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? B B 172.16.14. 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 HQ C C 172.16.14.32/27 172.16.14.96/27 A A 172.16.14.132/30 172.16.14.136/30 172.16.14.140/30

10. 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 Table 172.16.0.0/16 Routing Table 172.16.25.0/24 172.16.26.0/24 172.16.27.0/24 172.16.26.0/24 172.16.25.0/24 A B

11. 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 192.168.8.0/21 192.168.15.0/24 192.168.8.0/24 192.168.9.0/24 H HQ B

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

13. Autonomous Systems Autonomous System 100Autonomous 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

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

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 Yes No Process packet normally Determine interface Process entry according to filter configuration Is there a filter for this interface? End Is there an entry for this address? No Routing update Yes Drop packet

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

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

23. 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?

24. –Routes are learned from another routing protocol when a router redistributes the information between the protocols IP Routing Table I192.168.5.0 I172.16.1.0 I172.16.2.0 I172.16.3.0 IP Routing Table D EX172.16.0.0 D192.168.5.8 D 192.168.5.16 D 192.168.5.24 S1 Advertises Routes from EIGRP to IGRP S0 Advertises Routes from IGRP to EIGRP AS 200 IGRP 172.16.0.0 S0S1 AS 300 EIGRP 192.168.5.0 What Is Redistribution? ASBR A B C

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

26. Summary