You want to set the Type of Service (TOS) field in DLSw packets to ensure that they get preferential treatment in the network.
In many organizations, the SNA traffic that is encapsulated in DLSw is considered both mission critical and time sensitive. So these organizations don't want lower priority traffic to interfere with it. The simplest way to accomplish this is to tag these high priority packets using the standard IP Precedence field:
Router-A#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router-A(config)#ip local policy route-map dlswroutemap Router-A(config)#ip route-cache policy Router-A(config)#access-list 101 permit tcp any any eq 2065 Router-A(config)#access-list 101 permit tcp any eq 2065 any Router-A(config)#access-list 101 permit tcp any any eq 2067 Router-A(config)#access-list 101 permit tcp any eq 2067 any Router-A(config)#route-map dlswroutemap permit 10 Router-A(config-route-map)#match ip address 101 Router-A(config-route-map)#set ip precedence flash-override Router-A(config-route-map)#exit Router-A(config)#end Router-A#
The most important concept here is the idea that you should set the priority of a packet at the point where it enters the network. In this case, the DLSw packet is actually created by the router, so this is the perfect place to tag it. Then every other router in the network can simply react appropriately to this priority tag without having to look deeply into the packet to figure out how important it is.
This example just shows how to set the IP Precedence fieldit doesn't actually affect the forwarding behavior.You have to ensure that the interfaces on routers that need to forward this packet are configured to deal with IP Precedence properly by using Weighted Fair Queuing or some other appropriate queuing mechanism. Techniques for doing this are shown in Chapter 11. The actual tagging of the packet is done by using policy-based routing, which is described in more detail in Chapter 5.
The IP Precedence value is set for every TCP packet with a source or destination port of 2065 or 2067. These are the two ports commonly used for DLSw traffic, 2065 is used by DLSw Version 1, and 2067 by Version 2. Cisco's DLSw+ primarily uses 2065, although Recipe 15.11 shows an exception to this.
Chapter 11 demonstrates some more sophisticated options for setting QoS tag values.
Recipe 15.11; Chapter 11