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Overview of IBM Networking
STUN and BSTUN
BC-215
Cisco IOS Bridging and IBM Networking Configuration Guide
• Allows networks with IBM mainframes and communications controllers to share data using Cisco
routers and existing network links. As an SDLC function, STUN fully supports the IBM SNA and
allows IBM SDLC frames to be transmitted across the network media and shared serial links.
illustrates a typical network configuration without STUN and the same network configured with
STUN.
• Encapsulates SDLC frame traffic packets and routes them over any of the supported network media
(serial, FDDI, Ethernet, and Token Ring, X.25, SMDS, and T1/T3) using TCP/IP encapsulation.
Because TCP/IP encapsulation is used, you can use any of the Cisco routing protocols to route the
packets.
• Copies frames to destinations based on address. STUN in passthrough mode does not modify the
frames in any way or participate in SDLC windowing or retransmission; these functions are left to
the communicating hosts. However, STUN in local acknowledgment mode does participate in SDLC
windowing and retransmission through local termination of the SDLC session.
• Ensures reliable data transmission across serial media having minimal or predictable time delays.
With the advent of STUN and WAN backbones, serial links now can be separated by wide
geographic distances spanning countries and continents. As a result, these serial links have time
delays that are longer than SDLC allows for bidirectional communication between hosts. The STUN
local acknowledgment feature addresses the problems of unpredictable time delays, multiple
retransmissions, or loss of sessions.
• Allows for configuration of redundant links to provide transport paths if part of the network goes
down.