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Ch. 1 – Introduction to Classless Routing

Ch. 1 – Introduction to Classless Routing. CCNA 3 version 3.0 Rick Graziani Cabrillo College. Note to instructors. If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint.

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Ch. 1 – Introduction to Classless Routing

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  1. Ch. 1 – Introduction to Classless Routing CCNA 3 version 3.0 Rick Graziani Cabrillo College

  2. Note to instructors • If you have downloaded this presentation from the Cisco Networking Academy Community FTP Center, this may not be my latest version of this PowerPoint. • For the latest PowerPoints for all my CCNA, CCNP, and Wireless classes, please go to my web site: http://www.cabrillo.cc.ca.us/~rgraziani/ • The username is cisco and the password is perlman for all of my materials. • If you have any questions on any of my materials or the curriculum, please feel free to email me at graziani@cabrillo.edu (I really don’t mind helping.) Also, if you run across any typos or errors in my presentations, please let me know. • I will add “(Updated – date)” next to each presentation on my web site that has been updated since these have been uploaded to the FTP center. Thanks! Rick Rick Graziani graziani@cabrillo.edu

  3. Overview of Information in Module 1 • Define VLSM and briefly describe the reasons for its use • Divide a major network into subnets of different sizes using VLSM • Define route aggregation and summarization as they relate to VLSM • Configure a router using VLSM • Identify the key features of RIP v1 and RIP v2 • Identify the important differences between RIP v1 and RIP v2 • Configure RIP v2 • Verify and troubleshoot RIP v2 operation • Configure default routes using the ip route and ip default-network commands Rick Graziani graziani@cabrillo.edu

  4. Note • Much of the information in this module is in addition to the online curriculum. • The additional information was included it add clarity and make the topics more understandable. • Advanced IP Management • Subnetting • Classless interdomain routing (CIDR) • Variable length subnet masking (VLSM) • Route summarization • Network Address Translation (NAT) • Classless Routing Protocols • RIPv2 Rick Graziani graziani@cabrillo.edu

  5. Advanced IP Management

  6. IPv4 Address Classes Rick Graziani graziani@cabrillo.edu

  7. IPv4 Address Classes • No medium size host networks • In the early days of the Internet, IP addresses were allocated to organizations based on request rather than actual need. Rick Graziani graziani@cabrillo.edu

  8. IPv4 Address Classes Class D Addresses • A Class D address begins with binary 1110 in the first octet. • First octet range 224 to 239. • Class D address can be used to represent a group of hosts called a host group, or multicast group. Class E AddressesFirst octet of an IP address begins with 1111 • Class E addresses are reserved for experimental purposes and should not be used for addressing hosts or multicast groups.  Rick Graziani graziani@cabrillo.edu

  9. IP addressing crisis • Address Depletion • Internet Routing Table Explosion Rick Graziani graziani@cabrillo.edu

  10. IPv4 Addressing Subnet Mask • One solution to the IP address shortage was thought to be the subnet mask. • Formalized in 1985 (RFC 950), the subnet mask breaks a single class A, B or C network in to smaller pieces. Rick Graziani graziani@cabrillo.edu

  11. Network Network Subnet Host Subnet Example Using /24 subnet... 190.52.1.2 190.52.2.2 190.52.3.2 Given the Class B address 190.52.0.0 Class B Network Network Host Host Internet routers still “see” this net as 190.52.0.0 But internal routers think all these addresses are on different networks, called subnetworks Rick Graziani graziani@cabrillo.edu

  12. Network Network Subnet Host Subnet Example Using the 3rd octet, 190.52.0.0 was divided into: 190.52.1.0 190.52.2.0 190.52.3.0 190.52.4.0 190.52.5.0 190.52.6.0 190.52.7.0 190.52.8.0 190.52.9.0 190.52.10.0 190.52.11.0 190.52.12.0 190.52.13.0 190.52.14.0 190.52.15.0 190.52.16.0 190.52.17.0 190.52.18.0 190.52.19.0 and so on ... Rick Graziani graziani@cabrillo.edu

  13. 190 190 Network 190 190 190 190 190 52 52 52 52 52 52 Network 52 1 Subnet 0 255 2 3 254 Etc. Host Host Host Host Host Host Host Host Subnet Example Network address 190.52.0.0 with /16 network mask Using Subnets: subnet mask 255.255.255.0 or /24 Subnets 255 Subnets 28 - 1 Cannot use last subnet as it contains broadcast address Rick Graziani graziani@cabrillo.edu

