Ccna

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.

What Is QoS?

QoS stands for Quality of Service. The term is being used by Cisco to refer to IP-based features that allow specification and delivery of services much like the Quality of Service features in ATM.

When you get right down to it, there isn’t all that much a router can do to control traffic, since it is not the originator of most of the traffic. The router can drop traffic — although we’d prefer it didn’t do so. It can put some queued frames out an interface before others. It can be selective about accepting traffic — another form of dropped traffic. And, with TCP, it can selectively drop the occasional packet as an indirect signal to slow down. With cooperative hosts, the router can try to accept reservations and hold bandwidth for applications that need it.

OSPF Network Type

Please also read the previous article  OSPF and Route Summarization– Part II

OSPF has three network types:

• Broadcast networks (Ethernet, Token Ring, FDDI)
• Nonbroadcast multiaccess networks (SMDS, Frame Relay, X.25, Classic IP Over ATM)
• Point-to-point networks (HDLC, PPP)

You can configure an interface as any type you wish.

Broadcast is typically used on networks where broadcast and multicast are supported. The idea is to send out one OSPF multicast that reaches multiple receivers. This can be appropriate with X.25 and Frame Relay if you have a full mesh of PVC’s. Loss of a PVC can cause problems with OSPF broadcast networks, however, since some routers with a common subnet will no longer be able to transmit directly to each other.