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Routing Protocols and Concepts

Routing Protocols and Concepts. VLSM and CIDR Chapter 6 Modified by Pete Brierley. What will we Learn from chapter 6?. Compare and contrast classful and classless IP addressing. Review VLSM and explain the benefits of classless IP addressing.

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Routing Protocols and Concepts

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  1. Routing Protocols and Concepts VLSM and CIDR Chapter 6 Modified by Pete Brierley

  2. What will we Learn from chapter 6? • Compare and contrast classful and classless IP addressing. • Review VLSM and explain the benefits of classless IP addressing. • Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses

  3. 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 • 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 simply subnetting a subnet

  4. Classful and Classless IP Addressing • Classful IP addressing • As of January 2007, there are over 433 million 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)

  5. Classful and Classless IP Addressing • The High Order Bits These are the leftmost bits in a 32 bit address

  6. Classful and Classless IP Addressing • 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.

  7. Classful and Classless IP Addressing • The IPv4 Classful Addressing Structure (RFC 790) An IP address has 2 parts: -The network portion Found on the leftside of an IP address -The host portion Found on the right side of an IP address

  8. Classful and Classless IP Addressing

  9. Classful and Classless IP Addressing • Purpose of a subnet mask • It is used to determine the network portion of an IP address

  10. Classful and Classless IP Addressing • Classful Routing Updates -Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates The reason is that the Subnet mask is directly related to the network address

  11. Classful and Classless IP Addressing • 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

  12. Classful and Classless IP Addressing • Classless IP Addressing • CIDR & Route Summarization -Variable Length Subnet Masking (VLSM) -Allows a subnet to be further sub-netted according to individual needs -Prefix Aggregation a.k.a. Route Summarization -CIDR allows for routes to be summarized as a single route

  13. Classful and Classless IP Addressing • Classless Routing Protocol • Characteristics of classless routing protocols: -Routing updates include the subnet mask -Supports VLSM Supports Route Summarization

  14. Classful and Classless IP Addressing • Classless Routing Protocol

  15. VLSM • Classful routing • -only allows for one subnet mask for all networks • VLSM & classless routing -This is the process of subnetting a subnet -More than one subnet mask can be used -More efficient use of IP addresses as compared to classful IP addressing

  16. VLSM • VLSM – the process of sub-netting a subnet to fit your needs -Example: Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask. -Mask allows for 254 host addresses per subnet -Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24

  17. Classless Inter-Domain Routing (CIDR) • Route summarization done by CIDR -Routes are summarized with masks that are less than that of the default classful mask -Example: 172.16.0.0 / 13is the summarized route for the 172.16.0.0 / 16 to 172.23.0.0 / 16 classful networks

  18. Classless Inter-Domain Routing (CIDR) • Steps to calculate a route summary -List networks in binary format -Count number of left most matching bits to determine summary route’s mask -Copy the matching bits and add zero bits to determine the summarized network address

  19. CCNA 3/Module 1 An Introduction to Classless Routing

  20. Overview: Classful/Classless Routing • Classful routing - a network must use the same subnet mask for the entire network • Classless routing – using more than one subnet mask for a network address • “subnetting a subnet”

  21. Overview: (Classful) IPv4 Addressing Limits • IPv4 – 20 years old • IPv4 – even with subnetting, couldn’t handle the global demand for Internet connectivity • Class B space was on the verge of depletion. • Rapid and substantial increase in the size of the Internet's routing tables. • As more Class C's came online, the flood of new network information threatened Internet routers' capability to cope.

  22. Overview: (Classful) IPv4 Addressing Limits • Provides IP scheme with limitations: • Class A – 126 networks: 16,777,214 hosts each • Class B – 65,000 networks: 65,534 hosts each • Class C – 2 million networks: 254 hosts each • While available addresses were running out, only 3% of assigned addresses were actually being used! • Subnet zero, broadcast addresses, • pool of unused addresses at • Class A and B sites, etc.

  23. Overview: Scalability & Routing Tables • Maximum theoretical routing table size is 60,000 entries. • Classful addressing would have hit this capacity by mid-1994. • Internet growth would have ended.