  14. 190 Network 190 190 190 190 52 Network 52 52 52 52 1 255 Subnet Etc. 0 254 Host Host Host Host Host Host Subnet Example Subnet 0 (all 0’s subnet) issue: The address of the subnet, 190.52.0.0/24 is the same address as the major network, 190.52.0.0/16. Subnets 255 Subnets 28 - 1 Last subnet (all 1’s subnet) issue: The broadcast address for the subnet, 190.52.255.255 is the same as the broadcast address as the major network, 190.52.255.255. Rick Graziani graziani@cabrillo.edu

  15. All Zeros and All Ones Subnets Using the All Ones and All Zeroes Subnet • There is no command to enable or disable the use of the all-ones subnet, it is enabled by default. Router(config)#ip subnet-zero • The use of the all-ones subnet has always been explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS version 12.0. RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete! Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use, though, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems. CCO: Subnet Zero and the All-Ones Subnethttp://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml Rick Graziani graziani@cabrillo.edu

  16. Need a Subnet Review? • If you need a Review of Subnets, please review the following links on my web site: • Subnet Review (PowerPoint) • Subnets Explained (Word Doc) Rick Graziani graziani@cabrillo.edu

  17. Long Term Solution: IPv6 (coming) • IPv6, or IPng (IP – the Next Generation) uses a 128-bit address space, yielding 340,282,366,920,938,463,463,374,607,431,768,211,456 possible addresses. • IPv6 has been slow to arrive • IPv4 revitalized by new features, making IPv6 a luxury, and not a desperately needed fix • IPv6 requires new software; IT staffs must be retrained • IPv6 will most likely coexist with IPv4 for years to come. • Some experts believe IPv4 will remain for more than 10 years. Rick Graziani graziani@cabrillo.edu

  18. Short Term Solutions: IPv4 Enhancements • CIDR (Classless Inter-Domain Routing) – RFCs 1517, 1518, 1519, 1520 • VLSM (Variable Length Subnet Mask) – RFC 1009 • Private Addressing - RFC 1918 • NAT/PAT (Network Address Translation / Port Address Translation) – RFC Rick Graziani graziani@cabrillo.edu

  19. CIDR (Classless Inter-Domain Routing) • By 1992, members of the IETF were having serious concerns about the exponential growth of the Internet and the scalability of Internet routing tables. • The IETF was also concerned with the eventual exhaustion of 32-bit IPv4 address space. • Projections were that this problem would reach its critical state by 1994 or 1995. • IETF’s response was the concept of Supernetting or CIDR, “cider”. • To CIDR-compliant routers, address class is meaningless. • The network portion of the address is determined by the network subnet mask or prefix-length (/8, /19, etc.) • The first octet (first two bits) of the network address (or network-prefix) is NOT used to determine the network and host portion of the network address. • CIDR helped reduced the Internet routing table explosion with supernetting and reallocation of IPv4 address space. Rick Graziani graziani@cabrillo.edu

  20. Active BGP entries http://bgp.potaroo.net/ Report last updated at Thu, 16 Jan 2003 Rick Graziani graziani@cabrillo.edu

  21. CIDR (Classless Inter-Domain Routing) • First deployed in 1994, CIDR dramatically improves IPv4’s scalability and efficiency by providing the following: • Eliminates traditional Class A, B, C addresses allowing for more efficient allocation of IPv4 address space. • Supporting route aggregation (summarization), also known as supernetting, where thousands of routes could be represented by a single route in the routing table. • Route aggregation also helps prevent route flapping on Internet routers using BGP. Flapping routes can be a serious concern with Internet core routers. • CIDR allows routers to aggregate, or summarize, routing information and thus shrink the size of their routing tables. • Just one address and mask combination can represent the routes to multiple networks. • Used by IGP routers within an AS and EGP routers between AS. Rick Graziani graziani@cabrillo.edu

  22. Without CIDR, a router must maintain individual routing table entries for these class B networks. With CIDR, a router can summarize these routes using a single network address by using a 13-bit prefix: 172.24.0.0 /13 Steps: 1. Count the number of left-most matching bits, /13 (255.248.0.0) 2. Add all zeros after the last matching bit: 172.24.0.0 = 10101100 00011000 00000000 00000000 Rick Graziani graziani@cabrillo.edu

