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Configuring Transparent Bridging
Technology Overview
BC-27
Cisco IOS Bridging and IBM Networking Configuration Guide
FDDI-bridged interface. But for Internet Packet Exchange (IPX), Novell-ether encapsulation from the
bridge-group virtual interface is translated to raw-token or raw-FDDI when bridging IPX to a Token
Ring- or FDDI-bridged interface. Because this behavior is usually not what you want, you must
configure IPX SNAP or Service Advertisement Protocol (SAP) encapsulation on the bridge-group
virtual interface.
Other Considerations
The following are additional facts regarding the support of integrated routing and bridging:
• Integrated routing and bridging is not supported on cBus platforms (AGS+ and Cisco 7000 series).
• Integrated routing and bridging is supported for transparent bridging, but not for source-route
bridging (SRB).
• Integrated routing and bridging is supported on all media interfaces except X.25 and Integrated
Services Digital Network (ISDN) bridged interfaces.
• Integrated routing and bridging supports three protocols: IP, IPX, and AppleTalk in both
fast-switching and process-switching modes.
• Integrated routing and bridging and concurrent routing and bridging cannot operate at the same time.
• With integrated routing and bridging configured, associate Layer-3 attributes only on the
bridge-group virtual interface and not on the bridging interfaces. Having IP addresses both on the
bridge-group virtual interface and on the bridging interfaces is known to produce inconsistent
behavior.
SRT Bridging Features
Cisco routers support transparent bridging on Token Ring interfaces that support SRT bridging. Both
transparent and SRT bridging are supported on all Token Ring interface cards that can be configured for
either 4- or 16-MB transmission speeds.
As with other media, all the features that use bridge-group commands can be used on Token Ring
interfaces. As with other interface types, the bridge group can be configured to run either the IEEE or
DEC Spanning-Tree Protocols. When configured for the IEEE Spanning-Tree Protocol, the bridge
cooperates with other SRT bridges and constructs a loop-free topology across the entire extended LAN.
You can also run the DEC Spanning-Tree Protocol over Token Ring. Use it when you have other
non-IEEE bridges on other media and you do not have any SRT bridges on Token Ring. In this
configuration, all the Token Ring transparent bridges must be Cisco routers. This is because the DEC
Spanning-Tree Protocol has not been standardized on Token Ring.
As specified by the SRT bridging specification, only packets without a routing information field (RIF)
(RII = 0 in the SA field) are transparently bridged. Packets with a RIF (RII = 1) are passed to the SRB
module for handling. An SRT-capable Token Ring interface can have both SRB and transparent bridging
enabled at the same time. However, with SRT bridging, frames that did not have a RIF when they were
produced by their generating host never gain a RIF, and frames that did have a RIF when they were
produced never lose that RIF.
Note Because bridges running only SRT bridging never add or remove RIFs from frames, they
do not integrate SRB with transparent bridging. A host connected to a source-route bridge
that expects RIFs can never communicate with a device across a bridge that does not
understand RIFs. SRT bridging cannot tie in existing source-route bridges to a transparent