1 © 2004, Cisco Systems, Inc. All rights reserved. Introduction to Classless Routing

222 © 2004, Cisco Systems, Inc. All rights reserved. Objectives

333 © 2004, Cisco Systems, Inc. All rights reserved. What Is VLSM and Why Is It Used?

444 © 2004, Cisco Systems, Inc. All rights reserved. Variable-Length Subnet Masks Variable-Length Subnet Masks (VLSM) VLSM allows an organization to use more than one subnet mask within the same network address space. Network administrator can use: a long mask on networks with few hosts, and a short mask on subnets with many hosts; a routing protocol that supports VLSM;

555 © 2004, Cisco Systems, Inc. All rights reserved. Classful Routing Classful routing protocols require that a single network use the same subnet mask.

666 © 2004, Cisco Systems, Inc. All rights reserved. Classless Routing VLSM allows a single autonomous system to have networks with different subnet masks.

777 © 2004, Cisco Systems, Inc. All rights reserved. VLSM

888 © 2004, Cisco Systems, Inc. All rights reserved. VLSM

999 © 2004, Cisco Systems, Inc. All rights reserved. Calculating VLSMs

10 © 2004, Cisco Systems, Inc. All rights reserved. Waste of Space In the past: it has been recommended that the first and last subnet not be used; use of the first subnet (subnet zero), for host addressing was discouraged because of the confusion that can occur when a network and a subnet have the same addresses. Now: It has become acceptable practice to use the first and last subnets in a subnetted network in conjunction with VLSM.

11 © 2004, Cisco Systems, Inc. All rights reserved. Waste of Space

12 © 2004, Cisco Systems, Inc. All rights reserved. Waste of Space If the last three subnets are used for the WAN links, all of the available addresses will be used and there will be no room for growth; also have wasted the 28 host addresses from each subnet to simply address three point-to- point networks; this address scheme would waste one-third of the potential address space.

13 © 2004, Cisco Systems, Inc. All rights reserved. Waste of Space use subnet zero; there will be eight useable subnets; each subnet can support 30 hosts.

14 © 2004, Cisco Systems, Inc. All rights reserved. “IP subnet-zero” Command no ip subnet-zero command From Cisco IOS version 12.0, Cisco routers use subnet zero by default.

15 © 2004, Cisco Systems, Inc. All rights reserved. When to Use VLSM

16 © 2004, Cisco Systems, Inc. All rights reserved. When to Use VLSM Using VLSM Large subnets are created for addressing LANs; Very small subnets are created for WAN links and other special cases; A 30-bit mask: - used to create subnets with only two valid host addresses; - the best solution for the point-to-point connections.

17 © 2004, Cisco Systems, Inc. All rights reserved. Sub-subnets

18 © 2004, Cisco Systems, Inc. All rights reserved. Sub-subnets

19 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets VLSM allows for the setting of a subnet mask that suits the link or the segment requirements; should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another.

20 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets

21 © 2004, Cisco Systems, Inc. All rights reserved. Classful Addressing Scheme Classful routing protocol RIP v1, IGRP, and EGP do not support VLSM; the WAN link must be a subnet of the same Class B network; the WAN link would need the same subnet mask as the LAN segments; а 24-bit mask of can support 250 hosts.

22 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets

23 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets Use the VLSM: a 24-bit mask would still be applied on the LAN segments for the 250 hosts; a 30-bit mask could be used for the WAN link because only two host addresses are needed.

24 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets the subnet addresses are generated when the /20 subnet is divided into /26 subnets; the subnet addresses used on the WAN links, are calculated by further subneting of the one of the unused /26 subnets; /26 is further subnetted with a prefix of /30. This provides four more subnet bits and therefore 16 subnets for the WANs.

25 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets VLSM Example VLSM – can be used to subnet an already subnetted address Example - subnet address /20 and network that needs ten host addresses Using /20: - there are 2^12 – 2, or 4094 host addresses, most of which will be wasted. Using VLSM: it is possible to subnet /20 to create more network addresses with fewer hosts per network; /20 is subnetted to /26: - there is a gain of 2^6, or 64 subnets; - each subnet can support 2^6 – 2, or 62 hosts.

26 © 2004, Cisco Systems, Inc. All rights reserved. Calculating Sub-subnets

27 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example Allocated Network: /24

28 © 2004, Cisco Systems, Inc. All rights reserved.

