Enterasys Networks X-PeditionTM manual QoS and Link Fragmentation and Interleaving LFI

QoS and Link Fragmentation and Interleaving (LFI)

the dialer interface is pushed to binded serial and, when disconnected, is removed from the serial port. Refer to “Configuring PPP” on page 8-1.

Suggestions for Using QoS on the XSR

The XSR supports QoS on all interfaces but you should enable QoS only on the data path that actually requires it (generally on lower speed Frame Relay and PPP interfaces) because QoS is fairly processor intensive and may adversely impact router performance.

In a typical XSR environment, QoS may be enabled on the WAN link. The following lists two configuration scenarios:

•A standard office IP application, with no multi-media programs:

–Enable PQ or CBWFQ

•A complex office application, with multi-media applications:

–Use high Priority Queue for VoIP traffic with a cap on bandwidth it may consume

–Use medium Priority Queue for control packets with a cap on bandwidth

–Use CBWFQ queue for interactive traffic - Telnet, Web access

–Use CBWFQ with RED for remaining traffic

–Use Frame Relay fragmentation if the WAN link is Frame Relay with multiple DLCIs

Also, if the WAN link is running Frame Relay, you may enable generic traffic shaping to specify the Committed Information Rate (CIR), FECN and BECN options to control link throughput. Similarly, you can set policing to limit input and output rates of a PPP link (in situations where the delivery mechanism has higher throughput capability than the subscribed rate (e.g., the service could be delivery on 10 Mbps Ethernet, but you have subscribed only to 128 kbps). This ensures that the node conforms to the bandwidth level of any Service Level Agreement.

QoS and Link Fragmentation and Interleaving (LFI)

Latency sensitive traffic such as Voice over IP (VOIP) is susceptible to increased latency when the network processes large packets such as FTP transfers traversing a WAN. Packet delay is especially significant when the FTP packets are queued over slower links. A large 1500 byte packets takes 215 milliseconds to traverse a 56-kbps line, which exceeds the delay target of 150 milliseconds for real-time packets.

That being the case, Link Fragmentation and Interleaving (LFI), in association with QoS, reduces delay on slower-speed links by breaking up large datagrams and interleaving latency-sensitive packets with smaller packets resulting from the fragmented datagram. QoS classifies latency- sensitive packets into priority queues, schedules packets from the priority queues for transmission prior to packets from non-priority queues and delivers them to the LFI mechanism for fragmentation and interleaving with fragments from other traffic streams. The XSR implements LFI using Multi-link PPP (MLPPP) with multi-class or Frame Relay with FRF12.

Configuring QoS with MLPPP Multi-Class

Configuring QoS with MLPPP multi-class requires creating a MLPPP interface with multi-class. You set the fragment size using the fragment delay where a minimum value is 10 milliseconds. The XSR uses fragment delay and the link speed to calculate the size of the fragment, and splits all packets transiting this interface with this size. If a priority packet is fragmented, its fragments are interleaved with fragments from the other packets.

XSR User’s Guide 12-13