Chapter 9.Implementing Scalability

Features in Your Internetwork

Table of Contents

• Routing Principles• Extending IP Addressing Space• Connecting to ISPs• Controlling Routing Updates and

Policies• Route Redistribution• Summary

Routing Principles

• Routing Defined• Classful Routing• Classless Routing

What Is Routing?

•Routing is the process of forwarding an item from one location to another

•Routers forward traffic to a logical destination in a computer network

•Routers perform two major functions:– Routing

• Learning the logical topology of the network

– Switching• Forwarding packets from an inbound interface to an

outbound interface

Classful Routing

•Classful routing protocols are a consequence of the distance vector method of route calculation– RIPv1– IGRP

•Subnet masks are not carried within the routing updates

•Summary routes are automatically created at Class A, B, or C network boundaries

Classless Routing

•Classless routing protocols include the routing mask with the route advertisement– Open Shortest Path First (OSPF)– EIGRP– RIPv2– IS-IS– BGP

• Summary routes can be manually controlled within the network

Extending IP Addressing Space

• IP Addressing Solutions• VLSM Overview• Route Summarization Overview• CIDR Overview

IP Addressing Solutions

• Subnet masking, RFCs 950, 1812• Address allocation for private Internets,

RFC 1918

• Network Address Translation, RFC 1631

• Hierarchical addressing

• Variable-length subnet masks, RFC 1812

• Route summarization, RFC 1518

• Classless interdomain routing, RFCs 1518,

1519, 2050

What Is a Variable-Length Subnet Mask?

BB172.16.14. 64/27

– Subnet 172.16.14.0/24 is divided into smaller subnets:

• Subnet with one mask at first (/27)• Further subnet one of these subnets not used

elsewhere (/30)

172.16.0.0/16

172.16.1.0/24

172.16.2.0/24

HQHQ

CC

172.16.14.32/27

172.16.14.96/27

AA 172.16.14.132/30

172.16.14.136/30

172.16.14.140/30

What Is Route Summarization?

– Routing protocols can summarize addresses of several networks into one address

I can route to the 172.16.0.0/16 network.

Routing Table172.16.0.0/16Routing Table

172.16.25.0/24172.16.26.0/24172.16.27.0/24

172.16.27.0/24

172.16.26.0/24

172.16.25.0/24

A B

What Is CIDR?

192.168.8.0/24

192.168.9.0/24

192.168.15.0/24

– Networks 192.168.8.0/24 through 192.168.15.0/24 are summarized by HQ in one advertisement 192.168.8.0/21

A

192.168.8.0/21 192.168.8.0/21

192.168.15.0/2

4 192.168.15.0/2

4

192.168.8.0/

24 192.168.8.0/

24

192.168.9.0/24 192.168.9.0/24

H

HQ

B

Connecting to ISPs

• Autonomous Systems• BGP Characteristics• BGP Route Selection Decision Process• BGP Multihoming

Autonomous Systems

Autonomous System 100 Autonomous System 200

IGPs: RIP, IGRP, OSPF, EIGRP EGPs: BGP

• An autonomous system (AS) is a collection of networks under a a single technical administration

• IGPs operate within an autonomous system• EGPs connect different autonomous systems

BGP Characteristics

•BGP is a distance vector protocol with enhancements:– Reliable updates—BGP runs on top of TCP (port

179)– Incremental, triggered updates only– Periodic keepalives to verify TCP connectivity– Rich metrics (called path vectors or attributes)– Designed to scale to huge internetworks

BGP Route Selection Decision Process

• Consider only (synchronized) routes with no AS loops and a valid next-hop, and then:

– Prefer highest weight (local to router)– Prefer highest local preference (global within AS)– Prefer route originated by the local router– Prefer shortest AS-path– Prefer lowest origin code (IGP < EGP < incomplete)– Prefer lowest MED (from other AS)– Prefer EBGP path over IBGP path– Prefer the path through the closest IGP neighbor– Prefer the path with the lowest neighbor BGP

router ID

Multihoming Example

AS 64500

10.10.10.110.10.20.2

AS 65500

172.25.0.0

E

ISP

AS 65000 10.10.10.2

172.20.0.0

A

10.10.20.1

172.30.0.0ISP

AS 65250

CB

Controlling Routing Updates ad Policies

• Route Filters with Distribute Lists• Route Maps• Policy-Based Routing• BGP Policy Control

Route Filters with Distribute-List

Yes

No

Process packet normally

Determineinterface

Process entry according to filter

configuration

Is there a filter for thisinterface?

End

End

Is there anentry for this

address?

No

Routingupdate

Yes

Drop packet

Route Maps

– Filter for network advertisements– Offer detailed control over

advertisements– Complex access lists

•Complex conditional advertisement via match command

•Changes routing table parameters via set command

Policy-Based Routing

– Allows you to implement policies that selectively cause packets to take different paths

– Can also mark traffic with different TOS

– Since Cisco IOS Release 11.0

– Applied to incoming packets– Implemented using route maps

• To restrict routing information to and from BGP neighbors use

– Distribute lists (using access lists), or

– Prefix lists

BGP Policy Control

Route Redistribution

• When to Use Multiple Routing Protocols• Redistribution Overview• Redistribution Implementation

Guidelines

•Interim during conversion•Application-specific protocols

•One size does not always fit all

•Political boundaries•Groups that do not work and play nicely

with others

•Mismatch between devices•Multivendor interoperability•Host-based routers

When Do You Use Multiple Routing Protocols?

– Routes are learned from another routing protocol when a router redistributes the information between the protocols

IP Routing TableI 192.168.5.0I 172.16.1.0I 172.16.2.0I 172.16.3.0

IP Routing Table

D EX 172.16.0.0D 192.168.5.8D 192.168.5.16D 192.168.5.24

S1 Advertises Routes from EIGRP to IGRP

S0 Advertises Routes from IGRP to EIGRP

AS 200IGRP172.16.0.0

S0S1 AS 300EIGRP192.168.5.0

What Is Redistribution?

ASBR

ABC

Redistribution Implementation Guidelines

IGRP/OSPF

IGRP OSPFRedistribute

Default or Static

IGRP OSPFRedistribute

Redistribute and Filter or Change Administrative Distance

Summary