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CCNA 3/Module 1

CCNA 3/Module 1. Introduction to Classless Routing. 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”.

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CCNA 3/Module 1

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  1. CCNA 3/Module 1 Introduction to Classless Routing

  2. 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”

  3. 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.

  4. 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.

  5. 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.

  6. 1.1.1 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

  7. 1.1.1 What is VLSM and why is it used? Classfulrouting protocols use one subnet mask for a single network • Ex: 192.168.187.0, must use subnet mask 255.255.255.0 VLSMallows a single autonomous system to have networks withdifferent 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)

  8. 1.1.2 A waste of space • In classless routing, it was recommended that firstand lastsubnet not be used • First (SN 0) had same address for the network and subnet • Last subnet (all-1’s) was the broadcast • Always could have been used, was not recommended practice • Address depletion has lead to use of these subnets • Now acceptable practice to use the first and last subnets in conjunction with VLSM

  9. 1.1.2 A waste of space

  10. 1.1.2 A waste of space If subnet zero is used, there are 8useable subnets • Each subnet can support 30 hosts • Cisco routers use subnet zero by default IOS v. 12.0+ If no ip subnet-zero command is used on the router, there are 7useable subnets with 30 hosts per subnet • If supporting 4 routers (1 subnet each) that need3WAN 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

  11. 1.1.2 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

  12. 1.1.3 When to use VLSM Design addressing scheme that allows: • Growth • Doesn’t waste addresses on point-to-point links • 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

  13. 1.1.4 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

  14. 1.1.4 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

  15. 1.1.4 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 /25 • 50 /26 • 2 /30

  16. 1.1.4 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

  17. 1.1.4 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

  18. 1.1.4 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

  19. 1.1.5 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 aggregationby using Classless InterDomain Routing(CIDR) • 172.16.14.0 • 172.16.15.0 • Router needs to carry only one route 172.16.14.0/23

  20. 1.1.5 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 if the routers of a network run a classless routing protocol such as OSPF or EIGRP • Consists of IP address and bit mask in routing updates • The summary route uses prefix common to all addresses of organization

  21. 1.1.5 Route Aggregation w/VLSM Carefully assign addresses in a hierarchical fashion to share same high-order bits for summarization • A router must know subnets attached in detail • A router does not need to tell other routers about subnets • A router 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

  22. 1.1.6 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)

  23. 1.1.6 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 as follows for the 192.168.10.136/30 network (.136/30 - network address;.139/30 - broadcast address; .137/30 and 138/30 – host addresses: • (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

  24. 1.2.1 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 InteriorGateway Protocol Routing protocol used betweenAutonomous Systems is an Exterior Gateway Protocol RIP v1: • is an IGP that is classful • was 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

  25. 1.2.1 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 • Class B default classful mask is 255.255.0.0 • Class C default classful mask is 255.255.255.0

  26. 1.2.2 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)

  27. 1.2.3 Comparing RIP v1 & v2

  28. 1.2.4 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

  29. 1.2.5 Verifying RIP v2

  30. 1.2.5 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

  31. 1.2.6 Troubleshooting RIP v2

  32. 1.2.7 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

  33. 1.2.7 Default Routes Default Route Command: Default Route Command: FOSTER(config)# ip route 172.16.1.0255.255.255.0172.16.2.1 FOSTER(config)# ip route 172.16.1.0255.255.255.0172.16.2.1 Default NW Default NW Tells that 8 bits of subnetting in effect Tells that 8 bits of subnetting in effect Next hop router Next hop router

  34. 1.2.7 Default Routes DYNAMIC PROTOCOL Default Route Command Used to: 1. Give packets that are not ID’d in the routing table a place to go • Usually a router that connects to the Internet 2. Connect a router with a static default route FOSTER(config)# ip default-network 192.168.20.0 Default NW

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