Switching Basics and Intermediate Routing CCNA 3 Chapter

Switching Basics and Intermediate Routing CCNA 3 Chapter 1 www. ciscopress. com

VLSM • Variable-length subnet masks were developed to allow multiple levels of subnetted IP addresses within a single network • The routing protocol you use must support VLSM – Open Shortest Path First (OSPF) – Enhanced Interior Gateway Routing Protocol (EIGRP) – Routing Information Protocol version 2 (RIPv 2) • VLSM is crucial for an effective IP addressing plan www. ciscopress. com

VLSM Prefix Length • Prefix length is a shorthand way for expressing the subnet mask for a particular network – Number of 1 s in the binary representation of the subnet mask • When bits are taken from the host part of an address and added to the network part, the number of the bits in the host part decreases – You create additional subnets at the expense of the number of host devices on each network segment www. ciscopress. com

VLSM Prefix Length • Number of subnets can be calculated using the 2 s formula, where s is the number of bits by which the default mask is extended • In IOS releases prior to 12. 0, you must explicitly allow subnet 0 • In IOS releases 12. 0 and later, subnet 0 is enabled by default • The all-1 s subnet has always been allowed www. ciscopress. com

VLSM Prefix Length • Bits that are not part of the network or subnetwork portions of the address are the range of host address • Use the 2 h – 2 formula (where h is the number of host bits) to calculate available host addresses; all 0 s in host portion is the subnet identifier address, all 1 s in host portion is the subnet broadcast address www. ciscopress. com

VLSM Prefix Length Network Mask and IP Address for the Range 192. 168. 1. 64 Through 192. 168. 1. 79, with Host Bits Shaded • In the IP network number that accompanies the network mask, the following are true: – When the host bits are all binary 0 s, that address is the beginning of the address range – When the host bits are all binary 1 s, that address is at the end of the address range www. ciscopress. com

VLSM Prefix Length Fourth Octet for the Range 192. 168. 1. 64 Through 192. 168. 1. 79 (continued on next slide) www. ciscopress. com

VLSM Prefix Length (continued) Fourth Octet for the Range 192. 168. 1. 64 Through 192. 168. 1. 79 (continued) www. ciscopress. com

VLSM Prefix Length • In this example, PCs use the prefix length of 28 (the subnet mask 255. 240) to determine which other devices on their local network have their first 28 bits in common – A 28 -bit prefix length permits 14 hosts per subnet • The PC uses ARP to find the corresponding destination MAC address if communication with any of these devices is necessary • If the destination IP address is not in the range for the subnet, the packet is forwarded to the default gateway www. ciscopress. com

VLSM Prefix Length • A router works in a similar manner when it makes a routing decision – It compares the destination IP address of the packet to network entries in the routing table – The network entries have a prefix length associated with them – The router uses the prefix length to determine how many destination bits must match to send the packet out the corresponding outbound interface that is associated with the network number in the routing table www. ciscopress. com

VLSM Prefix Length • The router determines from the table where to send the packet destined for 192. 168. 1. 67 – In this table, there are four entries for network 192. 168. 1. 0 – The third entry is for the 192. 168. 1. 64 subnet, which is the subnet to which 192. 168. 1. 67 belongs – Note that the next subnet, 192. 168. 1. 80, begins with a number larger than 192. 168. 1. 67 www. ciscopress. com

VLSM Benefits of VLSM • More efficient use of IP addresses – Without use of VLSM, a single subnet mask must be implemented with an entire Class A, B, or C network • Greater capacity to use router summarization (discussed later in this chapter) – Allows more hierarchical levels within an addressing plan • Isolation of topology changes from other routers www. ciscopress. com

VLSM Benefits of VLSM Permits Flexible, Efficient Subnet Address Allocation www. ciscopress. com

