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Mobile Backhaul by Esa Metsala, Juha Salmelin

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7.2 Native Ethernet and Resilience

Native Ethernet concepts, MAC address learning, flooding of unknown unicast frames, and spanning tree based resilience are intended for the local area. Correspondingly, Carrier Ethernet and resilience in Metro and Wide area is addressed separately. Why would we then discuss native Ethernet further?

First, BTS or controller sites may deploy redundant L2 switches. Within this site LAN, native Ethernet functionality may be needed. One reason is cost: cost of an Ethernet port in a L2 bridge is typically lower than the cost of an Ethernet port in a router. And redundant switches are needed for high availability.

Similarly, as e.g. E-LAN service (multipoint) emulates a L2 bridge, it is useful to review how L2 bridging works. Rapid spanning tree protocol (RSTP) is also included in the MEF definitions as a possibility for supporting resilience.

7.2.1 Ethernet Bridging

At the Ethernet layer (L2), native Ethernet bridging relies on MAC address learning, flooding of unknown unicast traffic, and spanning tree protocol for ensuring a loop free topology. With this approach, a single active topology exists, and load sharing is not possible. In a redundant L2 topology, restoration occurs by spanning tree calculating a new loop-free L2 topology. The result is a tree topology (hub-and-spoke): Frames from all stations (leaves) pass through the hub (the root of the tree).

With L2 bridging, there has to be only a single L2 forwarding path at a time. If two Ethernet ...

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