Configuring Transparent Bridging
Transparent and SRT Bridging Configuration Task List
BC-28
Cisco IOS Bridging and IBM Networking Configuration Guide
bridged network. To tie in existing bridges, you must use source-route translational
bridging (SR/TLB) instead. SR/TLB is described in the chapter “Configuring
Source-Route Bridging.”
Bridging between Token Ring and other media requires certain packet transformations. In all cases, the
MAC addresses are bit-swapped because the bit ordering on Token Ring is different from that on other
media. In addition, Token Ring supports one packet format, logical link control (LLC), while Ethernet
supports two formats (LLC and Ethernet).
The transformation of LLC frames between media is simple. A length field is either created (when the
frame is transmitted to non-Token Ring) or removed (when the frame is transmitted to Token Ring).
When an Ethernet format frame is transmitted to Token Ring, the frame is translated into an LLC-1
SNAP packet. The destination service access point (DSAP) value is AA, the source service access point
(SSAP) value is AA, and the organizational unique identifier (OUI) value is 0000F8. Likewise, when a
packet in LLC-1 format is bridged onto Ethernet media, the packet is translated into Ethernet format.
Caution Bridging between dissimilar media presents several problems that can prevent
communication from occurring. These problems include bit order translation (or using
MAC addresses as data), maximum transmission unit (MTU) differences, frame status
differences, and multicast address usage. Some or all these problems might be present in a
multimedia bridged LAN. Because of differences in the way end nodes implement Token
Ring, these problems are most prevalent when bridging between Token Ring and Ethernet
or between Ethernet and FDDI LANs.
Problems currently occur with the following protocols when bridged between Token Ring and other
media: Novell IPX, DECnet Phase IV, AppleTalk, Banyan VINES, Xerox Network Systems (XNS), and
IP. Further, problems can occur with the Novell IPX and XNS protocols when bridged between FDDI
and other media. We recommend that these protocols be routed whenever possible.
Transparent and SRT Bridging Configuration Task List
To configure transparent bridging or SRT bridging on your router, complete one or more of the tasks in
the following sections:
•Configuring Transparent Bridging and SRT Bridging, page29
•Transparently Bridged VLANs for ISL, page30
•Routing between ISL VLANs, page 32
•Configuring a Subscriber Bridge Group, page 34
•Configuring Transparent Bridging over WANs, page 34
•Configuring Concurrent Routing and Bridging, page39
•Configuring Integrated Routing and Bridging, page39
•Configuring Transparent Bridging Options, page42
•Filtering Transparently Bridged Packets, page45
•Adjusting Spanning-Tree Parameters, page52
•Configuring Transparent and IRB Bridging on a PA-12E/2FE Ethernet Switch, page 55