Cisco

What is a LAN Switch?

Logically, we really need to start with the question, what is a LAN switch?

A LAN switch is a OSI model Layer 2 device, logically similar to a bridge. We’ll primarily focus on Ethernet (Fast Ethernet, Gigabit Ethernet) switches but much of what we say also applies to Token Ring and FDDI.

As you know, bridges learn where computers are by observing source MAC addresses in LAN frames as they pass through the bridge. After first powering on, a bridge forwards frames on all ports and is pretty much indistinguishable from a hub/repeater (except that it regenerates frames, so it does not count towards the Ethernet cable length limitation or hub/repeater 5-4-3 rule). As the bridge learns which MAC address was sourced on which port, it can then be selective about what port(s) a frame is forwarded to, only forwarding the frame to the port appropriate for the destination MAC address.

Introduction

This article follows up on last month’s article, Configuring SNMP in Cisco Routers. We’ll talk briefly about SNMP and switches, and also about Syslog.

previous article  Configuring SNMP in Cisco Routers

Syslog

Syslog is a UNIX process that handles error and status message logging on behalf of system and other processes (daemons). The Event Viewer subsystem on NT is a direct NT counterpart. And there are also free (and not-so-free) syslog daemons now available for NT. For example, Cisco Resource Manager Essentials  includes a syslog message receiver, and syslog reporting functionality.

Introduction

From doing this network management work, it is apparent that Cisco has been adding a little here, a little there, to the SNMP capabilities of the routers and switches. Furthermore, my previous articles on Configuration for Manageability only scratch the surface of the topic. So it seems like it might be useful to us all to take a look at Cisco SNMP features and what they buy us in the way of router/switch manageability. There’s enough to cover that this will be (at least) a two-article series.

Introduction

This month we continue looking at Quality of Service (QoS) in Cisco devices.

This article looks at

• Traffic Shaping
• Policy Routing
• QoS Policy Propagation via BGP (QPPB)

Traffic Shaping

About Traffic Shaping

Traffic Shaping comes in two forms: Generic Traffic Shaping and Frame Relay Traffic Shaping.

Traffic Shaping allows you to control how fast packets are sent out an interface, any interface. You might want to do this to avoid congestion either locally or elsewhere in your network, for example if you have a network with different access rates or if you are restricting some traffic to a fraction of the available bandwidth. For example, if one end of the link in a Frame Relay network is 256 Kbps and the other end of the link is only 128 Kbps, sending packets at 256 Kbps at the very least causes congestion. Somewhere.

In this article, we’ll look at the following topics:

• IP Precedence bits
• Random Early Detection (RED)
• Committed Access Rate (CAR)
• Some Cisco QoS Links

The IP Precedence bits allow us to specify what traffic gets what Class of Service. RED lets us selectively drop TCP packets to throttle the sender, providing better goodput. And CAR is input-side traffic admission control, allowing us to police inbound traffic.

IP Precedence Bits

Part of the new IP QoS tool kit in routers and switches is the IP Precedence bits. Devices at the edge of the network may classify traffic as deserving a certain Class of Service using these bits. Core devices can then use the bits to provide differing types of service to different flavors of traffic.