CCNA 1 Module 10 Routing Fundamentals and Subnets

Copyright © 2005 University of Bolton Today’s Topics Routed Protocols IP Routing Protocols The Mechanics of Subnetting

Copyright © 2005 University of Bolton Routable Protocols What is a protocol Routable protocol must have network addressing and host addressing IPX uses network address + MAC address IP uses subnet masks to indicate network and host parts of an address

Copyright © 2005 University of Bolton IP Protocol IP is a connectionless, unreliable, best-effort delivery protocol Connectionless: no fixed route, routers forward traffic based on routing protocol rules Unreliable/best effort: don’t mean IP isn’t very good, instead they mean that IP doesn’t check that data is delivered, that’s left to higher layers

Copyright © 2005 University of Bolton Encapsulation/de-encapsulation

Copyright © 2005 University of Bolton Connections? Connectionless protocol – e.g. Postal Service Connection-oriented – e.g. Telephone Call Internet is large connectionless network Routes vary due to variety of factors including congestion, route cost, available bandwidth

Copyright © 2005 University of Bolton IP Packet Format

Copyright © 2005 University of Bolton Routing Routing is an OSI layer 3 (network layer) function Routers maintain routing tables with information about other networks available on each interface Routers make decisions about forwarding packets based on layer 3 addresses IP most common routable protocol, also IPX/SPX and Appletalk but not NetBEUI

Copyright © 2005 University of Bolton Comparison of Routers and Switches Routers operate at layer 3, switches at layer 2 Compare to telephone exchange Switch maintains switching table based on MAC addresses which are essentially random Router maintains routing table based on IP addresses which are hierarchical

Copyright © 2005 University of Bolton Routing vs Routed Routed or Routable Protocols –Operate at Layer 3 (network layer) –Includes network layer address that allows a router to forward towards destination –E.g. IP, IPX, AppleTalk and others Routing Protocols –Communication between routers –Build up picture of network to allow routing decisions –E.g. RIP, OSPF, BGP and others

Copyright © 2005 University of Bolton Path determination Router builds a table of static and dynamic routes Contains Network addresses and interface # Router works down the table matching network portion of destination address with table (using netmask) If a match, packet is forwarded through interface, if not the router moves on to the next entry.

Copyright © 2005 University of Bolton Routing Tables Routing protocols determine contents of routing table based on periodic communication with other routers Table contains –Protocol type –Next hop (whether directly connected or not) –Routing Metric –Outbound Interface Routing metrics are calculated in different ways by protocols

Copyright © 2005 University of Bolton Routing Algorithms Process varies greatly due to design goals –Optimisation - Capacity to select best route –Simplicity and Low Overhead - for scalability –Robustness and Stability - cope with load and failures –Flexibility - adapt quickly to network changes e.g. BW –Rapid Convergence - calculate best routes quickly Depending on circumstances and priorities, users choose appropriate algorithm

Copyright © 2005 University of Bolton Routing Metrics Commonly used metrics include –Bandwidth –Delay –Load –Reliability (error rate) –Hop Count –Ticks –Cost (assigned)

Copyright © 2005 University of Bolton IGP and EGP Autonomous System (AS): network(s) under the common administrative control e.g. bolton.ac.uk Interior Gateway Protocols (IGP) route within AS –RIP and RIP v2 –IGRP and EIGRP –OSPF –IS-IS Exterior Gateway Protocols (EGP) route between A –BGP

Copyright © 2005 University of Bolton Distance Vector Protocols Routers use distance and direction (vector) to decide routing Routers exchange complete routing tables regularly to learn network structure Metric can be hop count or a more complex calculation Protocols: RIP, IGRP, EIGRP

Copyright © 2005 University of Bolton Link State Protocols Respond quickly to network changes Generate a Link State Advertisement (LSA) and send to adjacent routers LSAs propagated to other routers on the network Algorithms include IS-IS and OSPF

Copyright © 2005 University of Bolton RIP Protocol Distance vector protocol Uses hop count as metric Max 15 hops Fastest route may not be shortest Requires common subnet mask

Copyright © 2005 University of Bolton RIP v2 Protocol Can support multiple subnets on a network Known as classless routing Using different subnet masks is known as Variable Length Subnet Masking (VLSM)

Copyright © 2005 University of Bolton IGRP Cisco proprietary protocol Distance vector protocol Uses performance metrics such as bandwidth and delay rather than hop count

Copyright © 2005 University of Bolton OSPF Open Shortest Path First Industry standard protocol Link state algorithm Copes well with large networks

Copyright © 2005 University of Bolton EIGRP Cisco proprietary protocol Advanced algorithm with low overheads compared to IGRP Distance vector protocol with some link state features (hybrid protocol)

Copyright © 2005 University of Bolton Border Gateway Protocol (BGP) Exchanges information between AS Principal protocol used between major Internet outfits and ISPs Decisions based on policies rather than metrics

Mechanics of subnetting

Copyright © 2005 University of Bolton Introduction Extend the network + host address model to network + subnet + host Some host bits used to indicate subnetwork instead Advantages –Contain network broadcasts –Low level security –Traffic between subnets must pass through router

Copyright © 2005 University of Bolton Establishing Subnet Masks Need to know how max hosts on each subnet and map to nearest power of 2 Tells us how many host bits we need Remained can be used for subnet addresses n2 n

Copyright © 2005 University of Bolton Subnet Mask Calculation Subnet mask uses binary 1s to represent network portion of address and 0s for host portion Or Often represented using slash notation /26 meaning 26 network bits e.g /26

Copyright © 2005 University of Bolton Building a subnet table /27 3 bits used for subnet, 5 for host

Copyright © 2005 University of Bolton Network classes ClassNetwork bitsHost bits A824 B16 C248 Have scope for more subnets, or larger subnets with class A and class B addresses On a class B network, if we need 2000 subnets containing 25 hosts what is the netmask?

Copyright © 2005 University of Bolton Calculating subnets using AND Address: / Subnetwork Address: