PMP450 Bridge Learning App Note

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What is the purpose of Ethernet Bridge Learning?

To conserve resources by preventing unwanted traffic flooding on a multiport Ethernet Bridge. For example, if a bridge already learned that a MAC address (MAC‐A) is sourced from certain port (PORT‐A), it will forward all the traffic targeting MAC‐A to Port‐A, rather than flooding all the ports with the packets that only targeting MAC‐A.

What is the purpose of Ethernet Bridge Learning of PMP450?

The key purpose is to conserve the precious RF resource so that the radio link don’t get flooded from unwanted traffic.

PMP450 Bridging Explained

The following diagram showed a simplify diagram of PMP450 AP and SM from Ethernet Bridging’s perspective.

The PMP450 AP is connected to an Ethernet Switch at the core network side connected to two SMs from the RF side. For each SM, a virtual port is allocated.

The SM has two ports, the RF port connects to the AP while the LAN port connects to the end user’s home network (e.g. a HUB or switch with multiple user devices attached to it).

The AP and the SMs all maintain a MAC address forwarding table that can store up to 4096 MAC addresses.

While one might expect the Ethernet Bridge to learn and store the MAC address for traffic sourcing from all directions, there is no need for that. At the end of the day, the key purpose of learning is to conserve the RF resource to prevent unwanted flooding of the Ethernet traffic to the RF link.

PMP450 AP RF side (virtual port) MAC learning

Source address of the traffic coming from the radio side will be learned. (Note the mac table learn from the virtual ports is used to deal with egress traffic targeting the RF side, deciding whether to drop the packets or forward them to the appropriate SMs.)

By default, the AP will drop any packet coming from its LAN port, if the target MAC address of the packet is not already learned. However, the operator can enable bridge flooding so that even packet with unknown target MAC address will be sent over the air. But this would be a waste of RF resource. So, learning on this side prevents unwanted flooding of the traffic to the air – packet only with known MAC address (targeting the SMs) will be forwarded.

PMP450 AP LAN side (LAN Port) MAC learning

If learning is implemented for this part, source address of the traffic coming from the LAN/Network Core side should be learned.

MAC address table learned here is to deal with egress traffic targeting the LAN side, deciding whether to drop the packet or forward it.

The PMP450 is designed NOT to learn about the MAC address of the traffic coming from the Network Core. Here’s why.

Assume the AP receives a packet from the radio side. It has three options: 1. Drop it, 2. Send it to another virtual port to another SM, 3. Forward it up to the LAN port. If the AP already knows that the packet is to send to another SM (by the MAC table learned on the RF side), it will forward the packet to the target SM (if isolation is NOT enabled). If the AP cannot find the MAC address from the MAC table, we don’t want the AP to drop the packet, but rather, the default behavior is to forward the packet to the LAN port. Otherwise, the SM may have trouble initiating a TCP/IP communication with the core network side/Internet.

There is no benefit for the AP to perform learning for the LAN port side, the desire is to always forward the traffic coming from the radio side to the LAN port. In addition, not learning the downlink direction also preserves the bridge table space.

PMP450 SM LAN side (LAN Port) MAC learning

Source address of the traffic coming from the SM LAN side should be learned.

The PMP450 SM LAN side MAC learning is implemented so that unwanted traffic could be filtered from being forwarded to the RF side. Suppose user device 1 is trying to send a packet to user device 2. The packet will be forwarded by the HUB to the SM (SM1), since the HUB does not learn and so it will just flood whatever traffic to all ports. If the SM already learned that user device 2 is also on its LAN port side, it will just drop the packet so it will not waste the RF resource by unnecessarily forwarding the packet to the AP.

PMP450 SM RF side MAC learning

If learning is implemented for this part, source address of the traffic coming from the AP side will be learned.

Note the mac table learned (if we do learn) from the RF port is used to deal with egress traffic targeting the AP, deciding whether to drop the packets or forward them to the AP.

Learning of on this port is not needed, because if the SM already receives a packet from its LAN port and the packet is not filter (based on its MAC learning table), its only choice is to forward it up to the AP.

Also notice that ingress traffic coming from the RF port will forwarded to the LAN port of the SM.

In summary, we learn in the Uplink direction (SM‐to‐AP) only so that we will only forward downstream packets to known destinations to preserve RF spectrum as well as to preserve bridge table entries of uplink entities.

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