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Ch. 1 – Introduction to Classless Routing

Ch. 1 – Introduction to Classless Routing. CCNA 3 version 3.0. 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

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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

  2. 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

  3. Note • Much of the information in this module is in addition to the online curriculum. • The additional information was included to 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

  4. Advanced IP Management

  5. IPv4 Address Classes

  6. 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.

  7. 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. 

  8. IP addressing crisis • Address Depletion • Internet Routing Table Explosion

  9. 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.

  10. 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

  11. 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 ...

  12. 190 190 190 190 190 190 190 Network 52 52 Network 52 52 52 52 52 1 Subnet 0 255 254 Etc. 3 2 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

  13. 190 Network 190 190 190 190 52 52 52 52 Network 52 0 254 Subnet Etc. 255 1 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.

  14. 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

  15. 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)

  16. 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.

  17. 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)

  18. 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.

  19. Active BGP entries http://bgp.potaroo.net/ Report last updated at Thu, 16 Jan 2003

  20. 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.

  21. 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

  22. 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.

  23. ISP/NAP Hierarchy - “The Internet: Still hierarchical after all these years.” Jeff Doyle (Tries to be anyways!)

  24. 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

  25. 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.

  26. 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.

  27. Example from online curriculum Number of Networks Aggregated = 2^(network bits borrowed) Are we over summarizing here?

  28. Summarized and Specific Routes: Longest-bit Match (more later) • ISP receives a summarized /16 update from Sub1 and a more specific /24 update from Sub2. • ISP will include both routes in the routing table. • ISP will forward all packets matching at least the first 24 bits of 172.16.5.0 to Sub2 (172/16/5/0/24), longest-bit match. • ISP will forward all other packets matching at least the first 16 bits to Sub1 (172.16.0.0/16). ISP 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 Sub1 Sub2 172.16.2.0/24 172.16.10.0/24

  29. Another example from online curriculum

  30. 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.

  31. 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

  32. 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!”

  33. 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

  34. 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

  35. 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

  36. 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.

  37. 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.

  38. 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?

  39. 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.

  40. 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

  41. 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

  42. 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.

  43. 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

  44. Network Address Translation (NAT) NAT: Network Address Translatation • NAT, as defined by RFC 1631, is the process of swapping one address for another in the IP packet header. • In practice, NAT is used to allow hosts that are privately addressed to access the Internet.

  45. Network Address Translation (NAT) • NAT translations can occur dynamically or statically. • The most powerful feature of NAT routers is their capability to use port address translation (PAT), which allows multiple inside addresses to map to the same global address. • This is sometimes called a many-to-one NAT. • With PAT, or address overloading, literally hundreds of privately addressed nodes can access the Internet using only one global address. • The NAT router keeps track of the different conversations by mapping TCP and UDP port numbers. TCP Source Port 1026 2.2.2.2 TCP Source Port 1923 TCP Source Port 1026 2.2.2.2 TCP Source Port 1924

  46. Classless Routing ProtocolsRIPv2

  47. Classless routing protocols • The true defining characteristic of classless routing protocols is the capability to carry subnet masks in their route advertisements. • “One benefit of having a mask associated with each route is that the all-zeros and all-ones subnets are now available for use.” • Cisco allows the all-zeros and all-ones subnets to be used with classful routing protocols.

  48. Classless Routing Protocols “The true characteristic of a classless routing protocol is the ability to carry subnet masks in their route advertisements.” Jeff Doyle, Routing TCP/IP Benefits: • All-zeros and all-ones subnets • - Although some vendors, like Cisco, can also handle this with classful routing protocols. • VLSM • Can have discontiguous subnets • Better IP addressing allocation • CIDR • More control over route summarization

  49. Classless Routing Protocols Classless Routing Protocols: • RIPv2 • EIGRP • OSPF • IS-IS • BGPv4 Note: Remember classful/classless routing protocols is different than classful/classless routing behavior. Classlful/classless routing protocols (RIPv1, RIPv2, IGRP, EIGRP, OSPF, etc.) has to do with how routes get into the routing table; how the routing table gets built. Classful/classless routing behavior (no ip classless or ip classless) has to do with the lookup process of routes in the routing table (after the routing table has been built). It is possible to have a classful routing protocol and classless routing behavior or visa versa. It is also possible to have both a classful routing protocol and classful routing behavior; or both a classless routing protocol and classless routing behavior.

  50. RIP version 1 0 1 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | command (1) | version (1) | must be zero (2) | +---------------+---------------+-------------------------------+ | address family identifier (2) | must be zero (2) | +-------------------------------+-------------------------------+ | IP address (4) | +---------------------------------------------------------------+ | must be zero (4) | +---------------------------------------------------------------+ | must be zero (4) | +---------------------------------------------------------------+ | metric (4) | +---------------------------------------------------------------+ • Classful Routing Protocol, sent over UDP port 520 • Does not include the subnet mask in the routing updates. • Automatic summarization done at major network boundaries. • Updates sent as broadcasts unless the neighbor command is used which sends them as unicasts.

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