Chapter 10: Layer 2 Redundant Links; In Depth; Layer 2 Switching Overview; Frames - Cisco RJ-45-to-AUX Brochure

Chapter 10: Layer 2 Redundant Links

In Depth

If your company has ever experienced a critical work stoppage due to a downed server or a network outage,
you understand how critical it is to implement redundancy in your network. No matter what happens with an
individual link in your network, the other links should take over using redundant links.
The Spanning−Tree Protocol (STP) provides the components needed to ensure consistent network availability
when a problem occurs with a link in the network. Not only can you use these redundant links in the case of a
network failure, but—because your company is paying to implement the redundant links—it makes sense to
load balance over those links and use them to handle twice the traffic load when they are operating correctly.
This chapter will concentrate on using Layer 2 redundant links as well as using STP to block redundant links
to prevent data loops from forming in the network. Layer 3 devices, which are typically routers, give the
routing protocols the responsibility for making sure routing loops do not occur in the network.

Layer 2 Switching Overview

Here's a quick reminder of how Layer 2 switching works. The Open System Interconnection (OSI) Layer 2
uses the Network Interface Card's (NIC) physical address, called the Media Access Control (MAC) address,
which is assigned to the card by the vendor of the card. In Chapter 2, I covered how Application Specific
Integrated Circuits (ASICs) are used to build the switches' Content Addressable Memory (CAM)
table—similar to a router's routing table—which is used to calculate and maintain loop−free paths through the
network.
At Layer 3, you use software to create filtering tables and make filtering decisions. Switches use hardware
ASIC chips to help make filtering decisions—filtering with hardware is much faster.
Both switches and bridges track the source hardware address of each frame received on each port and enter
this information in their filtering table. This table allows the bridge or switch to make a forwarding or filtering
decision for data it receives based on the information learned.
Bridges have up to 16 ports but only one instance of STP for all the ports. Cisco switches can have hundreds
of ports and an instance of STP running for each virtual LAN (VLAN). On some Cisco Catalyst switches,
thousands of instances of STP can be running on the network.
When a frame is received on a switch port, the switch looks at the destination address in the frame header and
compares the address with known source addresses the switch has learned to see if it knows the port the
destination resides on. If the address is known, the switch forwards the frame to the destination port. If the
address is unknown, the switch forwards the frame to all its ports, which are members of the same broadcast
domain as the arriving switch port. This broadcast domain consists of all the members of the same VLAN as
the port of arrival. This domain does not necessarily include just VLAN members on the switch of arrival—it
can include all the switches in the network that share the same VLAN number.

Frames

A switch will typically receive three types of frames without a specific destination. These frames in turn
become a broadcast and are flooded out all the ports except for the port of arrival. These frame types are:
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