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538 Chapter 14: SNA Topologies

Figure 14-20 depicts this activity. In this example, SNA traffic is shown on DLCI 17, Telnet is
on DLCI 18, and FTP is on DLCI 19. Traffic is differentiated on up to four DLCIs with this
feature. DLCI 16 is high priority, and DLCI 17 is medium priority.

Figure 14-20

DLCI Prioritization

Using this technique, there can be different CIRs for each DLCI. For example, the Telnet DLCI
CIR could be 0, because Telnet is very low-bandwidth traffic, and the DLCI 16 could have a
CIR of 56 K.

DLSw Tuning

When designing for DLSw+ networks, CCDPs must know the tunable parameters if they are to
make the network really purr. This section discusses some of the major tunable parameters and
the impact that the commands can have.

IP MTU Path Discovery

This parameter specifies the largest packet size that may traverse the network. This is
determined during peer establishment. The maximum IP frame size then dictates the maximum
number of SNA bytes that can be stored in one frame. The default is 1450 bytes for a TCP/IP
network. Changing this value can result in better performance. A good example is to increase
the serial MTU to 4096 bytes to allow larger frame sizes to traverse the serial link and,
therefore, carry more SNA data.

The Cisco IOS global command for setting IP MTU path discovery is

ip tcp path-mtu-discovery

TCP Window Size

Another adjustment is to increase the TCP window size to allow more outstanding requests.
This command can also minimize packet fragmentation, because creating larger packets means
fewer smaller ones.

Token Ring

ISBN S/370

56K

Satellite

Router B

SNA

Host

Router A

87200333.book Page 538 Wednesday, August 22, 2001 1:41 PM