Transparent Bridging

Overview

Shiva® devices support transparent bridging using the Bridge Control Protocol (BCP). The bridges in a network operate as isolated entities, largely unaware of each others' presence. A transparent bridge maintains a forwarding database consisting of [address, interface] records, by saving the source address of each LAN transmission that it receives, along with the interface identifier for the interface on which it was received. It goes on to check whether the Destination Address is in the database, and if so, either discards the message when the destination and source are located at the same interface, or forwards the message to the indicated interface. A message whose Destination Address is not found in the table is forwarded to all interfaces except the interface on which it was received. .

Eligible Protocols

The following protocols may be configured for bridging using BCP (on a per-device basis):

AppleTalk* - ETHER2 and SNAP encapsulations are supported

Internet Protocol (IP) - ETHER2 encapsulation is supported

Logic Link Control (LLC) - the Service Access Point (SAP) types to be bridged are specified by the existing [Bridge] LLCSapTypes parameter. No BCP options are negotiated. The current LLC bridging using Net Beui Control Protocol (NBCP) and NBFCP is still supported.

Up to 256 remote nodes may be transparently bridged by a LanRover™ Access Switch; up to 16 remote nodes may be transparently bridged by a LanRover. If there are multiple nodes on the LAN of an AccessPort that is dialed into a LanRover Access Switch, then each node counts towards the 256 (or 16) limit. The Ethernet version of the LanRover Access Switch does not operate in promiscuous mode.

MAC Addresses

LanRover Access Switches perform MAC address translation. This means the MAC address of every remote node is mapped to one of the LanRover Access Switch's 256 bridged MAC addresses. The LanRover Access Switch substitutes the translated MAC address in the Address Resolution Protocol (ARP) portion of IP ARP and AppleTalk ARP packets. No other packet types have the protocol portion of the packet translated.

The LanRover Access Switch learns the MAC addresses of remote nodes dynamically by snooping packets it receives on dial-up connections. When it receives a packet from a remote node that it has no record of, it creates a map of that node's MAC address to one of the LanRover Access Switch's 256 MAC addresses. This means that each remote node is unreachable using a unicast MAC address from the LanRover Access Switch's LAN until that remote node generates a packet to the LanRover Access Switch's LAN.

Considerations
When using transparent bridging, remember to consider the following

The Shiva device allows bridging of one protocol and routing of another protocol on the same dial-up connection
BCP may be run on either single-user or LAN-to-LAN connections with identical functionality.
BCP bridging is not be supported on the Token-Ring version of the LanRover Access Switch.
Virtual connections with bridging are not supported.
BCP cannot run on the same connection as either NBCP or NBFCP
The Shiva device cannot initiate BCP connections.
The same protocol may be bridged on one dialup connection and routed on another.
When bridging IP, the Shiva device can run as an IP concentrator, but cannot run in IP router mode.


Bridging over AppleTalk*

AppleTalk nodes on the LAN of one Shiva® AccessPort that is dialed into a LanRover Access Switch cannot communicate with AppleTalk nodes on another AccessPort LAN dialed into the same LanRover Access Switch. When bridging AppleTalk, the Shiva device can run as an AppleTalk end-node concentrator, but cannot run in AppleTalk router mode.

AccessPort Interoperability

Bridging is not guaranteed to work if a device other than an AccessPort dials into the Shiva device. Specifically, devices performing 802.1d (Spanning Tree) do not work when dialing into the Shiva device with BCP. When an AccessPort dials into a Shiva device, the AccessPort can not be running in Spanning Tree mode. Multiple AccessPorts on the same LAN cannot dial in to the same Shiva device while bridging. AccessPort to Shiva device AppleTalk bridging works for MAC IP as long as the Shiva device is not a MAC IP gateway