29 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 1 Subnets: /26 SubnetNetwork address Broadcast address Range / / / /

30 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 1 Subnet: /26 62 usable hosts Perth: /26 Uses 60 host addresses from 62 usable host addresses !!! The requirements for the router Perth are carried out.

31 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 3 VLSM за /27: SubnetNetwork address Broadcast address Range / / / / /

32 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 2 For the router Kuala Lumpur is used the subnet : / 27 The requirements for the router Kuala Lumpur are carried out.

33 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 3 Routers Sydney и Singapore each requires 12 hosts The next unused subnet is /27

34 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 3 VLSM за /28: SubnetNetwork address Broadcast address Range / / / … /

35 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 3 For the router Sydney is used the sub-network: /28 For the router Singapore is used the sub-network: /28

36 © 2004, Cisco Systems, Inc. All rights reserved. VLSM Example - Step 4 VLSM за /30: SubnetNetwork address Broadcast address Range / / / ….

37 © 2004, Cisco Systems, Inc. All rights reserved. Route Aggregation using VLSM: try to keep the subnetwork numbers grouped together in the network to allow for aggregation. keeping networks like and near one another so that the routers need only carry a route for /23.

38 © 2004, Cisco Systems, Inc. All rights reserved. Route Aggregation

39 © 2004, Cisco Systems, Inc. All rights reserved. Route Summarization

40 © 2004, Cisco Systems, Inc. All rights reserved. Route Summarization Route summarization, or supernetting is only possible if the routers of a network run a classless routing protocol (OSPF, EIGRP) Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates

41 © 2004, Cisco Systems, Inc. All rights reserved. Route Summarization Remember the following rules: 1.A router must know in detail the subnet numbers attached to it. 2.A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers. 3.A router using aggregate routes would have fewer entries in its routing table.

42 © 2004, Cisco Systems, Inc. All rights reserved. Route Summarization VLSM allows for the summarization of routes; increases flexibly by basing the summarization entirely on the higher- order bits shared on the left, even if the networks are not contiguous.

43 © 2004, Cisco Systems, Inc. All rights reserved. Route Summarization

44 © 2004, Cisco Systems, Inc. All rights reserved. Configuring VLSM Singapore(config)# interface serial 0 Singapore(config-if)# ip address KualaLumpur(config)# interface serial 1 KualaLumpur(config-if)# ip address

45 © 2004, Cisco Systems, Inc. All rights reserved. History of RIP Autonomous System (AS) the Internet is a collection of autonomous systems; is generally administered by a single entity; will have its own routing technology, which may differ from other autonomous systems. Routing protocols Interior Gateway Protocol (IGP) - used within an AS; Exterior Gateway Protocol (EGP) - is used to transfer routing information between autonomous systems. RIP designed to work as an IGP in a moderate-sized AS; is not intended for use in more complex environments.

46 © 2004, Cisco Systems, Inc. All rights reserved. RIP Features IGP Classful Distance vector routing protocol (broadcasts its entire routing table to each neighbor router at default interval of 30 seconds) Hop count as a metric - the maximum number of hops 15

47 © 2004, Cisco Systems, Inc. All rights reserved. RIP and Different Subnets If the router receives information about a network, and the receiving interface belongs to the same network but is on a different subnet, the router applies the one subnet mask that is configured on the receiving interface: Class A addresses - default classful mask ; Class B addresses - default classful mask ; Class C addresses - default classful mask

48 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 Limitations RIP v1 limitations: It does not send subnet mask information in its updates; It sends updates as broadcasts on ; It does not support authentication; It is not able to support VLSM or classless interdomain routing (CIDR).

49 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 Configuration RIP v1 is a popular routing protocol - virtually all IP routers support it; simplicity; universal compatibility; is capable of load balancing over as many as six equal- cost paths, with four paths as the default.

50 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Features RIP v2 features: improved version of RIP v1; distance vector protocol; uses a hop count metric; uses holddown timers to prevent routing loops – default is 180 seconds; uses split horizon to prevent routing loops; uses 16 hops as a metric for infinite distance.

51 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Features RIP v2 features: Provides prefix routing - allows it to send out subnet mask information with the route update; Supports the classless routing - different subnets within the same network can use different subnet masks(VLSM); Supports multicasts routing updates using the Class D address better efficiency

52 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Features RIP v2 features: Provides for authentication in its updates: - used a set of keys on an interface as an authentication check; - allows for a choice of the type of authentication clear text - default Message-Digest 5 (MD5) encryption: - used to authenticate the source of a routing update; - used to encrypt enable secret passwords.

