploying VoIP over ISDN.

For information on recommended configurations and related caveats for applying Class-Based Weighted Fair Queueing (CBWFQ) and Low Latency Queueing (LLQ) to MLPPP bundle interfaces and dialer interfaces, refer to Configuring CBWFQ and LLQ on MLPPP and Dialer Interfaces.

For supported releases across platforms, refer to the Software Advisor.

Version history
Revision #:
1 of 1
Last update:
‎06-22-2009 03:34 PM
Updated by: