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? B B /27 –Subnet /24 is divided into smaller subnets: Subnet with one mask at first (/27) Further subnet one of these subnets not used elsewhere (/30) / / /24 HQ C C / /27 A A / / /30

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

What Is CIDR? / / /24 –Networks /24 through /24 are summarized by HQ in one advertisement /21 A / / / /24 H HQ B

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

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

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 AS E ISP AS A ISP AS C B

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 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

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 Table I I I I IP Routing Table D EX D D D S1 Advertises Routes from EIGRP to IGRP S0 Advertises Routes from IGRP to EIGRP AS 200 IGRP S0S1 AS 300 EIGRP What Is Redistribution? ASBR A B C

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

Summary