Ccnp

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.

This day, we’ll see how to think of summarizable address blocks in a slightly different way. And then we’ll see how this all applies to configuring route summarization with OSPF.

Please also read the previous article  OSPF — Part III

Thinking About How Big Blocks of Addresses Are

Let’s start by getting our bearings as far as size of address blocks. A Class C network address is a /24, with subnet mask 255.255.255.0. Let’s draw that as a “basic sized” rectangle, as shown in the figure.

Introduction

The OSPF routing protocol is playing an increasingly important role in modern networks. It is a multi-vendor standard. We’ll take a look at OSPF basics in this article.

Running OSPF so your routers can exchange information with NT servers is probably not a Good Thing. I personally don’t want servers sending routing information to my “official routing protocol”, since I’ve seen too many servers with duplicate IP addresses or other oddities. If a server administrator goofs and duplicates the server farm subnet on some workgroup server somewhere else, and it advertises it into your network, you’re in for a serious outage! If your group controls the servers, or if NT servers are acting as low-end routers at remote sites, well, then there’s a reason to run OSPF. I’ll just mention in passing here that OSPF has an authentication feature that allows you to easily exclude any “unofficial” routers from your OSPF Autonomous System (“group of consenting routers”).