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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? 172.16.14.32/27 172.16.14. 64/27 A B HQ 172.16.14.96/27 172.16.0.0/16 C – 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) 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 172.16.27.0/24 Routing Table 172.16.25.0/24 172.16.26.0/24 172.16.27.0/24 B Routing Table 172.16.0.0/16 – Routing protocols can summarize addresses of several networks into one address What Is CIDR? 192.168.8.0/24 192.168.9.0/24 A B 192.168.9.0/24 192.168.15.0/24 192.168.8.0/21 HQ H – Networks 192.168.8.0/24 through 192.168.15.0/24 are summarized by HQ in one advertisement 192.168.8.0/21 Connecting to ISPs • • • • Autonomous Systems BGP Characteristics BGP Route Selection Decision Process BGP Multihoming 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 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 172.25.0.0 AS 65500 ISP AS 65000 E 172.20.0.0 172.30.0.0 B C 10.10.20.1 10.10.10.2 10.10.20.2 10.10.10.1 A AS 64500 ISP AS 65250 Controlling Routing Updates ad Policies • • • • Route Filters with Distribute Lists Route Maps Policy-Based Routing BGP Policy Control Route Filters with Distribute-List Routing update Determine interface Is there a filter for this interface? No Process packet normally End Yes Is there an entry for this address? Yes Process entry according to filter configuration No End 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 BGP Policy Control • To restrict routing information to and from BGP neighbors use – Distribute lists (using access lists), or – Prefix lists Route Redistribution • When to Use Multiple Routing Protocols • Redistribution Overview • Redistribution Implementation Guidelines 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 What Is Redistribution? ASBR AS 200 IGRP 172.16.0.0 IP Routing Table I 192.168.5.0 I 172.16.1.0 I 172.16.2.0 I 172.16.3.0 S1 C S0 A B S1 Advertises Routes from EIGRP to IGRP S0 Advertises Routes from IGRP to EIGRP AS 300 EIGRP 192.168.5.0 IP Routing Table D EX D D D 172.16.0.0 192.168.5.8 192.168.5.16 192.168.5.24 – Routes are learned from another routing protocol when a router redistributes the information between the protocols Redistribution Implementation Guidelines IGRP/OSPF IGRP Redistribute OSPF Default or Static IGRP Redistribute OSPF Redistribute and Filter or Change Administrative Distance Summary