Maximum frame-size parameter requirements for frame relay

This topic helps you determine the value of the maximum frame size.

The frame-relay network provider defines and typically establishes a tariff for the frame information field size provided. This size is referred to as N203. Use the N203 value to determine the value specified for the IBM^® eServer iSeries™ maximum frame size (MAXFRAME parameter).

The MAXFRAME parameter specifies the maximum protocol data unit (PDU) that the system transmits or receives. The iSeries system supports MAXFRAME values from 262 to 8192 bytes. The common N203 frame size that all networks and attached equipment support is 1600 bytes.

When configuring the MAXFRAME value, you must take into account the amount of overhead required for the various line types and their associated protocol headers. The length of these headers varies, depending on the type of frame-relay connection. For example, bridged connections typically require a longer header than that used for Systems Network Architecture (SNA) direct because these headers must include source and destination adapter addresses. Distributed data interface (DDI) and token-ring connections also include up to 18 bytes of routing information.

Table 1 shows the number of bytes required for the various frame-relay header types.

Table 1. Lengths for 802.2, remote LAN, and frame-relay headers
Connection type and protocol	802.2 LLC header	Remote LAN headers				Frame-relay headers			Total bytes
Connection type and protocol	802.2 LLC header	RI	AC/AF	Type/Length	DA/SA	C/NLPID	OUI/PID	L2/L3	Total bytes
SNA direct	4					2		4	10
IP direct						2			2
HPR						2		4	6
Bridged token-ring	4	18	2		12	3	5		44
Bridged DDI	4	18	2		12	3	5		44
Bridged IEEE 802.3	4			2	12	3	5		26
Bridged Ethernet V2				2	12	3	5		22
Abbreviations used: RI Routing information AC/FC Access control (AC) and frame control (FC) DA/SA Destination address (DA) and source address (SA) C/NLPID Q.922 control field (C), Network Level Protocol Identifier (NLPID), and optional PAD OUI/PID Organization Unique ID (OUI) and Protocol Identifier (PID) L2/L3 Layer 2 (L2) and Layer 3 (L3) Protocol HPR High-Performance Routing

The MAXFRAME value configured for the line description must equal the N203 value subscribed to from the network minus the total length of the header for the connection type and protocol. For example:

If the N203 subscription is 1600 and you are configuring for SNA direct communications, the MAXFRAME value specified on the CRTLINFR command should be set to 1590 (1600 – 10 bytes).
If the N203 subscription is 8192 and you are configuring a bridged frame-relay connection to a token-ring network, the MAXFRAME value specified on the CRTLINTRN command should be set to 8148 (8192 – 44 bytes).