IBM BC-201 - Manual (Page 8)

Overview of IBM Networking
DLSw+
BC-208
Cisco IOS Bridging and IBM Networking Configuration Guide
78-11737-02
Figure 86 illustrates an LLC2 session in which a 37x5 on a LAN segment communicates with a 3x74 on
a different LAN segment separated via a wide-area backbone network. Frames are transported between
Router A and Router B by means of DLSw+. However, the LLC2 session between the 37x5 and the 3x74
is still end-to-end; that is, every frame generated by the 37x5 traverses the backbone network to the 3x74,
and the 3x74, on receipt of the frame, acknowledges it.
Figure 86 LLC2 Session Without Local Acknowledgment
On backbone networks consisting of slow serial links, the T1 timer on end hosts could expire before the
frames reach the remote hosts, causing the end host to resend. Resending results in duplicate frames
reaching the remote host at the same time as the first frame reaches the remote host. Such frame
duplication breaks the LLC2 protocol, resulting in the loss of sessions between the two IBM machines.
One way to solve this time delay is to increase the timeout value on the end nodes to account for the
maximum transit time between the two end machines. However, in networks consisting of hundreds or
even thousands of nodes, every machine would need to be reconfigured with new values. With local
acknowledgment for LLC2 enabled, the LLC2 session between the two end nodes would not be not
end-to-end, but instead, would terminate at two local routers.
Figure 87 shows the LLC2 session with
the 37x5 ending at Router A and the LLC2 session with the 3x74 ending at Router B. Both Router A and
Router B execute the full LLC2 protocol as part of local acknowledgment for LLC2.
Figure 87 LLC2 Session with Local Acknowledgment
With local acknowledgment for LLC2 enabled in both routers, Router A acknowledges frames received
from the 37x5. The 37x5 still operates as if the acknowledgments it receives are from the 3x74. Router
A looks like the 3x74 to the 37x5. Similarly, Router B acknowledges frames received from the 3x74. The
3x74
S1106a
WAN
SNA session
Token
Ring
Token
Ring
Router A Router B
LLC2 session
37x5
Router A Router B
S1107a
WAN
SNA session
Token
Ring
Token
Ring
LLC2 session LLC2 session
TCP session
3x74
37x5