Chapter 9. Implementing Scalability Features in Your Internetwork

1. Chapter 9.Implementing Scalability Features in Your Internetwork

2. Table of Contents • Routing Principles • Extending IP Addressing Space • Connecting to ISPs • Controlling Routing Updates and Policies • Route Redistribution • Summary

3. Routing Principles • Routing Defined • Classful Routing • Classless Routing

4. 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

5. 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

6. 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

7. Extending IP Addressing Space • IP Addressing Solutions • VLSM Overview • Route Summarization Overview • CIDR Overview

8. 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

9. What Is a Variable-Length Subnet Mask? 172.16.14.32/27 A 172.16.14.132/30 172.16.1.0/24 172.16.14. 64/27 172.16.14.136/30 B 172.16.0.0/16 HQ 172.16.2.0/24 • 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) 172.16.14.96/27 C 172.16.14.140/30

10. What Is Route Summarization? 172.16.25.0/24 I can route to the 172.16.0.0/16 network. 172.16.26.0/24 A B Routing Table 172.16.0.0/16 172.16.27.0/24 Routing Table 172.16.25.0/24 172.16.26.0/24 172.16.27.0/24 • Routing protocols can summarize addresses of several networks into one address

11. 192.168.15.0/24 192.168.8.0/21 192.168.8.0/24 192.168.9.0/24 What Is CIDR? 192.168.8.0/24 A B 192.168.9.0/24 HQ • Networks 192.168.8.0/24 through 192.168.15.0/24 are summarized by HQ in one advertisement 192.168.8.0/21 192.168.15.0/24 H

12. Connecting to ISPs • Autonomous Systems • BGP Characteristics • BGP Route Selection Decision Process • BGP Multihoming

13. Autonomous Systems IGPs: RIP, IGRP, OSPF, EIGRP EGPs: BGP Autonomous System 100 Autonomous System 200 • 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

14. 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

15. 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

16. Multihoming Example E B C A 172.25.0.0 AS 65500 172.20.0.0 172.30.0.0 ISP AS 65000 ISP AS 65250 10.10.10.2 10.10.20.1 10.10.20.2 10.10.10.1 AS 64500

17. Controlling Routing Updates ad Policies • Route Filters with Distribute Lists • Route Maps • Policy-Based Routing • BGP Policy Control

18. Route Filters with Distribute-List Routing update Yes Yes Is there a filter for thisinterface? Process entry according to filterconfiguration Is there anentry for thisaddress? Determine interface No No End Process packet normally Drop packet End

19. 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

20. 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

21. BGP Policy Control • To restrict routing information to and from BGP neighbors use • Distribute lists (using access lists), or • Prefix lists

22. Route Redistribution • When to Use Multiple Routing Protocols • Redistribution Overview • Redistribution Implementation Guidelines

23. When Do You Use Multiple Routing Protocols? • 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

24. B C What Is Redistribution? ASBR AS 200IGRP172.16.0.0 AS 300EIGRP192.168.5.0 S1 S0 A IP Routing Table IP Routing Table S1 Advertises Routes from EIGRP to IGRP I 192.168.5.0 D EX 172.16.0.0 I 172.16.1.0 • Routes are learned from another routing protocol when a router redistributes the information between the protocols D 192.168.5.8 I 172.16.2.0 S0 Advertises Routes from IGRP to EIGRP 192.168.5.16 D I 172.16.3.0 192.168.5.24 D

25. Redistribution Implementation Guidelines IGRP/OSPF Redistribute IGRP OSPF Default or Static Redistribute IGRP OSPF Redistribute and Filter or Change Administrative Distance

26. Summary