  23. CIDR (Classless Inter-Domain Routing) • By using a prefix address to summarizes routes, administrators can keep routing table entries manageable, which means the following • More efficient routing • A reduced number of CPU cycles when recalculating a routing table, or when sorting through the routing table entries to find a match • Reduced router memory requirements • Route summarization is also known as: • Route aggregation • Supernetting • Supernetting is essentially the inverse of subnetting. • CIDR moves the responsibility of allocation addresses away from a centralized authority (InterNIC). • Instead, ISPs can be assigned blocks of address space, which they can then parcel out to customers. Rick Graziani graziani@cabrillo.edu

  24. ISP/NAP Hierarchy - “The Internet: Still hierarchical after all these years.” Jeff Doyle (Tries to be anyways!) Rick Graziani graziani@cabrillo.edu

  25. Supernetting Example • Company XYZ needs to address 400 hosts. • Its ISP gives them two contiguous Class C addresses: • 207.21.54.0/24 • 207.21.55.0/24 • Company XYZ can use a prefix of 207.21.54.0 /23 to supernet these two contiguous networks. (Yielding 510 hosts) • 207.21.54.0 /23 • 207.21.54.0/24 • 207.21.55.0/24 23 bits in common Rick Graziani graziani@cabrillo.edu

  26. Supernetting Example • With the ISP acting as the addressing authority for a CIDR block of addresses, the ISP’s customer networks, which include XYZ, can be advertised among Internet routers as a single supernet. Rick Graziani graziani@cabrillo.edu

  27. CIDR and the Provider Another example of route aggregation. Rick Graziani graziani@cabrillo.edu

  28. CIDR and the provider 200.199.48.0/25 Even Better: 200.199.48.32/27 11001000 11000111 00110000 0 0100000 200.199.48.64/27 11001000 11000111 00110000 0 1000000 200.199.48.96/27 11001000 11000111 00110000 0 1100000 200.199.48.0/25 11001000 11000111 00110000 0 0000000 (As long as there are no other routes elsewhere within this range, well…) 200.199.56.0/24 11001000 11000111 0011100 0 00000000 200.199.57.0/24 11001000 11000111 0011100 1 00000000 200.199.56.0/23 11001000 11000111 0011100 0 00000000 Summarization from the customer networks to their provider. 200.199.56.0/23 Rick Graziani graziani@cabrillo.edu

  29. CIDR and the provider 200.199.48.0/25 200.199.48.0/25 11001000 11000111 0011 0000 00000000 200.199.49.0/25 11001000 11000111 0011 0001 00000000 200.199.56.0/23 11001000 11000111 0011 1000 00000000 200.199.48.0/20 11001000 11000111 0011 0000 00000000 20 bits in common Further summarization happens with the next upstream provider. 200.199.56.0/23 Rick Graziani graziani@cabrillo.edu

  30. CIDR Restrictions • Dynamic routing protocols must send network address and mask (prefix-length) information in their routing updates. • In other words, CIDR requires classless routing protocols for dynamic routing. • However, you can still configure summarized static routes, after all, that is what a 0.0.0.0/0 route is. Rick Graziani graziani@cabrillo.edu

  31. Summarized and Specific Routes: Longest-bit Match (more later) • Merida receives a summarized /16 update from Quito and a more specific /24 update from Cartago. • Merida will include both routes in the routing table. • Merida will forward all packets matching at least the first 24 bits of 172.16.5.0 to Cartago (172/16/5/0/24), longest-bit match. • Merida will forward all other packets matching at least the first 16 bits to Quito (172.16.0.0/16). Merida Summarized Update Specific Route Update 172.16.0.0/16 172.16.5.0/24 172.16.5.0/24 172.16.1.0/24 Quito Cartago 172.16.2.0/24 172.16.10.0/24 Rick Graziani graziani@cabrillo.edu

  32. Example from online curriculum Rick Graziani graziani@cabrillo.edu

  33. Another example from online curriculum Rick Graziani graziani@cabrillo.edu

  34. Short Term Solutions: IPv4 Enhancements • CIDR (Classless Inter-Domain Routing) – RFCs 1517, 1518, 1519, 1520 • VLSM (Variable Length Subnet Mask) – RFC 1009 • Private Addressing - RFC 1918 • NAT/PAT (Network Address Translation / Port Address Translation) – RFC Rick Graziani graziani@cabrillo.edu

  35. VLSM (Variable Length Subnet Mask) • Limitation of using only a single subnet mask across a given network-prefix (network address, the number of bits in the mask) was that an organization is locked into a fixed-number of of fixed-sized subnets. • 1987, RFC 1009 specified how a subnetted network could use more than one subnet mask. • VLSM = Subnetting a Subnet • “If you know how to subnet, you can do VLSM!” Rick Graziani graziani@cabrillo.edu

