Understanding Linux Network Internals

Spanning Tree Protocol (STP)

We saw in Chapter 15 how the STP works. In this chapter, we will mainly see how:

• Ingress BPDUs are processed

• Egress BPDUs are transmitted

• Timers are handled

Key Spanning Tree Routines

Here is a list of the key routines used by the spanning tree code to implement the logic described in Chapter 15:

br_become_root_bridge

br_is_root_bridge

br_become_root_bridge makes a nonroot bridge the root bridge. This task consists of stopping the TCN timer, because it should not run on the root bridge, and starting the Hello timer, which runs only on the root bridge. The function also updates other timers to locally configured values, and starts a topology change. br_is_root_bridge returns 1 when the input bridge is the root bridge, and 0 otherwise.

br_should_become_designated_port

br_designated_port_selection

br_become_designated_port

br_is_designated_port

br_is_designated_for_some_port

br_should_become_designated_port returns 1 if the input port should be assigned the designated role, and 0 otherwise. br_designated_port_selection loops over all the bridge ports and it assigns the designated role to those that deserve it (see the section "Designated Port Selection" in Chapter 15).

br_become_designated_port assigns the designated role to a bridge port. br_is_designated_port returns 1 when the input port is a designated port, and 0 otherwise. Given a bridge, br_is_designated_for_some_port returns 1 if the bridge has at least one port with the designated role, and 0 otherwise. ...