The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

9.4. Issues in Link Aggregation

Compared to some of the other advanced switch features discussed in this book (for example, VLANs, Multicast Pruning, and Priority Operation), link aggregation is fairly straightforward. However, there are a few issues that you need to examine to fully understand the workings and limitations of aggregated links, including:

• Address assignment

• Transmission and distribution of frames across the links in an aggregation

• Constraints on the technologies employed in an aggregation

• Configuration management

9.4.1. Addressing

In a traditional, non-aggregated context, each network interface controller (NIC) has associated with it a globally unique 48-bit MAC address. This address is used for two purposes:

• As the unicast address for received frames (i.e., Destination Address comparison)

• As the Source Address in transmitted frames

Figure 9-3 depicts the model of how this unique address is configured and used in a typical NIC application. The NIC includes both a MAC controller (either a discrete integrated circuit or a function embedded into a larger, system-level component) and a ROM containing the unique address. The ROM is programmed ("burned") with the address at the time the NIC is built; the manufacturer is responsible for ensuring address uniqueness.^[]

^[] While early NICs often used a discrete ROM dedicated to storing the address, many products today store the address in a larger memory that may contain diagnostic code, a bootstrap loader, and other software. ...