Understanding Linux Network Internals

Forwarding Database

Each bridge instance has it own forwarding database , which is used regardless of whether STP is enabled or disabled. We will see later in this chapter exactly when the database is consulted and updated. Let's first see its implementation and the core functions for manipulating it. All of the routines used to manage forwarding databases are located in net/bridge/br_fdb.c.

The database is embedded in the net_bridge data structure and is defined as a hash table (see Figure 16-6). An instance of a net_bridge_fdb_entry data structure is added to the database for each MAC address that is learned on any of the bridge's ports.

The bridge forwarding database subsystem is initialized with br_fdb_init, which simply creates the br_fdb_cache cache that will be used for the allocation of net_bridge_fdb_entry instances.

Allocations are done with fdb_create, which also initializes a few fields of net_bridge_fdb_entry according to its input parameters.

Lookups

Elements of the forwarding database are identified by their MAC addresses. A lookup in the table consists of selecting the right hash table bucket with br_mac_hash and browsing the bucket's list of net_bridge_fdb_entry instances to find one that matches a given MAC address.

There are two main lookup routines:

fdb_find

This simply searches net_bridge_fdb_entry for a given MAC address. It is not used to forward data traffic. It is mainly used by bridging management functions.

_ _br_fdb_get

Similar to fdb_find, this is called by ...