Introduction to Routing and Packet Forwarding

1. Introduction to Routing and Packet Forwarding Routing Protocols and Concepts – Chapter 1

2. Introduction to Routing and Packet forwarding • Routers are computers that specialize in sending data over a network whose functions are: • Path determination • Packet forwarding • Routers are composed of: • Hardware i.e. CPU, Memory, System bus, Interfaces • Software used to direct the routing process • IOS • Configuration file • Routers operate at layer1, 2 and 3 of the OSI model • Connecting Networks • WANs • Serial cables are connected to router serial ports. • In the lab environment clock rates must be configured for DCE • LANs • Straight through cables or cross over cables are used to connect to fastethernet port. (The type of cable used depends on what devices are being connected)

3. Introduction to routing and Packet forwarding • Routers need to be configured. Basic configuration consists of: • Router name • Router banner • Password(s) • Line console • Telnet lines • Privileged mode • Interface configurations i.e. IP address and subnet mask, enabling the interface • Routing tables contain routes information containing the following • Directly connected networks • Remotely connected networks • Network addresses and subnet masks • IP address of next hop router • Metrics

4. Introduction to routing and packet forwarding • Routing principles • Every router makes its decision alone • Routers have different routing table • A router may know a route to a destination but may never know a return route/path. • Routers determine a packet’s path to its destination by doing the following • Receiving an encapsulated frame & examining destination MAC address. • If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address. • If destination IP address is in routing table or there is a static route then Router determines next hop IP address. Router will re-encapsulate packet with appropriate layer 2 frame and send it out to next destination. • Process continues until packet reaches destination. • Note - only the MAC addresses will change the source and destination IP addresses do not change.

5. Static Routing Routing Protocols and Concepts – Chapter 2

6. Static Routes • Purpose of a static route • A manually configured route used when routing from a network to a stub network • Static routes can be • default routes • Summary routes • Configuring static routes • Use the ip routecommand to configure a static route • Advantages of static routing • It can backup multiple interfaces/networks on a router • Easy to configure • No extra resources are needed • More secure • Disadvantages of static routing • Network changes require manual reconfiguration • Does not scale well in large topologies

7. Introduction to Dynamic Routing Protocol Routing Protocols and Concepts – Chapter 3

8. Dynamic Routing Protocols • Functions of Dynamic Routing Protocols: • Dynamically share information between routers. • Automatically update routing table when topology changes. • Maintaining up-to-date routing information • Discover remote networks • Choosing the best path to destination networks • Ability to find a new best path if the current path is no longer available • Components of a routing protocol • Algorithm • Routing protocol algorithms are used for facilitating routing information and best path determination • Routing protocol messages • These are messages for discovering neighbors and exchange of routing information

9. Dynamic Routing Protocols • Routing protocols are grouped as either • Interior gateway protocols (IGP)Or Exterior gateway protocols(EGP) • Classless routing protocols or Classful routing protocols • Distance Vector or Link state routing protocols • Metric used for each routing protocol • RIP - hop count • IGRP & EIGRP - Bandwidth (used by default), Delay (used by default), Load, Reliability • IS-IS & OSPF – Cost, Bandwidth (Cisco’s implementation) • Load balancing • This is the ability of a router to distribute packets among multiple same cost paths

10. Distance Vector Routing Protocols Routing Protocols and Concepts – Chapter 4

11. Distance Vector Routing Protocols • A router using distance vector routing protocols knows 2 things: • Distance to final destination • Vector, or direction, traffic should be directed • Examples of Distance Vector routing protocols: • Routing Information Protocol (RIP) • Interior Gateway Routing Protocol (IGRP) • Enhanced Interior Gateway Routing Protocol (EIGRP) • Characteristics • Periodic updates • Neighbors • Broadcast updates • Entire routing table is included with routing update

