Cisco – S1C10

Routers

All You Ever Wanted To Know

But Were Afraid to Ask

Routers – Intelligent Devices

• Make best path decisions• Use Layer 3 addresses to make forwarding

decisions• Facilitate ARP – Address Resolution Protocol to

learn MAC addresses• Facilitate RARP – helping a device learn its IP

address• Uses Routing Protocols (determine best path) on

Routed Protocols (protocols that can be routed)

Review of Layer 2 Devices

• Bridges and Switches operate at Layer 2– Use MAC (Physical) address to filter and forward

– Used to connect segments of a network

• Routers operate at Layer 3– Use IP (logical) addresses to forward and switch

– Used to connect separate networks and to access WWW

– Provide end-to-end routing

Router Connections

• Router Interfaces must have addresses• Functions Performed

– Strips off data link header carried by frame (contains MAC addresses)

– Examines network layer addresses (IP) to determine destination network

– Consults routing tables to determine which interface (port) to use to forward packet

• Each interface requires separate, unique network address

– Encapsulates packet into proper frame to route• Token Ring, Ethernet, FDDI, etc.

Assigning IP Addresses

• Static– Configure each device with IP address

• Keep meticulous records– Some operating systems, such as Windows 95 and

Windows NT, send an ARP request to check for a duplicate IP address when they attempt to initialize TCP/IP

– Generate error message and won’t initialize TCP/IP if they find duplicate IP address

Addressing

• Dynamic– RARP – Reverse Address Resolution Protocol

• Binds MAC address to IP device• Requires RARP server to answer RARP requests• MAC and IP headers and operation code are difference from

ARP request• Broadcast goes to all devices on network

– BOOTstrap Protocol (BOOTP)• Uses UDP to carry messages; UDP encapsulated in IP datagram• Takes IP address that matches its MAC address

– Operates in a client-server environment– Can contain IP address, router address, server address

Addressing - DHCP

• Dynamic Host Configuration Protocol– Allows host to obtain an IP quickly– Defined range of IP addresses reside on DHCP

server– Hosts request address from server– Server chooses address and assigns it – Server can also send subnet mask

DHCP Process

• Sends CHCPDiscover broadcast message– UDP packets with port number set to BOOTP port

– Client moves into select state and collects DCPOFFER responses from server

– Client selects first response and negotiates lease time (length of time to keep address without renewing it) by sending DHCPREQUEST packet

– Acknowledgement from server enters client into BOUND state

ARP Request and ICMP

• ARP Address Resolution Protocol– Uses to find IP address when MAC address is

known

• ICMP (Internet Control Message Protocol)– Used to report a problem to sender of message

• E.g. undeliverable packet

• Echo request/echo reply - pinging

ARP Tables

• Contain MAC and IP addresses of devices on same LAN – Map IP address to IP address (in RAM )

• When source host locates entry in its table, it binds or associates the IP address to the MAC address and uses it to encapsulate the data

Using ARP to Determine IP Address

• If MAC-IP address bond is not in ARP table– Host sends broadcast FF FF FF FF FF FF to all

hosts on network segment– If IP address is on segment, host responds by

sending its MAC address – ARP Reply

• If IP address is not discovered– Gateway IP is used

• Router sends ARP using its MAC address

Routers and ARP Requests

• If Gateway (router) recognizes that IP address is on same subnet as source host, it discards packet

• If subnet address is different, router responds with its MAC address (proxy ARP)

• Router then sends the ARP request to the proper subnet

• If necessary, Router sends ARP request to another router --- INDIRECT ROUTING

Routed or Routable Protocols

• Protocols that provide support for the network layer

• IPX, IP, Appletalk, DecNET

• NOT ROUTABLE– NetBEUI – small, fast, efficient, but limited to

running on one segment

Routing Algorithms

• Design goals include:– Optimization– Simplicity and low overhead– Robustness and stability– Rapid convergence– Flexibility– scalability

Routing Metrics

• Bandwidth

• Delay

• Load

• Reliability

• Hop count

• Cost

Routing ProtocolsAll Interior

• Determine the paths that routed protocols follow to destinations

• RIP Routing Information Protocol– Hops

• IGRP Cisco – uses several metrics

• OSPF Open Shortest Path First

• EIGRP Cisco - hybrid

Exterior Gateway Protocol - EGP

• Used to connect to outside world– Internet– Other networks– BGP – Border Gateway Protocol is primary

exterior routing protocol used on the Internet

RIP

• 15 hops is maximum• Belongs to Interior Gateway Protocols

– Used only in autonomous networks

• Updates routing table every 30 seconds• Create large amounts of network traffic

because are constantly connecting to neighboring routers

• Uses one metric – distance vector

IGRP

• Cisco protocol

• Distance vector protocol

• Uses other metrics as well– Bandwidth– Load– Delay– reliability

EIGRP

• Enhanced Interior Gateway Routing Protocol

• Hybrid of Link-State and Distance Vector– Basic difference is way they update routing

tables and metrics used

• Provides superior operating efficiency• Combines advantages of link-state

protocols and distance vector protocols

OSPFLINK State Protocol

• Open Shortest Path First– Determines optimum path– Uses several criteria to determine best route

• Cost

• Route speed

• Traffic

• Reliability

• Security

IS-ISLink State

• Intermediate System – Intermediate System– Based on Decnet– Intermediate system is a router– Uses CLNP – connectionless Network Protocol

in a CLNS – connectionless Network environment

– Alternative to OSPF• Mixes CLNP and IP routing in one protocol

Encapsulation

• If router strips frame header and can’t find destination IP address, it may drop the packet

IP Packet

• Version• IP Header Length• TOS – type of service• Total length• ID• Flag• Fragment Offset• TTL – counter that decreases (increments of 1)• Protocol• Source and Destination IP address• Options, Padding, and Data

Multiple Protocols

• Routers can support many routed protocols– IPX, IP, AppleTalk, DecNet– IPX is Novell protocol– IP is Internet protocol– AppleTalk is Macintosh protocol– DecNet is DEC protocol

Static Routes

• Manual entries in routing tables– Entered by network administrator– Can be used to test a link on the network– Also used to conserve wide area bandwidth– Preferred way when there is only one path to

destination network• Referred to as STUB network

Dynamic Routing

• Eliminates need for network administrators to manually enter information

• Works best when bandwidth and large amounts of network traffic are not issues

• RIP, IGRP, EIGRP, and OSPF can handle dynamic routing

• Internet would be impossible without dynamic routing

Connectionless vs Connection Oriented

• Connectionless– Destination not contacted before packet is sent

• Postal System – Packet Switched

• Connection Oriented– Destination contacted before packet is sent

• Telephone system – Circuit Switched

Subnetting

• Borrows Host bits to form subnets• Must borrow 2 bits and must leave 2 bits• Class C

– Can borrow from 2-6 bits

• Class B– Can borrow from 2-14 bits

• Class C– Can borrow from 2-22 bits

Subnet Mask

• Network Address all 1 bits• Subnet address all 1 bits• Host portion of address 0 bits

• Network is 192.16.20.0• Borrow 3 bits from host octet for subnets• 11111111.11111111.11111111.11100000• 255.255.255.224

Subnet Mask Examples

• 10.0.0.0 is network• Octets 2 and 3 are used for subnets• Subnet mask is 11111111.11111111.11111111.00000000• 255.255.255.0• 172.16.0.0 is network• Half of octet 3 is used for subnets• Subnet mask is• 11111111.11111111.11110000.0000• 255.255.240.0.0