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modifying the control field parameters, you can determine the number of acknowledgments sent for 
frames received and the level of polling used to determine available stations. In this manner, you can set 
the amount of resources used for frame checking and optimize the network load. 

SDLC is used as the primary SNA link-layer protocol for WAN links. SDLC defines two types of 
network nodes: primary and secondary. Primary nodes poll secondary nodes in a predetermined order. 
Secondary nodes then send any outgoing data. When configured as primary and secondary nodes, our 
routers are established as SDLC stations. 

The Cisco LLC2 implementation supports the following features: 

Local acknowledgment for RSRB

This feature is used in our implementation of RSRB as described in the chapter “Configuring 
Source-Route Bridging.” 

Because LANs are now connected through RSRB and WAN backbones, the delays that occur are 
longer than LLC2 allows for bidirectional communication between hosts. Our local 
acknowledgment feature addresses the problem of delays, resending, and loss of user sessions. 

IBM LNM support

Routers using 4- or 16-Mbps Token Ring interfaces configured for SRB support Lan Network 
Manager (LNM) and provide all IBM bridge program functions. With LNM, a router appears as an 
IBM source-route bridge, and can manage or monitor any connected Token Ring interface. 

LNM support is described in the chapter “Configuring Source-Route Bridging.” 

SDLLC media translation

The SDLLC feature provides media translation between the serial lines running SDLC and Token 
Rings running LLC2. SDLLC consolidates the IBM SNA networks running SDLC into a 
LAN-based, multiprotocol, multimedia backbone network. 

SDLLC is described in the chapter “Configuring IBM Network Media Translation.” 

ISO Connection-Mode Network Service (CMNS) 

The Cisco CMNS implementation runs X.25 packets over LLC2 so that X.25 can be extended to 
Ethernet, FDDI, and Token Ring media.