  36. VLSM – Simple Example • Subnetting a /8 subnet using a /16 mask gives us 256 subnets with 65,536 hosts per subnet. • Let’s take the 10.2.0.0/16 subnet and subnet it further… 1st octet 2nd octet 3rd octet 4th octet 10.0.0.0/8 10 Host Host Host 10.0.0.0/16 10 Subnet Host Host 10.0.0.0/16 10 0 Host Host 10.1.0.0/16 10 1 Host Host 10.2.0.0/16 10 2 Host Host 10.n.0.0/16 10 … Host Host 10.255.0.0/16 10 255 Host Host Rick Graziani graziani@cabrillo.edu

  37. VLSM – Simple Example • Note: 10.2.0.0/16 is now a summary of all of the 10.2.0.0/24 subnets. • Summarization coming soon! Network Subnet Host Host 10.2.0.0/16 10 2 Host Host 10.2.0.0/24 10 2 Subnet Host 10.2.0.0/24 10 2 0 Host 10.2.1.0/24 10 2 1 Host 10.2.n.0/24 10 2 … Host 10.2.255.0/24 10 2 255 Host Rick Graziani graziani@cabrillo.edu

  38. VLSM – Simple Example 10.0.0.0/8 “subnetted using /16” Subnet1st hostLast hostBroadcast 10.0.0.0/16 10.0.0.1 10.0.255.254 10.0.255.255 10.1.0.0/16 10.1.0.1 10.1.255.254 10.1.255.255 10.2.0.0/16 “sub-subnetted using /24” • Subnet1st host Last hostBroadcast • 10.2.0.0/24 10.2.0.1 10.2.0.254 10.2.0.255 • 10.2.1.0/24 10.2.1.1 10.2.1.254 10.2.1.255 • 10.2.2.0/24 10.2.2.1 10.2.2.254 10.2.2.255 • Etc. • 10.2.255.0/24 10.2.255.1 10.2.255.254 10.2.255.255 10.3.0.0/16 10.3.0.1 10.3.255.254 10.0.255.255 Etc. 10.255.0.0/16 10.255.0.1 10.255.255.254 10.255.255.255 Rick Graziani graziani@cabrillo.edu

  39. VLSM – Simple Example • Your network can now have 255 /16 subnets with 65,534 hosts each AND256 /24 subnets with 254 hosts each. • All you need to make it work is a classless routing protocol that passes the subnet mask with the network address in the routing updates. • Classless routing protocols: RIPv2, EIGRP, OSPF, IS-IS, BGPv4 (coming) An example of VLSM, NOT of good network design. Subnets 10.0.0.0/16 10.1.0.0/16 10.2.0.0/16 10.2.0.0/24 • 10.2.1.0/24 • 10.2.2.0/24 • Etc. • 10.2.255.0/24 10.3.0.0/16 Etc. 10.255.0.0/16 10.2.0.0/24 10.1.0.0/16 10.7.0.0/16 10.2.1.0/24 10.3.0.0/16 10.2.6.0/24 10.2.8.0/24 10.8.0.0/16 10.4.0.0/16 10.5.0.0/16 10.2.4.0/24 10.2.3.0/24 10.2.5.0/24 10.6.0.0/16 Rick Graziani graziani@cabrillo.edu

  40. Another VLSM Example using /30 subnets • This network has seven /27 subnets with 30 hosts each ANDeight /30 subnets with 2 hosts each. • /30 subnets are very useful for serial networks. 207.21.24.0/24 network subnetted into eight /27 (255.255.255.224) subnets 207.21.24.192/27 subnet, subnetted into eight /30 (255.255.255.252) subnets Rick Graziani graziani@cabrillo.edu

  41. 207.21.24.192/27 207.21.24. 11000000 /30 Hosts Bcast 2 Hosts 0 207.21.24.192/30 207.21.24. 110 00000 01 10 11 .193 & .194 1 207.21.24.196/30 207.21.24. 110 00100 01 10 11 .197 & .198 2 207.21.24.200/30 207.21.24.110 01000 01 10 11 .201 & .202 3 207.21.24.204/30 207.21.24.110 01100 01 10 11 .205 & .206 4 207.21.24.208/30 207.21.24. 110 10000 01 10 11 .209 & .210 5 207.21.24.212/30 207.21.24.110 10100 01 10 11 .213 & .214 6 207.21.24.216/30 207.21.24.110 11000 01 10 11 .217 & .218 7 207.21.24.220/30 207.21.24.110 11100 01 10 11 .221 & .222 Rick Graziani graziani@cabrillo.edu

