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Miscellaneous Bridge Features

This document describes bridging features that are available with the ASRT bridge. It includes the following sections:

Protocol Filtering

IBM RT Feature for SNA Traffic

Multiple Spanning Tree Protocol Options

Protocol Filtering

A single platform can perform both bridging and routing. Protocol filtering determines whether the incoming data is routed or bridged, based on the contents of the destination address field of incoming frames.

Table 2 shows how the destination address field determines "Bridge or Route?" question.

Table 2 Route/Bridge Decision Table

If Destination MAC Addressing Contains: Action the Bridge Takes

Interface Address Passes the frame to the configured protocol that routes the frame.

Multicast or Broadcast Address If the frame belongs to a configured protocol, passes the frame to the protocol forwarder. Otherwise, bridges the frame.

Other Unicast If the frame belongs to a configured protocol, discards the frame. Otherwise, bridges the frame.

Table 2 Route/Bridge Decision Table
If Destination MAC Addressing Contains:	Action the Bridge Takes
Interface Address	Passes the frame to the configured protocol that routes the frame.
Multicast or Broadcast Address	If the frame belongs to a configured protocol, passes the frame to the protocol forwarder. Otherwise, bridges the frame.
Other Unicast	If the frame belongs to a configured protocol, discards the frame. Otherwise, bridges the frame.

IBM RT Feature for SNA Traffic

Some IBM PCs (RT PC running OS/2/EE) run SNA over Ethernet Type 2 instead of 802.3 Ethernet. This requires an additional header that contains the length of the MAC user data followed by the 802.2 (LLC) header.

You can enable or disable the processing of these frames on a per port basis. If enabled, the bridge learns the source station's behavior and generates the correct frame format. But if there is no information about the station's behavior (multicast or unknown stations), the bridge produces duplicate frames, one in 802.3 and 802.2 format, and the other with the IBM-RT header.

Multiple Spanning Tree Protocol Options

ASRT lets you extend spanning tree protocol options to cover as many configuration options as possible. The next sections describe these features.

Multiple Spanning Tree Protocol Problems

Bridging technology employs different spanning tree algorithms to support different bridging methods. The common purpose of each algorithm is to produce a loop-free topology.

In the spanning tree algorithm used by Transparent Bridges (TB), Hello Bridge Protocol Data Units (BPDUs) and Topology Change Notification (TCN) BPDUs are sent in a transparent frame to well-known group addresses of all participating media (Token Ring, Ethernet, FDDI, etc.). Tables are built from this exchanged information and a loop free topology is calculated.

SRB uses transparent frames to determine a loop free topology. The algorithm sends Hello BPDUs in a transparent frame to a well-known functional address. SRB bridges do not use TCN BDPUs. The port state setting created as a result of this spanning tree algorithm does not affect All Route Explorer (ARE) Frame and Specifically Routed Frame (SRF) traffic.

In bridging configurations using IBM 8209 bridges, a different spanning tree method is used to detect parallel 8209 bridges. This algorithm uses Hello BPDUs sent as STE frames to IEEE 802.1D group address on the Token Ring. On the Ethernet, Hello BPDUs are sent as transparent frames to the same group address. This method allows 8209s to build spanning trees with transparent bridges and other IBM 8209 bridges. It does not participate in the SRB spanning tree protocol however, and Hello BPDUs sent by SRBs are filtered. As such, there is no way to prevent the 8209 from becoming the root bridge. If the 8209 bridge is selected as the root, then traffic between two STB domains may have to pass through Token Ring/SRB domains.

As you can see, running multiple spanning tree protocols can cause compatibility problems because each algorithm creates its own loop-free topology.

Enhanced STP

The enhanced STP bridging feature allows you to further extend the Spanning Tree protocol. Based on the bridge personality, it allows bridges to participate in the appropriate STP. Previously, SRB bridges allowed only manual configuration of a loop-free tree over the Token Ring. This was the only mechanism to prevent loops in the case of parallel SRB bridges. With the addition of the enhanced STP feature the following spanning tree algorithm combinations are possible:

Pure Transparent Bridge (STB) - IEEE 802.1D Spanning Tree protocol.
Pure Source Route Bridge (SRB) - IBM SRB Spanning Tree protocol.
Transparent and Source Route Bridges as separate entities - IEEE 802.1D Spanning Tree protocol for STB and manual configuration for SRB loop-free topology.
SR-TB Bridge - IEEE 802.1D Spanning Tree protocol for STB ports and IBM 8209 BPDUs on SRB ports to form a single tree of STBs and SR-TBs. SRB Hello BPDUs are allowed to pass on the SR domain but are not processed.

IBM 8209 bridges filter such frames but this is allowed as it is a two-port bridge with the other port being a transparent bridge port.

Pure SRT Bridge - Only IEEE 802.1D Spanning Tree protocol is used. SRB Hello BPDUs and IBM 8209 BPDUs are allowed to pass but are not processed.
ASRT Bridge - IEEE 802.1D Spanning Tree protocol is used to make a tree with STBs and SRT bridges. 8209-like BPDUs are also generated on all SR interfaces to make tree with SR-TB and IBM 8209 bridges.

These BPDUs are processed as soon as they are received. This causes two BPDUs to be generated and received on all SR and STB interfaces. Since both BPDUs carry the same information, there is no conflict of port information. This lets the ASRT bridge create a spanning tree with IBM 8209 and SR-TB bridges along with other STBs and SRT bridges.

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