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Core Issue

Bridging is necessary when non-routable protocols like Network Basic Input/Output System (NetBIOS) and Local-Area Transport (LAT) are used to communicate between devices connected across different segments. Bridging can also be configured for routable protocols instead of being routed. Transparent Bridging (TB) is used to bridge networks using similar media. TB is mostly used in an Ethernet environment. A device configured for TB initially learns the location of the devices by looking at the source MAC addresses in the frames. Next it builds a bridging table containing the MAC addresses and the ports through which they are reachable. To forward or filter traffic on the ports, the destination address in the frames is compared with information in this table. When there are multiple paths available between any two segments for providing redundancy, the spanning tree algorithm is used. This prevents loops. TB can either be remote or local. Local bridges provide direct connections between LAN segments in the same site. Remote bridges connect LAN segments at different sites, usually over WAN links.


To resolve this issue, perform these steps:

  1. To configure bridging for a protocol, disable routing for that protocol. Cisco routers route IP by default and bridge all other protocols. To bridge IP, disable IP routing by issuing the no ip routing command from global configuration mode.
  2. To configure bridging, create a bridge group by issuing the bridge bridge-group protocol {dec | ieee} command from global configuration mode.
  3. To identify redundant paths and prevent bridging loops, choose either DEC or IEEE as the Spanning Tree Protocol (STP).
  4. The router can act as multiple bridges and frames are bridged only among interfaces belonging to the same bridge group. This grouping feature is commonly used to separate networks or users. To include interfaces under the defined bridge group, issue the bridge-group bridge-group command under the interfaces. This command can be used on WAN interfaces using any PPP (like High-Level Data Link Control [HDLC]) for remote bridging across the link.
  5. Remote TB can also be configured over an X.25 or Frame Relay network. Issue the x25 map bridge  or frame-relay map bridge command under the specific physical interface or multipoint subinterface.

For sample configurations, refer to Configuring Transparent Bridging.

This document includes the following configuration scenarios:       Example 1: Simple Transparent Bridging       Example 2: Transparent Bridging with Multiple Bridge Groups       Example 3: Bridging Over a Wide Area Network       Example 4: Remote Transparent Bridging Over X.25       Example 5: Remote Transparent Bridging Over Frame Relay With No Multicast       Example 6: Remote Transparent Bridging Over Frame Relay With Multicast       Example 7: Remote Transparent Bridging Over Frame Relay With Multi Sub-Interfaces       Example 8: Remote Transparent Bridging Over Switched Multimegabit Data Service (SMDS)       Example 9: Remote Transparent Bridging With Circuit Group

How are Neighbors Connected

Connected via Frame Relay

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