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)
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.
match access-group 102
!--- Traffic that matches ACL 102 is classified as class data.
priority percent 50
bandwidth percent 25
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.
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