Cisco IOS Cookbook, 2nd Edition

Controlling DLSw Packet Fragmentation

Problem

You want to control packet fragmentation in DLSw to improve throughput.

Solution

There are two methods for controlling packet fragmentation when using DLSw. The first is to set an MTU for the bridge, as mentioned above in the section on bridging Token Ring to Ethernet:

Router-A(config)#dlsw remote-peer 0 tcp 10.1.1.5 lf 1470 lsap-output-list 200

This is used primarily when connecting media with different MTU values. However, it is also common to connect two high MTU media such as Token Rings via an intervening network that has low MTU links. In this situation, you should take advantage of DLSw’s TCP transport and the following command:

Router-A(config)#ip tcp path-mtu-discovery

Discussion

These two different commands work at different levels and accomplish different goals. The first one sets the MTU of packets that pass through the bridge. However the DLSw packets themselves need not have the same MTU. In fact, DLSw+ is able to break up a large Token Ring packet, carry it in a series of several DLSw packets, and then reassemble the large packet at the other end. So the first command above instructs DLSw not to accept any packets for bridging if they are larger than the specified size.

The most serious performance problems happen when the DLSw packets themselves must be fragmented in the network. In general, the DLSw routers will use the largest MTU that they can. This will usually wind up being the MTU of the first link into the IP network heading ...