53 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Features

54 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 Behavior

55 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 Behavior

56 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 Behavior RIP v1 allows maximum hop count of 15 to prevent a packet from looping infinitely; If the destination network is more than 15 routers away, the network is considered unreachable and the packet is dropped. uses split horizon to prevent loops: - RIP v1 advertises routes out an interface only if the routes were not learned from updates entering that interface. uses holddown timers to prevent routing loops: - Holddowns ignore any new information about a subnet indicating a poorer metric for a time equal to the holddown timer.

57 © 2004, Cisco Systems, Inc. All rights reserved. RIP v1 versus RIP v2

58 © 2004, Cisco Systems, Inc. All rights reserved. IP Configuration Tasks

59 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Configuration Select a routing protocol - RIP v2. Assign the IP network numbers without specifying the subnet values. Assign the network or subnet addresses and the appropriate subnet mask to the interfaces.

60 © 2004, Cisco Systems, Inc. All rights reserved. RIP v2 Configuration the interfaces on Router A connected to networks and , or their subnets, will send and receive RIP v2 updates. These routing updates allow the router to learn the network topology. Routers B and C have similar RIP configurations but with different network numbers specified. Enables RIP as the routing protocol Identifies RIP version 2 Specify directly connected networks

61 © 2004, Cisco Systems, Inc. All rights reserved. Configuring RIPv2 network command allows the routing process to determine which interfaces will participate in the sending and receiving of routing updates; starts up the routing protocol on all interfaces that the router has in the specified network; allows the router to advertise that network.

62 © 2004, Cisco Systems, Inc. All rights reserved. Verify RIP v2 Configuration show ip protocols command displays values about routing protocols and routing protocol timer information; show ip route command displays the contents of the IP routing table. the routing table contains: - entries for all known networks and subnetworks; - code that indicates how that information was learned.

63 © 2004, Cisco Systems, Inc. All rights reserved. Verifying the RIP Configuration show ip protocols command

64 © 2004, Cisco Systems, Inc. All rights reserved. Holddown Timer RIP sends updated routing table information every 30 seconds. This interval is configurable. The router running RIP does not receive an update from another router for 180 seconds or more, the first router marks the routes served by the non-updating router as being invalid.

65 © 2004, Cisco Systems, Inc. All rights reserved. Flush Timer If there is still no update after 240 seconds the router removes the routing table entries.

66 © 2004, Cisco Systems, Inc. All rights reserved. Displaying the IP Routing Table show ip route command

67 © 2004, Cisco Systems, Inc. All rights reserved. Verify RIP v2 Configuration show ip interface brief command used to list a summary of the information and status of an interface show ip proticols command show running-config command examine the output to see if the routing table is populated with routing information; examine the output if entries are missing, routing information is not being exchanged.

68 © 2004, Cisco Systems, Inc. All rights reserved. Troubleshooting RIP v2 Debug ip rip command to display RIP routing updates as they are sent and received

69 © 2004, Cisco Systems, Inc. All rights reserved. “debug ip rip” Router being debugged: has received updates from one router at source address The router at source address sent information about two destinations in the routing table update; sent updates, in both cases to the multicast address as the destination; The number in parentheses is the source address encapsulated into the IP header.

70 © 2004, Cisco Systems, Inc. All rights reserved. Troubleshooting RIPv2 debug ip rip Outputs and Meanings

71 © 2004, Cisco Systems, Inc. All rights reserved. Routes By default, routers learn paths to destinations three different ways: Static routes manually defined by the system administrator the static routes as the next hop to a destination; are useful for security and traffic reduction, as no other route is known. Default routes manually defined by the system administrator as the path to take when there is no known route to the destination; keep routing tables shorter. When an entry for a destination network does not exist in a routing table, the packet is sent to the default network. Dynamic routes Dynamic routing means that the router learns of paths to destinations by receiving periodic updates from other routers.

72 © 2004, Cisco Systems, Inc. All rights reserved. IP route command ip route command indicates the default route

73 © 2004, Cisco Systems, Inc. All rights reserved. “ip default-network “ Command ip default-network command establishes a default route in networks using dynamic routing protocols. is usually configured on the routers that connect to a router with a static default route.

74 © 2004, Cisco Systems, Inc. All rights reserved. Default Route Hong Kong 2 and Hong Kong 3 would use Hong Kong 4 as the default gateway. Hong Kong 4 would use interface as its default gateway. Hong Kong 1 would route packets to the Internet for all internal hosts. HongKong1(config)# ip route

75 © 2004, Cisco Systems, Inc. All rights reserved. Summary