Understanding Routing

Agenda What Is Routing? Network Addressing Routing Protocols

© 1999, Cisco Systems, Inc. What is Routing?

What Is Routing? Routing is: Finding a path between a source and destination (path determination) Moving information across an internetwork from a source to a destination (switching*) Very complex in large networks because of the many potential intermediate nodes A router is: A network layer device that forwards packets from one network to another and determines the optimal path for forwarding network traffic * The term “switching”, when used to describe a router’s function, is different from a switch (the network device).

Routing Table Frame Relay Ethernet FDDI Network FDDI Network Frame Relay Remote Location Network Ethernet Main Site Routers—Layer 3

LAN-to-LAN connectivity LAN-to-WAN connectivity Remote access Where are Routers Used?

LAN-to-LAN Connectivity Routers encapsulate and de-encapsulate data packets as they are transferred from system X to system Y XY A A B B C C Presentation Data Link Physical Data Link Physical A B C Data Link Physical Data Link Network Transport Session Presentation Application Physical Data Link Network Transport Session Application Physical Network

Path Determination Routers find the best path through the network Routing tables contain route information Network addresses represent the path of media connections to a destination Which Path?

Multiprotocol Routing IP IP IP Routing Tables IP Token Ring Token Ring AppleTalk AppleTalk Apple IPX 4b ab13 IPX 3a ab Novell DECnet 5.8 DECnet 10.1 DEC VAX

Routing Tables Routing algorithms Initialize and maintain routing tables to help with path determination Route information types Destination/next-hop associations Path desirability Vary depending on routing algorithm Message = Routing table maintenance communications Routing update messages Link-state advertisement To ReachSend Network:To: 27Node A 57Node B 17Node C 24Node B 11Node B 72Node A To ReachSend Network:To: 27Node A 57Node B 17Node C 24Node B 11Node B 72Node A

Routing Algorithm Goals Optimality Selecting the best route based on metrics and metric weightings used in the calculation Simplicity and low overhead Efficient routing algorithm functionality with a minimum of software and utilization overhead Robustness and stability Correct performance in the face of unusual or unforeseen circumstances (e.g., high load) Rapid convergence Fast agreement, by all routers, on optimal routes Flexibility Quick and accurate adaptation to changes in router availability, bandwidth, queue size, etc.

Routing Metrics Path length Total hop count or sum of cost per network link Reliability Dependability (bit error rate) of each network link Delay Useful because it depends on bandwidth, queues, network congestion, and physical distance Communication cost Operating expenses of links (private versus public) Bandwidth and load

Network Addressing

Network and Node Addresses Network Node Network address—Path part used by the router Node address—Specific port or device on the network 1 2 3

Addressing Examples NetworkNode/Host Protocol AddressAddress General1.4 TCP/IP Novell IPX1aceb0b0000.0c00.6e25 AppleTalk10.1. X.25DNICNTN NTN: National Terminal Number

INTERNET Subnetwork Addressing Manufacturing R&D Subdividing address space into smaller blocks Helps organize network Security (keeps HR separately addressable) Scalability—Keeps traffic to appropriate segments Allows single, summarized routing entry ( ) to be advertised to external networks Specific route entries ( ) required only for routers in the subnetted block HR

Routing Algorithm Types Single-path versus multi-path Flat versus hierarchical Host-intelligent versus router-intelligent Intradomain versus interdomain Static versus dynamic routing Link state versus distance vector

Static Routing “Stub” Network A B Manual table updates by a network administrator Benefits Reflects administrator’s special topology knowledge Private—Not conveyed to other routers in updates Avoids the overhead of dynamic routing Stub network When a node is accessible by only one path, a static route is sufficient Point-to-point or circuit-switched connection

Dynamic Routing A network change blocks the established path... A B C D X A B C D X …and an alternate route is found dynamically. Most internetworks use dynamic routing

Distance Vector versus Link State Distance vector Sends routing table info only to neighbors, so change communication may need one min/router Also called “routing by rumor” Easy to configure, but slow Link state Floods routing information about itself to all nodes, so changes are known immediately Efficient, but complex to configure Cisco’s EIGRP hybrid Efficient and easy to configure

Routing Protocols

Routed versus Routing Protocols Routed protocols used between routers to direct user traffic; also called network protocols –Examples: IP, IPX, DECnet, AppleTalk, NetWare, OSI, VINES Destination Network Protocol Protocol name Exit Port to Use Routing protocols used between routers to maintain routing tables –Examples: RIP, IGRP, OSPF, BGP, EIGRP

Routing Protocol Evolutions Distance Vector Link State RIP Distance vector Most common IGP Uses hop count RIP Distance vector Most common IGP Uses hop count IGRP Distance vector Developed by Cisco Addresses problems in large, heterogeneous networks IGRP Distance vector Developed by Cisco Addresses problems in large, heterogeneous networks OSPF Link state, hierarchical Successor to RIP Uses least-cost routing, multipath routing, and load balancing Derived from IS-IS OSPF Link state, hierarchical Successor to RIP Uses least-cost routing, multipath routing, and load balancing Derived from IS-IS EIGRP Hybrid protocol Developed by Cisco Superior convergence and operating efficiency Merges benefits of link state & distance vector EIGRP Hybrid protocol Developed by Cisco Superior convergence and operating efficiency Merges benefits of link state & distance vector Hybrid

RIP and IGRP RIP Industry standard that selects the path with the fewest hops 19.2 k 64k IGRP Cisco protocol that selects the fastest path (using load, distance, etc.) 19.2 k 64k

OSPF and EIGRP AspectOSPFEIGRP TopologyHierarchicalNot restricted Memory & CPU requirements HighModerate Routing table sizeLargeModerate Controlling bodyIndustry standardCisco proprietary ConvergenceFast Supported protocols IP IPX AppleTalk ConfigurationDifficultEasy