  24. What is VLSM and why is it used? • The purpose of VLSM is to alleviate the shortage of IP addresses • VLSM allows: • More than one subnet mask within the same NW • Or . . . Multiple SNMasks with ONE IP Address • Use of long mask on networks with few hosts • Use of short mask on networks with many hosts • In order to use VLSM, the routing protocol must support it. • Cisco routers with the following routing protocols support VLSM: • OSPF (Open Shortest Path First) • IS-IS (Integrated Intermediate System to Intermediate System) • EIGRP (Enhanced Interior Gateway Routing Protocol) • RIP v2 • Static Routing

  25. What is VLSM and why is it used? Classful routing protocols use one subnet mask for a single network • Ex: 192.168.187.0, must use subnet mask 255.255.255.0 VLSM allows a single autonomous system to have networks with different subnet masks, for example: • Use a 30-bit subnet mask on network connections • (255.255.255.252) • Use a 24-bit subnet mask for user networks up to 250 users • (255.255.255.0) • Use a 22-bit subnet mask for user networks up to 1000 users • (255.255.252.0)

  26. A waste of space • In classless routing, recommended that first and last subnets have special use; not be used for host addresses • First (SN 0) had same address for the network and subnet • Last subnet (all-1’s) was the broadcast • Address depletion has lead to use of these subnets • Now acceptable practice to use the first and last subnets in conjunction with VLSM

  27. A waste of space

  28. A waste of space If subnet zerois used, there are 8 useable subnets • Each subnet can support 30 hosts • Cisco routers use subnet zero by default IOS v. 12.0+ Ifno ip subnet-zerocommand is used on the router, there are 7 useable subnets with 30 hosts per subnet • If supporting 4 routers (1 subnet each) that need 3 WAN links to each other, all subnets are used • No room for growth • Waste of 28 host addresses for each WAN (point-to-point) links or 1/3 of potential address space

  29. A waste of space FOSTER(config)#no ip subnet-zero • Disables the capability to use subnets that include the network address of the unsubnetted network

  30. When to use VLSM Networking design addressing scheme that allows: • Growth • Doesn’t waste addresses on point-to-point links

  31. When to use VLSM • VLSM addressing applied instead results in: • Variable sized subnets • Take 1 of the 3 subnets and subnet it again • Example 192.168.187.224(last subnet) • Apply a 30 bit mask (225.225.225.252) • Creates a possible8 rangesof addresses with30 bits • Best solution forpoint-to-point links – use 2 host addresses instead of 30

  32. Calculating subnets with VLSM • VLSM helps to manage IP addresses • VLSM can use one SNM for a point-to-point link and • one SNM for a LAN

  33. Calculating subnets with VLSM Foster’sFabulousFilms • 2 routers • 1 in Hollywood (100 hosts) • 1 in Ravenna (50 hosts) • 1 WAN link (2 needed) • IP/NW Address: 192.16.10.0 • Class C Use the BIGGEST first: 100 50 2

  34. Calculating subnets with VLSM If VLSM were used instead of classful routing: • A 24-bit mask could be used for LAN segments for 250 hosts • A 30-bit mask could be used for WAN segments for 2 hosts • 172.16.32.0/20 (would accommodate 4094 hosts) • Binary = 10101100.00010000.00100000.00000000 • SNM = 11111111.11111111.11110000.00000000 • VLSM address172.16.32.0/26 (needed for 62 hosts) • Binary = 10101100.00010000.00100000.00000000 • SNM = 11111111.11111111.11111111.11000000 • If 172.16.32.0/20 used, but only 10 hosts on segment, would provide 4094 hosts and waste 4084 addresses • By further subnetting /20 to /26, gain 64 subnets (26) each supporting 62 hosts

  35. Calculating Subnets w/VLSM Procedure to subnet a subnet /20 to /26 using VLSM: • 1. Write 172.16.32.0 in binary form • Binary = 10101100.00010000.00100000.00000000 • Draw a vertical line between the 20th and 21st bits (the original • subnet boundary) • 3. Draw a vertical line between the 26th and 27th bits extending the bits to segment/host needs • 4. Calculate the number of subnet addresses between the two vertical lines (lowest to highest) in value

  36. Calculating Subnets w/VLSM • Keep in mind that only unused subnets can be further subnetted • If any address for a subnet is used cannot be further subnetted

  37. Route Aggregation w/VLSM • Every network needs a separate entry in routing table • Each subnet needs a separate entry • Aggregation will reduce routing table size • When using VLSM keep subnetwork numbers grouped together in the network to allow for aggregation by using Classless InterDomain Routing (CIDR) • 172.16.14.0 • 172.16.15.0 • Router needs to hold only one route 172.16.14.0/23

  38. Route Aggregation w/VLSM • Using CIDR and VLSM prevents address waste and promotes route aggregation or summarization • Without summarization, Internet would collapse • Summarization reduces burden on upstream routers • This process of summarization continues until entire network is advertised as a single aggregate route • Summarization is also called supernetting • Possible only if the routers of a network run a classless routing protocol such as OSPF or EIGRP • IP address and bit mask included in routing updates • The summary routeuses a prefix common toall addresses of an organizational group