  42. This network has seven /27 subnets with 30 hosts each AND seven /30 subnets with 2 hosts each (one left over). • /30 subnets with 2 hosts per subnet do not waste host addresses on serial networks . 207.21.24.192/30 207.21.24.204/30 207.21.24.216/30 207.21.24.128/27 207.21.24.96/27 207.21.24.64/27 207.21.24.208/30 207.21.24.212/30 207.21.24.196/30 207.21.24.200/30 207.21.24.32/27 207.21.24.0/27 207.21.24.160/27 207.21.24.224/27 Rick Graziani graziani@cabrillo.edu

  43. VLSM and the Routing Table Displays one subnet mask for all child routes. Classful mask is assumed for the parent route. Routing Table without VLSM RouterX#show ip route 207.21.24.0/27 is subnetted, 4 subnets C 207.21.24.192is directly connected, Serial0  C 207.21.24.196 is directly connected, Serial1 C 207.21.24.200 is directly connected, Serial2 C 207.21.24.204 is directly connected, FastEthernet0 Routing Table with VLSM RouterX#show ip route 207.21.24.0/24 is variably subnetted, 4 subnets, 2 masks C 207.21.24.192 /30 is directly connected, Serial0  C 207.21.24.196 /30 is directly connected, Serial1 C 207.21.24.200 /30 is directly connected, Serial2 C 207.21.24.96 /27 is directly connected, FastEthernet0 Each child routes displays its own subnet mask. Classful mask is included for the parent route. • Parent Route shows classful mask instead of subnet mask of the child routes. • Each Child Routes includes its subnet mask. Rick Graziani graziani@cabrillo.edu

  44. Final Notes on VLSM • Whenever possible it is best to group contiguous routes together so they can be summarized (aggregated) by upstream routers. (coming soon!) • Even if not all of the contiguous routes are together, routing tables use the longest-bit match which allows the router to choose the more specific route over a summarized route. • Coming soon! • You can keep on sub-subnetting as many times and as “deep” as you want to go. • You can have various sizes of subnets with VLSM. Rick Graziani graziani@cabrillo.edu

  45. Route flapping • Route flapping occurs when a router interface alternates rapidly between the up and down states. • Route flapping can cripple a router with excessive updates and recalculations. • However, the summarization configuration prevents the RTC route flapping from affecting any other routers. • The loss of one network does not invalidate the route to the supernet. • While RTC may be kept busy dealing with its own route flap, RTZ, and all upstream routers, are unaware of any downstream problem. • Summarization effectively insulates the other routers from the problem of route flapping. Rick Graziani graziani@cabrillo.edu

  46. Short Term Solutions: IPv4 Enhancements • CIDR (Classless Inter-Domain Routing) – RFCs 1517, 1518, 1519, 1520 • VLSM (Variable Length Subnet Mask) – RFC 1009 • Private Addressing - RFC 1918 • NAT/PAT (Network Address Translation / Port Address Translation) – RFC Rick Graziani graziani@cabrillo.edu

  47. Private IP addresses (RFC 1918) If addressing any of the following, these private addresses can be used instead of globally unique addresses: • A non-public intranet • A test lab • A home network Global addresses must be obtained from a provider or a registry at some expense. Rick Graziani graziani@cabrillo.edu

  48. Discontiguous subnets • “Mixing private addresses with globally unique addresses can create discontiguous subnets.” – Not the main cause however… • Discontiguous subnets, are subnets from the same major network that are separated by a completely different major network or subnet. • Question: If a classful routing protocol like RIPv1 or IGRP is being used, what do the routing updates look like between Site A router and Site B router? Rick Graziani graziani@cabrillo.edu

  49. Discontiguous subnets • Classful routing protocols, notably RIPv1 and IGRP, can’t support discontiguous subnets, because the subnet mask is not included in routing updates. • RIPv1 and IGRP automatically summarize on classful boundaries. • Site A and Site B are all sending each other the classful address of 207.21.24.0/24. • A classless routing protocol (RIPv2, EIGRP, OSPF) would be needed: • to not summarize the classful network address and • to include the subnet mask in the routing updates. Rick Graziani graziani@cabrillo.edu

  50. Discontiguous subnets • RIPv2 and EIGRP automatically summarize on classful boundaries. • When using RIPv2 and EIGRP, to disable automatic summarization (on both routers): Router(config-router)#no auto-summary • SiteB now receives 207.21.24.0/27 • SiteB now receives 207.21.24.32/27 Rick Graziani graziani@cabrillo.edu

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