VLSM Calculations • VLSM is used to maximize number of possible IP addresses available for a network – Point-to-point serial links require only two host addresses, so a /30 subnet does not waste scarce subnet addresses • With VLSM, you can subnet a subnet! • Next slide will show the subnet 172. 16. 32. 0/20 is further subnetted with a /26 prefix www. ciscopress. com

VLSM Calculations Further Subnetting 172. 16. 32. 0/20 to /26 Prefixes www. ciscopress. com

VLSM Example VLSM Used to Define Subnets of 172. 16. 32. 0 Across the Boundary Between Octets Three and Four www. ciscopress. com

VLSM CIDR and Route Summarization • The definition of classless inter-domain routing (CIDR): – Allocation of one or more blocks of Class C network numbers to each network service provider – Organizations using the network service provider for Internet connectivity are allocated bitmask-oriented subsets of the provider’s address space as required • CIDR (“cider”) was developed to address the problem of IP address space running out and core Internet routers running out of capacity • Route summarization is the representation by a single network of a group of contiguous networks www. ciscopress. com

VLSM CIDR and Route Summarization of Contiguous Subnets of a Class B Network www. ciscopress. com

VLSM CIDR and Route Summarization of Contiguous Subnets of a Class B Network (continued) • Router D in previous slide has these networks in its routing table – – 172. 16. 12. 0/24 172. 16. 13. 0/24 172. 16. 14. 0/24 172. 16. 15. 0/24 • To calculate the summary route: – Find the number of highest-order bits that match in all addresses – Locate where the common pattern of digits ends – Count the number of common bits; this is the length of the summary route www. ciscopress. com

VLSM CIDR and Route Summarization of Contiguous Subnets of a Class B Network (continued) • Follow these guidelines when calculating summary routes: – Addresses that do not share the same number of bits as the prefix length of the summary route are not included in the summarization block – The IP addressing plan is hierarchical in nature to allow router to aggregate the largest number of IP addresses into a single summary route – IP networks can only be summarized in 2 n networks (for some n), where the last octet of the first network in the sequence is divisible by 2 n www. ciscopress. com

VLSM Route Aggregation • By using a prefix length instead of an address class to determine the network portion of the address, CIDR allows routers to aggregate routing information – Shrinks routing table – One address and mask combination can represent the routes to multiple networks • Route aggregation is used more loosely than CIDR; describes the summarization of classful networks • Without CIDR, routers must maintain tables for individual networks www. ciscopress. com

VLSM Route Aggregation CIDR Permits the Aggregation of Contiguous Class B Networks www. ciscopress. com

VLSM Route Aggregation Summarization Employs the Furthest-to-the-Right Principle www. ciscopress. com

VLSM Route Aggregation • In previous slide, the router can summarize routes to these networks using a 13 -bit prefix which these 8 networks share – 10101100 00011000 00000000 = 172. 24. 0. 0 – 11111000 00000000 = 255. 248. 0. 0 • A single address and mask define a classless prefix that summarizes routes to the eight networks: 172. 24. 0. 0/13 www. ciscopress. com

VLSM Route Aggregation • Using a prefix to summarize routes results in the following: – More efficient routing – A reduced number of CPU cycles when calculating a routing table or sorting through routing table entries to find a match – Reduced router memory requirements www. ciscopress. com

VLSM Supernetting • The practice of using a summary network to group multiple classful networks into a single address is called supernetting – Subnetting breaks down a classful network – Supernetting pastes together classful networks • With Class A and B address space almost exhausted, large organizations requested multiple Class C network addresses from their service providers • A block of contiguous Class C addresses can appear as a single large network, or supernet www. ciscopress. com

VLSM Supernetting • Supernetting and route aggregation are similar – Route aggregation is used in the context of summarizing routes with BGP – Supernetting is a term used when the summarized networks are under common administrative control • Many networking professionals use the terms “route summarization” and “route aggregation” interchangeably www. ciscopress. com

VLSM CIDR Example CIDR Permits the Aggregation of Several Classful Networks into a Single Route Advertisement www. ciscopress. com