12. Advantages and Disadvantages

13. Routing Loops • A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination. • Routing loops may be caused by: • Incorrectly configured static routes • Incorrectly configured route redistribution • Slow convergence • Incorrectly configured discard routes • Routing loops can create the following issues • Excess use of bandwidth • CPU resources may be strained • Network convergence is degraded • Routing updates may be lost or not processed in a timely manner • Preventing routing loops • Setting a maximum • Distance Vector routing protocols set a specified metric value to indicate infinity • Holddown timers • Holddown timers allow a router to not accept any changes to a route for a specified period of time. • Split Horizon rule: • A router should not advertise a network through the interface from which the update came. • Split horizon with poison reverse states • Once a router learns of an unreachable route through an interface, advertise it as unreachable back through the same interface • IP & TTL • The TTL field is found in an IP header and is used to prevent packets from endlessly traveling on a network

14. RIP version 1 Routing Protocols and Concepts – Chapter 5

15. RIPv1 • RIP Characteristics • A classful, Distance Vector (DV) routing protocol • Metric = hop count • Routes with a hop count > 15 are unreachable • Updates are broadcast every 30 seconds • Administrative Distance • RIP’s default administrative distance is 120 • RIP Message Format • RIP header - divided into 3 fields • Command field (specifies the message type request or response) • Version field (version of the protocol 1 or 2) • Must be zero (future expansion) • Route Entry - composed of 3 fields • Address family identifier • IP address • Metric

16. Commands used by RIP

17. VLSM and CIDR Routing Protocols and Concepts – Chapter 6

18. Introduction • Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address • In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes • Classes of IP addresses are identified by the decimal number of the 1st octet • Class A address begin with a 0 bit • Range of class A addresses = 0.0.0.0 to 127.255.255.255 • Class B address begin with a 1 bit and a 0 bit • Range of class B addresses = 128.0.0.0 to 191.255.255.255 • Class C addresses begin with two 1 bits & a 0 bit • Range of class C addresses = 192.0.0.0 to 223.255.255.255 • IP address space was depleting rapidly • The Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR) • CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space. • VLSM is subnetting a subnet

19. Classless IP Addressing • As of July 2012, there are over 908,585,739 hosts on internet • Initiatives to conserve IPv4 address space include: • VLSM & CIDR notation (1993, RFC 1519) • Network Address Translation (1994, RFC 1631) • Private Addressing (1996, RFC 1918) • IPv6 • Classless Inter-domain Routing (CIDR – RFC 1517) • Advantage of CIDR : • More efficient use of IPv4 address space • Route summarization • Requires subnet mask to be included in routing update because address class is meaningless • Recall purpose of a subnet mask: To determine the network and host portion of an IP address • Use ip classless command to turn on classless routing. • ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later

20. RIPv2 Routing Protocols and Concepts – Chapter 7

21. RIPv1 vs RIPv2 • Similarities between RIPv1 & RIPv2 • Both are distance vector routing protocols • Use of timers to prevent routing loops • Use of split horizon or split horizon with poison reverse • Use of triggered updates • Maximum hop count of 15 • Difference between RIPv1 & RIPv2 • RIPv1 • Is as classful distance vector routing protocol • Does not support discontiguoussubnets • Does not support VLSM • Does not send subnet mask in routing update • Routing updates are broadcast • summarize routes to the Classful boundary • RIPv2 • A classless distance vector routing protocol • Next hop address is included in updates • Routing updates are multicast • The use of authentication is an option

22. RIPv2 • Comparing RIPv1 & RIPv2 Message Formats • RIPv2 Message format is similar to RIPv1 but has 2 extensions • 1st extension is the subnet mask field • 2nd extension is the addition of next hop address • Configuring RIPv2 on a Cisco router • Requires using the version 2 command • RIPv2 ignores RIPv1 updates • To verify RIPv2 is configured use the • show ip protocolscommand

23. The Routing Table: A Closer Look Routing Protocols and Concepts – Chapter 8

24. Routing Table Structure • The routing table is a hierarchical structure that is used to locate routes and forward packets. • Routing table entries come from: • Directly connected networks, Static routes, Dynamic routing protocols • Level 1 routes have a subnet mask equal to or less than the classful mask of the network address. • A Level 1 Route is an Ultimate Route if it Includes either a next-hop address or an exit interface • Level 1 route can function as: Default route, Supernet route , Network route • A level 2 route is a route that is a subnet of a classful network address. • Parent Route is a level 1 route • A parent route does not contain any next-hop IP address or exit interface information • A parent route is created whenever a route with a mask greater than the classful mask is entered into the routing table • Child routes are level 2 routes • Child routes are a subnet of a classful network address

