Understanding Linux Network Internals

Topology Changes

A topology change is an event that changes which systems are on an L2 network, or how their ports are connected. When the topology changes, an address that used to be reachable through a given path may now be reachable through a different one. So a change in the topology must be handled properly to keep the network loop free and update the forwarding databases. In terms of graphs and trees, the topology changes when you add or remove a link, or select a different node as the tree's root (remember that the spanning tree is calculated based on the selection of the tree's root).

Let's first see the events that trigger a topology change, and then how they are handled:

A nonforwarding bridge port changes state to forwarding, or vice versa

This case includes a disabled port that is enabled and a port that simply changes state due to a protocol decision. From a tree's perspective, this is equivalent to adding a link to the tree or removing one.

The root bridge ID changes

This can happen, for example, because the current root bridge has been shut down (and therefore another one has taken over the root role), or because a better one has been enabled, or because either the current root bridge or another bridge has changed its priority. A change of the root bridge can, depending on where the new root bridge is located with respect to the old one, trigger quite a few changes of port state and roles all over the network. In theory, a change of root node can produce a very different ...