In Europe, the cheapest WAN links start around 2 Mbps these days. While this makes some WAN optimizations covered in Cisco’s QoS guides unnecessary, it’s good to know them and the effects of slower links on traffic.

Serialization delay
Putting a frame on the wire from a switch or router requires time. The amount of time is directly related to the line speed of the link. Note ‘line speed’: the actual negotiated speed on layer 1. For a 1 Gbps interface negotiated to 100 Mbps Full Duplex which is QoS rate-limited at 10 Mbps, the line speed is 100 Mbps. The formula is the following:


This means that if you have a 1514 bytes frame (standard MTU of 1500 bytes plus the layer 2 header of 14 bytes) and send it out of a 100 Mbps interface it will take (1,500*8)/(10^8)= 0.12 ms or 121 µs. If a small voice frame arrives in the egress queue of a switch or router it can incur op to 121 µs of additional latency if a 1514 bytes frame is just being sent out. Consequence: even under near perfect conditions and good QoS configuration where voice frames are given absolute priority over the network, there’s a possible jitter per hop. The higher the general bandwidth throughout the network, the lower the jitter, so latency-sensitive traffic does benefit from high bandwidth and fewer hops. Over a 10 GE interface that same frame would be serialized in just 1.21 µs per hop.

There are some consequences for WAN links: at slower speeds, the serialization delay increases rapidly. At 2 Mbps for a 1514 byte frame it’s 6 ms. At 64 kbps, it’s 190 ms. And in case you’re enabling jumbo frames: 9014 bytes over 10 Mbps is 7.2 ms.

Link Fragmentation and interleaving
Generally, at 768 kbps and below, jitter for voice becomes unacceptable. This is where Link Fragmentation and Interleaving (LFI). It works by splitting up large frames into smaller parts and putting low-latency packets between these parts.


Configuration of LFI on a Cisco router is as following:

Router(config)#interface Multilink1
Router(config-if)#ip address
Router(config-if)#ppp multilink
Router(config-if)#ppp multilink fragment delay 3
Router(config-if)#ppp multilink interleave
Router(config-if)#ppp multilink group 1
Router(config)#interface Serial0/0
Router(config-if)#encapsulation ppp
Router(config-if)#clock rate 768000
Router(config-if)#ppp multilink
Router(config-if)#ppp multilink group 1

First, create a Multilink interface. It will serve as an overlay for the actual physical interface, as this is where the LFI will be configured on. The Multilink interface will have all configuration: IP address, service policies,… Except for the layer 1 configuration (notice the clock rate command on the serial interface).

The ‘ppp multilink interleave’ activates LFI. The ‘ppp multilink fragment delay 3’ means LFI will automatically try to split up large packets so no packet has to wait longer than 3 ms while another is serialized. On the serial interface, encapsulation has to be set to ppp first. Next, it becomes possible to associate the interface with a Multilink overlay interface using the ‘ppp multilink group’ command.

The configuration has to be done on both sides of the WAN link, of course. The other side needs to use PPP encapsulation as well, and needs to have LFI enabled to reassemble to split up packets.

This concludes the series of QoS articles on this blog. Up next, I’ll try out different attacks on a Catalyst switch and see how it reacts.