Overview of IBM Networking
IBM Network Media Translation
Figure 95 QLLC Conversion Running on a Router with an Intermediate IP Network
X.25/QLLC session | TCP session |
| Running QLLC |
X.25 network | IP network |
| Router A |
LLC2 session 
Running RSRB
Token
Ring
Router B
S3031
Cisco’s Implementation of QLLC Conversion
SNA uses QLLC and X.25 as link layer protocols to provide a reliable connection. QLLC itself processes QLLC control packets. In a Token Ring environment, SNA uses LLC to provide a reliable connection. The
Figure 96 shows the simplest QLLC conversion topology: a single Token Ring device (for example, a 37x5 FEP) communicates with a single remote X.25 device (in this case a 3x74 cluster controller). In this example, a router connects the Token Ring network to the X.25 network.
Figure 96 QLLC Conversion Between a Single 37x5 and a Single 3x74
LLC2 session |
| QLLC/X.25 session | |
Token | T0 | S0 | X.25 |
37x5 | Ring | |
Router | ||
|
SNA session
= Virtual Token Ring
3x74 3270
S1391a
In Figure 96, each IBM end node has no indication that its counterpart is connected to a different medium running a different protocol. The 37x5 FEP responds as if the 3x74 cluster controller were communicating over a Token Ring, whereas the 3x74 responds as though the 37x5 FEP were communicating over an X.25 network. This is accomplished by configuring the router’s X.25 interface as a virtual Token Ring, so that the X.25 virtual circuit appears to the Token Ring device (and to the router itself) as if it were a Token Ring to which the remote X.25 device is attached.
Also in this figure, the LLC2 connection extends from the 37x5 FEP across the Token Ring network to the router. The QLLC/X.25 session extends from the router across the X.25 network to the 3x74 cluster controller. Only the SNA session extends across the Token Ring and X.25 networks to provide an
Cisco IOS Bridging and IBM Networking Configuration Guide