  39. Route Aggregation w/VLSM • Carefully assign addresses in a hierarchical fashion to share same high-order bits for summarization • A router: • Must know subnetsattached in detail • Does not need to tell other routers about subnets • Using aggregate routes has fewer entries in routing table • VLSM allows for summarization of routes • Works even if networks are not contiguous • VLSM increases flexibly by summarization on higher-order bits • Used to calculate the network number of the summary route • Uses only shared highest-order bits

  40. Configuring VLSM • If VLSM is chosen, it must be configured correctly • Example: 192.168.10.0 • One router has to support 60 hosts, needs 6 bits in host portion of address to provide 62 possible address • (26 = 64 – 2 = 60) • 192.168.10.0/26 (leaves 6 bits for hosts) • One router has to support 28 hosts, needs 5 bits in host portion of address to provide 30 possible hosts • (25 = 32 – 2 = 30) 192.168.10.64/27 (leaves 5 bits for hosts) • Two routers have to support 12 hosts each, needs 4 bits in host portion of address to provide 14 possible hosts • (24 = 16 – 2 = 14) 192.168.10.96/28 (leaves 4 bits for hosts) • 192.168.10.112/28 (leaves 4 bits for hosts)

  41. Configuring VLSM • Point-to-point connections are: • 192.168.10.128/30 (2 address required, 2 bits = 2 host addresses) • 192.168.10.132/30 (2 address required, 2 bits = 2 host addresses) • 192.168.10.136/30 (2 address required, 2 bits = 2 host addresses) • Choices = .136 .137 .138 .139 • Configuration of the 192.168.10.136/30 subnet • (.136/30 - network address; .137/30 and 138/30 – host addresses .139/30 - broadcast address; : • (config)#interface serial 0 • (config-if)#ip address 192.168.10.137 255.255.255.252 • (config)#interface serial1 • (config-if)#ip address 192.168.10.138 255.255.255.252

  42. RIP History Internet is a collection of autonomous systems (AS) • Each AS is administered by a single entity • Each AS has its own routing technology Routing protocol used within AS is Interior Gateway Protocol Routing protocol used between Autonomous Systems is an Exterior Gateway Protocol RIP v1: • is an IGP that is classful • designed to work within moderate-sized AS • is a distance vector routing protocol • by default, broadcasts entire routing table every 30 seconds • uses hop count as metric (16 max) • is capable of load balancing 6 equal-cost paths (4 default) • Does not send subnet mask information in its updates • Is not able to support VLSM or CIDR

  43. RIP History If the router receives information about a network, and the receiving interface belongs to same network but is on a different subnet, the router applies the one subnet mask configured on the receiving interface • Class A default classful mask is 255.0.0.0 or /8 • Class B default classful mask is 255.255.0.0 or /16 • Class C default classful mask is 255.255.255.0 or /24

  44. RIP v2 Features • RIP v2 is an Improved version of RIP v1 with following features: • Distance vector protocol • Uses hop count as metric • Uses hold-down timers (prevent routing loops), default 180 sec. • Uses split horizon to prevent routing loops • Uses 16 hops as infinite distance • Provides prefix routing (sends subnet mask with route update) • Supports use of classless routing (VLSM) • Multicasts updates using 224.0.0.9 address for better efficiency • Provides authentication in updates • Clear text - default • MD5 encryption – typically used to encrypt enable secret passwords (Message-Digest 5)

  45. Comparing RIP v1 & v2

  46. Configuring RIP v2 To enable a dynamic routing protocol: 1. Select routing protocol • FOSTER(config)#router rip • FOSTER(config-router)#version 2 • Configure routing protocol with the network IP address (identify physically connected network that will receive routing tables) • FOSTER(config-router)#network 10.0.0.0 • FOSTER(config-router)#network 172.16.0.0 • 3. Assign IP/SNM to interfaces

  47. Verifying RIP v2

  48. Verifying RIP v2 • RIP updates table every 30 seconds • If no update received in 180 seconds, route marked as down • If no update after 240 seconds, removes from routing table entry

  49. 1.2.6 Troubleshooting RIP v2

  50. Default Routes Three ways a router learns about paths: 1. Static routes– manual configuration of routes (next hop) • Uses ip route command 2. Default routes– manually defined path to take when there is no known route to a destination 3. Dynamic routes– routers lean paths by receiving updates from other routers

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