Supporting SNA Priorities

Problem

You want DLSw to preserve and support the SNA or APPN Class of Service definitions for forwarding packets through your IP network.

Solution

To configure DLSw to follow the SNA or APPN priorities defined in the traffic flow, you must configure the peer relationship to allow multiple distinct data streams:

Router-A#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router-A(config)#dlsw remote-peer 0 tcp 10.1.1.5 lsap-output-list 200 priority
Router-A(config)#end
Router-A#

You can then go further and map the individual priority streams to specific IP Precedence values, as in the previous example. The default values are shown in Table 15-4:

Router-A#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router-A(config)#dlsw tos map low 0 normal 1 medium 2 high 3
Router-A(config)#end
Router-A#

The TOS map does not need to match at both ends of a DLSw connection. But if you use the priority option on the dlsw remote-peer command on one router, you must also use it on the other.

Discussion

Recipe 15.10 showed how to configure the router to tag the IP Precedence field in all DLSw packets for preferential treatment through the network. This example allows for the creation of four separate DLSw priority levels that follow the SNA priorities. This is useful, for example, if the SNA traffic stream includes both bulk data transfers and interactive traffic.

As soon as you enable SNA prioritization in the dlsw remote-peer command, DLSw forms four TCP connections instead of just one. So it is critical you enable this option on both ends if it is required, or the remote router will simply reject the DLSw peer connection. DLSw then starts using the TCP Port numbers, as shown in Table 15-4.

Table 15-4. Default mapping of SNA priority to IP TOS and TCP ports
IP TOS	Value	SNA priority	DLSw TCP port
Routine	0
Priority	1
Immediate	2	Low	1983
Flash	3	Normal	1982
Flash Override	4	Medium	1981
Critical	5	High	2065
Internetwork Control	6
Network Control	7

Note that the highest priority SNA traffic has an IP TOS value of 5 (Critical) by default when SNA Priority is enabled. This is not a good choice in many networks. The routers need to reserve the top two Precedence values, Internetwork Control (6) and Network Control (7), for vital functions like routing protocols. Giving high priority SNA traffic a Precedence value of 5 means that there is no room for other high priority traffic such as voice. This is why we have included the dlsw tos map command in the recipe example. This command allows you to select more appropriate TOS values for the four SNA priorities.

Whether you use this method or the one in Recipe 15.10 to set up DLSw QoS is mostly a matter of whether you need to preserve the native SNA priority scheme. Opening 4 TCP connections, as in this recipe, causes the router to use more memory and CPU resources. This might become an issue on heavily loaded routers, particularly when many routers use a common central DLSw peer.