Classful and Classless Routing • Behavior of classful routing is limited compared to classless routing – Classful routing protocols(RIPv 1, IGRP) cannot do VLSM • Make routing decisions and send routing updates according to Class A, B, and C constructs – Classless routing protocols work independently of Class A, B, and C addresses • In the “real world, ” classful routing protocols are close to becoming irrelevant www. ciscopress. com

Classful and Classless Routing Classful Routing • RIPv 1 and IGRP are the two classful routing protocols – Rare to see either of these employed on a router today – Classful routing protocols do not include subnet mask information in their updates • The router applies two options when receiving a routing update packet – If the routing update information contains the same major network number as configured on the receiving interface, the router applies the subnet mask that is configured on that interface – If the routing update information contains a different major network than the one configured on the receiving interface, the router applies the default subnet mask www. ciscopress. com

Classful and Classless Routing Classful Routing • The router applies two options when receiving a routing update packet (continued) – The default classful masks are: • Class A: 255. 0. 0. 0 • Class B: 255. 0. 0 • Class C: 255. 0 • All subnets of the same major network (Classes A, B, and C) must use the same mask when using a classful routing protocol www. ciscopress. com

Classful and Classless Routing Classful Routing • Routers running a classful routing protocol perform automatic route summarization across network boundaries – They make assumptions about networks based on their IP address class – These assumptions lead to automatic summarization of routes when routers send routing updates across major classful network boundaries • Routers send update packets to other connected routers – Routers sends entire subnet address (without mask); assume the network and the interface use the same subnet mask www. ciscopress. com

Classful and Classless Routing Classful Routing • Router receiving the update makes the same assumption – If different masks are used, router would have wrong information in routing table – Important to use the same subnet mask on all interfaces that belong to the same classful network • When a router using a classful protocol sends an update regarding information of a subnet of a classful network across an interface belonging to a different classful network, the router assumes the remote router will use the default subnet mask for that IP address class www. ciscopress. com

Classful and Classless Routing Classful Routing Automatic Summarization Occurs at Classful Boundaries with RIPv 1 and IGRP www. ciscopress. com

Classful and Classless Routing Classful Routing • The process in the previous slide is automatic summarization across the network boundary – Router sends a summary of all the subnets by sending only major network information – Classful routing protocols automatically create a classful summary route at major network boundaries – Classful routing protocols do not allow summarization at other points within the major network space www. ciscopress. com

Classful and Classless Routing Classful Routing • The router that receives the updates behaves in a similar fashion – When a routing update contains information about a different classful network than the one that is in use on its interface, the router applies the default classful mask to that update • When using classful routing protocols, assigning the same subnet mask to all subnets is called fixed-length subnet masking (FLSM) – sometimes called static-length subnet masking www. ciscopress. com

Classful and Classless Routing Discontiguous Subnets • A classical problem with classful routing protocols: – Discontiguous subnets occur when a major network separates subnets of a major network – This can cause erroneous entries in routing tables – Traffic will not always reach its destination • Do not permit the use of discontiguous networks when using a classful routing protocol www. ciscopress. com

Classful and Classless Routing Discontiguous Subnets Present a Problem with Classful Routing www. ciscopress. com

Classful and Classless Routing Default Routes • Routers learn paths to destinations in three ways: – The system administrator defines static routes via an attached interface or the next hop to a destination – The network engineer manually defines default routes as the path to take when no known route exists to the destination; default routes minimize the size of the routing table – Dynamic routing occurs when the router learns of paths to destinations by receiving routing updates from other routers via a routing protocol www. ciscopress. com

Classful and Classless Routing Default Routes • You can define a static route with the ip route command: • You can define a default route with the ip default-network command: www. ciscopress. com

Classful and Classless Routing Default Routes A Default Network is Configured Pointing Toward the Internet www. ciscopress. com

Classful and Classless Routing Default Routes • You can define a default route to work with either static or dynamic routing: • The 0 s represent any destination with any mask • Default routes are often referred to as quad-zero routes www. ciscopress. com

