Link Fragmentation and Interleaving 497

Multilink PPP LFI

The core concept behind LFI, and its benefits, is very straightforward. The details, however, can be a little confusing, mainly because IOS LFI tools interact directly with IOS queuing tools. In addition, the two LFI tools covered on the Cisco QoS exams happen to behave differently as to how they interact with queuing tools. So to understand where LFI functions take place, you need to examine each tool specifically. This section covers multilink PPP LFI (MLP LFI), with Frame Relay fragmentation (FRF) covered in the next section of this chapter.

Figure 7-7 depicts how MLP LFI works with a queuing tool on an interface.

Figure 7-7 MLP LFI Interaction with Queuing

1500 Byte Packet Arrives, Followed by One 60 Byte Packet

The figure outlines a lot of the detailed concepts behind LFI. In this example, a 1500-byte packet first arrives at R1, followed by a 60-byte packet. The fragmentation logic has been configured to fragment the frames down to a little more than 300 bytes, to make room for 300 bytes from the packet, and a little more for the data-link headers and trailers. After fragmentation, the queuing tool on the interface classifies the frames into their respective queues, which in this example happens to be two different queues. (The queuing tool’s classification step works exactly as described in chapter 4, “Congestion Management.”)

Now look to the far right side of the figure. The TX Queue is shown, with a queue length of 2. In this example, an assumption has been made that the small packet arrived after IOS had placed the first two fragments of the large packet into the two available slots in the TX Queue, with the last three fragments being placed into Queue 2. The TX Queue is always absolutely a single FIFO queue, as described in Chapter 4. In other words, the small packet does not interrupt the router while it is in the middle of sending fragment 1, nor does the small packet have a chance to be sent before fragment 2, because fragment 2 is already in the TX Queue. The best behavior the small packet can hope for is to be the next packet placed onto the end of the TX Queue. Therefore, for now, the small packet has been placed into Queue 1.