Implementing IBM System Networking 10Gb Ethernet Switches

58 Implementing IBM System Networking 10Gb Ethernet Switches

2.2 Spanning Tree Protocol

In high-availability environments, a redundant design is often introduced to minimize any

network downtime. The redundancy is implemented on many layers, from physical cabling to

redundant switches, to ensure continuous operations.

For more information about network availability protocols and technologies, see 2.7, “High

availability” on page 77.

In a redundant multi-path network, Ethernet broadcast and unknown unicast flooding

mechanisms can lead to forwarding loops.

In Figure 2-6, imagine a situation where Server A wants to communicate with Server B, and

the MAC address of Server B is unknown.

The frame with the source MAC address of Server A’s NIC arrives at SW-1, and because the

destination MAC address (Server B’s NIC MAC address) is unknown, the frame is flooded out

on all links to SW-3 and SW-4. SW-3 and SW-4 and all of these switches follow the behavior

of forwarding the frame out of all their interfaces and the frame finally arrives at SW-1 and is

flooded again.

Figure 2-6 Redundant Ethernet environment

This unwanted behavior can lead to the depletion of switch resources and the network

becoming non-operational. A mechanism is needed to prevent forwarding loops in a switched

Ethernet environment. The mechanism is part of the IEEE 802.1d standard and is known as

the Spanning Tree Protocol (STP).

Aggregation Layer

Access Layer

Server A Server B

SW-1

SW-2

SW-3 SW-4