Classful and Classless Routing Classful Routing Table • What does a router running a classful routing protocol do with packets that lie in subnets that have no entry in the routing table? – The router discards the packets! • This can be overcome by using the ip classless command – Causes the router using a classful routing protocol to evaluate all packets using the longest-match criterion – As a last resort, the router uses a configured default route www. ciscopress. com

Classful and Classless Routing • All routing protocols except RIPv 1 and IGRP are classless routing protocols • RIPv 2, OSPF, IS-IS, EIGRP, and BGPv 4 are classless routing protocols that support VLSM and CIDR • With classless routing protocols, different subnets in the same major network can have different subnet masks – Maximizes use of addresses www. ciscopress. com

Classful and Classless Routing • Classful routing protocols automatically summarize to the classful network boundary; classless routing protocols allow you to control the route summarization process manually (might be needed to limit size of routing tables) • Classless routing protocols do not automatically advertise every subnet • By default, classless routing protocols perform automatic network summarization at classful boundaries, just like classful protocols www. ciscopress. com

Classful and Classless Routing • Difference between classless routing protocols and their predecessors is that you can manually turn off automatic summarization – Use the no auto-summary command – Not needed with OSPF or IS-IS • Automatic summarization cause problems in networks with discontiguous subnets – This can be fixed by turning off automatic summarization www. ciscopress. com

Classful and Classless Routing Discontiguous Subnets Presenting a Problem with Classless Routing www. ciscopress. com

Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary • Beginning with IOS Release 12. 2(8)T, EIGRP and BGP had auto-summary enabled by default • RIPv 2 has always had auto-summary enabled by default Default Behavior of RIPv 2 is to Automatically Summarize at the Network Boundary www. ciscopress. com

Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary RIPv 2 Supports VLSM with Automatic Summarization Disabled www. ciscopress. com

Classful and Classless Routing Effect of Auto-Summary and No Auto-Summary • To disable auto-summary in RIPv 2, use the no auto-summary command as seen below www. ciscopress. com

RIP Version 2 • RIP Version 1 characteristics – Uses hop count as the metric for path selection – Maximum allowable hop count is 15, so infinite distance equals 16 hops – Uses hold-down timers to prevent routing loops with a default of 180 seconds – Employs split horizon to prevent routing loops – Failure to receive routing updates in a timely manner results in removal of routes previously learned from a neighbor www. ciscopress. com

RIP Version 2 • RIP Version 1 characteristics (continued) – The administrative distance is 120 – Routing updates are broadcast every 30 seconds by default – Is capable of load-balancing over as many as six equal-cost paths; four is the default – Does not support authentication – Does not support VLSM because it is a classful routing protocol www. ciscopress. com

RIP Version 2 • RIP Version 2 characteristics – Uses hop count as the metric for path selection – Maximum allowable hop count is 15, so infinite distance equals 16 hops – Uses hold-down timers to prevent routing loops with a default of 180 seconds – Employs split horizon to prevent routing loops – Failure to receive routing updates in a timely manner results in removal of routes previously learned from a neighbor www. ciscopress. com

RIP Version 2 • RIP Version 2 characteristics (continued) – The administrative distance is 120 – Routing updates are multicast every 30 seconds by default – Is capable of load-balancing over as many as six equal -cost paths; four is the default – Supports clear text and Message Digest 5 (MD 5) authentication – Supports VLSM because it is a classless routing protocol – Supports manual route summarization www. ciscopress. com

RIP Version 2 • Major improvements with RIPv 2: – Support of authentication • Clear text is the default • MD 5 used to encrypt enable secret passwords – VLSM use – Sending subnet masks in updates – Multicasting routing updates • Uses 224. 0. 0. 9 as destination • Keeps PCs and servers from having to process the broadcast www. ciscopress. com

