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Bridge Apps

A bridge app implements a basic Ethernet bridge with split-horizon semantics. It has an arbitrary number of ports. For each input port there must exist an output port with the same name. Each port name is a member of at most one split-horizon group. If it is not a member of a split-horizon group, the port is also called a free port. Packets arriving on a free input port may be forwarded to all other output ports. Packets arriving on an input port that belongs to a split-horizon group are never forwarded to any output port belonging to the same split-horizon group. There are two bridge implementations available: apps.bridge.flooding and apps.bridge.learning.

DIAGRAM: bridge
port1 ---->*          *----> port1
           |          |
port2 ---->*          *----> port2
           |  bridge  |
...   ---->*          *----> ...
           |          |
portN ---->*          *----> portN


A bridge app accepts a table as its configuration argument. The following keys are defined:

— Key ports

Optional. An array of free port names. The default is no free ports.

— Key split_horizon_groups

Optional. A table mapping split-horizon groups to arrays of port names. The default is no split-horizon groups.

— Key config

Optional. The configuration of the actual bridge implementation.

Flooding bridge (apps.bridge.flooding)

The flooding bridge app implements the simplest possible bridge, which floods a packet arriving on an input port to all output ports within its scope according to the split-horizon topology.


The flooding bridge app ignores the config key of its configuration.

Learning bridge (apps.bridge.learning)

The learning bridge app implements a learning bridge using a custom hash table to store the set of MAC source addresses of packets arriving on each input port. When a packet is received it is forwarded to all output ports whose corresponding input ports match the packet's destination MAC address. When no input port matches, the packet is flooded to all output ports. Multicast MAC addresses are always flooded to all output ports associated with the input port. The scoping rules according to the split-horizon topology apply unchanged.


The learning bridge app accepts a table as the value of the config key of its configuration. The following keys are defined:

— Key mac_table

Optional. This is a table that defines the characteristics of the MAC table. The following keys are defined

— Key size

Optional. The number of MAC addresses to be stored in the table. Default is 256. The size of the table is increased automatically if this limit is reached or if an overflow in one of the hash buckets occurs. This value is capped by resize_max.

— Key timeout

Optional. Timeout for learned MAC addresses in seconds. Default is 60.

— Key verbose

Optional. A boolean value. If true, statistics about table usage is logged during each timeout interval. Default is false.

— Key copy_on_resize

Optional. A boolean value. If true, the contents of the table is copied to the newly allocated table after a resize operation. Default is true.

— Key resize_max

Optional. An upper bound for the size of the table. Default is 65536.