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IP Routing Basics

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IP Routing Basics

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  1. IP Routing Basics BXR-48000 Switch Router

  2. Objectives • Explain the concepts of IP routing • Describe the similarities and differences among OSPF, IS-IS and BGP • Explain routing policy configuration • Describe the handling of routing information on the BXR-48000

  3. General IP Routing Concepts • Internetworking occurs among Autonomous Systems (AS) • An Interior Gateway Protocol (IGP) operates within the AS to distribute IP destination information • An Exterior Gateway Protocol (EGP) operates from AS to AS to distribute IP destination information • Each router determines the best next hop for an IP packet • Each router uniquely identified (router ID, system ID)

  4. + IGP IGP EGP EGP EGP IGP IGP IP Routing Architecture Service Providers AS 20 AS 30 AS 40 AS 10 Subscribers

  5. OSPF Overview • An IGP using Shortest Path First algorithm to select best route to known destinations • Fast convergence of a link state database • Runs directly over IP • Supports Classless Inter-domain Routing (CIDR) • Allows the AS to be divided into areas • Discovers neighbors and forms adjacencies

  6. OSPF Architecture Area0.0.0.0 AS BoundaryRouter Area BorderRouter Area BorderRouter Area0.0.0.5 Area0.0.0.1 Autonomous System

  7. IS-IS Overview • An IGP originally for ISO protocol family • Modified to route IP with RFC 1195 • Hierarchy formed with Level 2 backbone and Level 1 areas • Separate Level 1 and Level 2 link state databases contributes to scalability • Employs the Shortest Path First (SPF) algorithm • Discovers neighbors, forms adjacencies, floods information according to Level

  8. IS-IS Architecture L1 L1 Autonomous System Level 1 Area L1L2 Level 2 Backbone Level 1 Area L2 Level 1 Area L1 L1L2 L1L2 L1

  9. BGP Overview • BGP exchanges network reachability information for specific network prefixes • Additional attributes are communicated to assist in determining the best route to a particular destination • The information exchange is explicitly defined as a point-to-point link • Peer is configured, not discovered • Peer relationship proceeds through defined states Tied to a TCP session R1 R2

  10. BGP Architecture • Originally designed to work between Autonomous Systems, which are registered by number • Developed into external and internal versions • EBGP – AS to AS • IBGP – within an AS • Provides tight control of information flow with limited overhead InternalBGP ExternalBGP AS 10 AS 30 AS 20 ExternalBGP

  11. BGP Activity • Establish connections • Describe routes with specific prefix lengths • Name available routes • Withdraw routes no longer active • Indicate the route’s origin • Express preference among multiple routes • List the Autonomous Systems traversed • Indicate next hop address • Identify aggregated prefixes • Report errors • Keep session alive

  12. System Route Use • System Control Processor handles these non-real time tasks • Each protocol marks its best route for each destination • Routing algorithm selects one route per destination • Selected routes are sent to the forwarding table, the table is copied to the port cards; forwarding decisions are made at the port cards OSPFRoutes StaticRoutes IS-ISRoutes RoutingTable DirectRoutes BGPRoutes ForwardingTable Port cards Port cards

  13. Routing Policy Control • What to do with all this information? BGP Advertisements OSPF LSAs Static Routes ---------- ---- ----- ------ ---------- ---- ----- ------ ---------- ---- ----- ------ ---------- ---- ----- ------ Direct Routes

  14. Destination Protocol OSPF IS-IS BGP Static block block block Direct block block block OSPF announce block block Source Protocol IS-IS block announce block I-BGP block block announce E-BGP block block block Default Policy Behavior • With no other configuration, the BXR-48000’s route treatment follows this chart • Anything else requires configuration

  15. Defining IP Routing Policies • In order to control the handling of routing information, the system has a Routing Policy Configuration Language (RPCL) shell • The shell permits the user to define behavior rules to enforce policies in the network • The language is based on RPSL, defined in RFC 2622 (and expanded upon in informational RFC 2650) • The shell has commands for inserting or removing policies, as well as loading and saving policy configuration files which are stored in the SCP’s flash file system

  16. RPCL in a Nutshell • Enter the shell by typing rpcl at the AMI prompt • The table is the location of the rules, so we insert new rules or remove others • table insert – add a rule • table remove – delete a rule • General syntax is [verb] [policy]-[from protocol]-[to protocol] • table insert export-direct-ospf • Command syntax is followed by a {macro} or specific action • {announce any}sends all routes to the target protocol • {announce {3.3.3 /24}} sends only one specific route • table insert export-direct-ospf {announce any} • table insert import-bgp {from accept any} • This syntax is followed by a line number to order the rules • table insert export-direct-ospf {announce any} 25

  17. Make the Changes Effective • All rule changes remain idle until the changes commit command is issued • Then changes affect the table • The table is used immediately but is not permanently stored • To make changes permanent, save the file to flash under the name rpcl.cfg • This file is read at boot • save rpcl.cfg • To exit the shell, type exit

  18. Deciding Among Multiple Routes • Route type listed with preference • Lower value is a higher preference • Direct routes (0) • Static routes (1) • OSPF routes (96) • IS-IS routes (120) • BGP routes (224) Protocol preferences can be modified with routing policies

  19. Summary • Explained the concepts of IP routing • Described the similarities and differences among OSPF, IS-IS and BGP • Explained routing policy configuration • Described the handling of routing information on the BXR-48000