25. Routing table structure • Routing table lookup process • Begins with examining level 1 routes for best match with packet’s destination IP • If the best match = an ultimate route then Packet is forwarded -Else- Parent route is examined • If parent route & destination IP match then Level 2 (child) routes are examined • Level 2 route examination • If a match between destination IP and child route found then Packet forwarded -Else • If Router is using classful routing behavior then Packet is dropped –Else • If router is using classless routing behavior then Router searches Level 1 supernet & default routes for a match • If a match is found then Packet if forwarded –Else Packet is dropped • Routing behavior refers to how a routing table is searched • Classful routing behavior • Indicated by the use of the no ip classless command • Router will not look beyond child routes for a lesser match • Classless routing behavior • Indicated by the use of the ip classless command • Router will look beyond child routes for a lesser match

26. EIGRP Routing Protocols and Concepts – Chapter 9

27. EIGRP • Roots of EIGRP: IGRP - Developed in 1985 to overcome RIPv1’s limited hop count - Distance vector routing protocol - Metrics used by IGRP include :bandwidth (used by default), Delay (used by default), reliability, and load • Discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S • EIGRP default administrative distances: Summary routes = 5, Internal routes = 90, Imported routes = 170 • EIGRP Header • Data link frame header - contains source and destination MAC address • IP packet header - contains source & destination IP address • EIGRP packet header - contains AS number • Type/Length/Field - data portion of EIGRP message • EIGRP terms and characteristics • EIGPR uses RTP to transmit & receive EIGRP packets • EIGRP has 5 packet type: Hello, Update, Acknowledgement, Query packets, Reply packets • Supports VLSM & CIDR • EIGRP uses a hello protocol to discover & establish adjacencies • EIGRP routing updates are periodic, Partial and bounded • Converges faster

28. Basic EIGRP Configuration and verification • The global command that enables eigrp is • router eigrp autonomous-system • All routers in the EIGRP routing domain must use the same process ID number (autonomous-system number) • The networkcommand enables interfaces to transmit & receive EIGRP updates • Includes network or subnet in EIGRP updates • R1(config-router)#network network-address wildcard mask • Commands used verify that EIGRP • show ip eigrp neighbors • show ip protocols • show ip route

29. Link-State Routing Protocols Routing Protocols and Concepts – Chapter 10

30. Link state routing • Link State Routing protocols are also known as Shortest Path First protocols • Link is an interface on the router • Link State is information about an interface such as; IP address, Subnet mask, Type of network, Cost associated with link, Neighboring routers on the link • Link State Packets • After initial flooding, additional LSP are sent out when a change in topology occurs • Examples of link state routing protocols include OSPF and IS-IS • The link state routing process • Routers 1st learn of directly connected networks • Routers then say “hello” to neighbors • Routers then build link state packets • Routers then flood LSPs to all neighbors • Routers use LSP database to build a network topology map & calculate the best path to each destination

31. OSPF Routing Protocols and Concepts – Chapter 11

32. OSPF • OSPF Characteristics • A commonly deployed link state routing protocol • Supports VLSM and CIDR • Equal cost load balancing • Auto-summerisation at border routers • Uses areas with area 0 as the backbone • Supports authentication • Employs DRs & BDRs on multi-access networks • DRs & BDRs are elected • DR & BDRs are used to transmit and receive LSAs • Uses 5 packet types:HELLO, DATABASEDESCRIPTION, LINKSTATEREQUEST, LINK STATEUPDATE and LINKSTATEACKNOWLEDGEMENT • OSPF Metric = cost • Lowest cost = best path

33. OSPF configuration and verification • Configuration • Enable OSPF on a router using the following command • R1(config)#router ospfprocess-id • Use the network command to define which interfaces will participate in a given OSPF process • Router(config-router)#network network-address wildcard-mask area area-id • Verifying OSPF configuration • Use the following commands • show ip protocol • show ip route • show ip ospf interface • show ip ospf neighbor