RIP Version 2 – Multicasting routing updates (continued) • Keeps PCs and servers from having to process the broadcast (continued) – IP sends the packet to the User Datagram Protocol (UDP) and UDP checks whether RIP port 520 is available; most PCs and servers do not have a process running on this port and discard the packet – Sometimes it is running as a gateway discovery technique in TCP/IP services, such as UNIX or Windows www. ciscopress. com

RIP Version 2 • Broadcast disadvantages of RIPv 1 – RIPv 1 can fit up to 25 networks/subnets in each update; updates are sent every 30 seconds • If the routing table has 1000 subnets, 40 packets will be sent every 30 seconds • Each of these broadcasts will have to be looked at by all devices on the network www. ciscopress. com

RIP Version 2 • Multicast advantages of RIPv 2 – The IP multicast address for RIPv 2 has its own MAC address: 0 x 0100. 5 e 00. 0009 – Devices such as PCs and servers read this MAC address and determine it is not for them; they discard the frame – If a device can’t distinguish this MAC address, the packet will be discarded at the IP layer (OSI network layer) as the multicast IP address is not the IP address of the device www. ciscopress. com

RIPv 2 Configuration • The router rip command starts a RIP routing process; the network command causes the implementation of these three functions: – Routing updates are multicast out an interface – Routing updates are processed if they enter that same interface – The subnet that is directly connected to that interface is advertised www. ciscopress. com

RIPv 2 Configuration Sample Network and Configuration of RIPv 2 www. ciscopress. com

RIPv 2 Configuration • In the previous slide, these commands were used to configure Router A: – Enable RIP as the routing protocol: router RIP – Identify Version 2 as the RIP being used: version 2 – Specifying a directly connected network: network 172. 16. 0. 0 – Specifying a directly connected network: network 10. 0 www. ciscopress. com

Verifying RIP Configuration Sample Network for Verifying RIP Configuration www. ciscopress. com

Verifying RIP Configuration • Most common commands for verifying RIP Configuration: – Display parameters for routing protocols: show ip protocols – Summary of IP information and status of all interfaces: show ip interface brief – Ensure that appropriate commands are configured for the RIP network: show running-config – Display contents of routing table: show ip route www. ciscopress. com

Verifying RIP Configuration www. ciscopress. com

Verifying RIP Configuration www. ciscopress. com

Verifying RIP Configuration Fields in the Routing Table Defined www. ciscopress. com

Troubleshooting RIP Configuration Sample Network for Troubleshooting RIP Configuration The debug ip rip command displays real-time RIP routing updates as they are sent and received To turn off debugging, use the no debug ip rip or the undebug all (u all) commands www. ciscopress. com

Troubleshooting RIP Configuration The debug ip rip command www. ciscopress. com

Troubleshooting RIP Configuration Sample debug ip rip output www. ciscopress. com

Summary • Classless IP addressing is implemented with: – VLSM: the ability to subnet and use different subnet masks in the same classful network – CIDR: the allocation of blocks of contiguous address space to customers by ISPs – Route summarization: a generic term that describes the use of a single network to represent a sequence of logically contiguous networks – Route aggregation: a generalized form of supernetting – Supernetting: pasting together classful networks into supernets www. ciscopress. com

Summary • Classful routing protocols: – RIPv 1 – IGRP • Classless routing protocols: – – – RIPv 2 EIGRP OSPF IS-IS BGPv 4 www. ciscopress. com

Summary • RIPv 2, EIGRP, and BGPv 4 can turn automatic route summarization on and off • RIPv 2 is an improvement to RIPv 1 – Adds authentication, VLSM support, passing of subnet masks in routing updates, and multicasting of routing updates • Configuring RIPv 2 requires adding the version 2 command; adding no auto-summary is recommended • All connected networks participating in RIP are defined with the network command in the form of classful networks www. ciscopress. com

Summary • RIP configuration can be verified with several commands: show ip protocols, show ip interface brief, show running-config, and show ip route • You can troubleshoot RIP with the debug ip rip command www. ciscopress. com