Download ccna1-ch10

Chapter 10
Routing fundamentals
& subnets
Contents:
•Routable and routed protocols
•IP Routing Protocols
•The Mechanics of Subnetting
Routable and routed protocols
•What is a protocol ?
•Is a set of rules that determines how
computers communicate with each other.
•A protocol describes:
•Message format.
•The way in which computers must exchange
a message.
•Routed protocol allows the router to
forward data between networks.
•Routed (routable) protocol must provide
network address and host address (IP/
IPX.
•IP routed protocol requires a subnet
mask.
•Subnet mask is used to obtain network
address from the host IP address.
•Subnet mask allows groups of
sequential addresses to be treated as a
single unit..
•IP as a routed protocol:
•Connectionless, unreliable, best-efforts
delivery protocol.
- Connectionless:
•No dedicated circuit is established prior
to transmission.
- Best-effort delivery:
•IP doesn’t verify that the data reached
its destination.
Packet propagation and switching
within a router
•As a packet moves through routers,
frame headers and trailers are replaced
at every router.
•This because frames are for local
addressing.
•While IP is for end to end addressing.
Internet protocol IP:
•Two type of delivery services:
- Connectionless service.
- Connection – oriented service.
•Connectionless service: (postal system)
•No actual (physical ) circuit is
established in advance.
•Destination is not contacted before a
packet is sent.
•No dedicated path between source and
destination.
•Packets may follow different paths to
destination.
•Packets may arrive out of order.
•Connectionless network processes are
Packet switched
•Connection oriented service:
•A connection is established between the
sender and the recipient before any data
is transferred. (Telephone system)
•Second phase: data transmission.
•Third phase: circuit termination..
•Packets follow the same path to
destination.
•Packets arrive in order
•Connection oriented network processes
are called circuit switched.
IP routing protocol
•Routing is a layer 3 function.
•Routers pass packets between networks
based on network addresses.
•The destination address (host address)
is needed only to deliver packet to its
final destination.
•Routing is the process of finding the
most efficient path to destination.
•Two key functions of a router:
•Routers build and maintain routing
tables (routing protocols) and,
exchange information with other routers
within the network topology.
•Packet switching.
•Router (routing protocol) use one or
more metrics to choose the best path to
destination network.
•Non routable protocols:
Do not support layer 3 addressing.
Examples : NetBUI
•Routing versus switching:
•Switching occurs at: Layer 2
•Routing occurs at: Layer 3
•Switch recognizes MAC addresses.
•Switch keeps a table of MAC addresses
•Router keeps a table of IP addresses
•Switched networks don't block
broadcasts.
•Broadcast storms only affect the
broadcast domain from which it
originated.
•Routers don’t pass broadcasts.
•Routed versus Routing:
•Routed protocol:
Transfer data from one host to another.
•Routing protocols:
Choose the best path from source to
destination.
•Routed protocol functions:
•provides enough information to allow
routers to forward it to final destination.
•Defines the format and use of the fields
within a packet
(IP, IPX, DECnet, ApplTalk..)
•Routing protocol functions:
•Build routing tables.
•Exchanges routing tables with other
routers.
•Enable routers to route routed
protocols.
(RIP, IGRP, OSPF, BGR, EIGRP)
•Path determination:
•Path determination is to choose the best
route among multiple routes available
in the routing table.
•Two types of routes:
Static routes.
Dynamic routes.
•Router uses path determination to
decide to which port the packet must be
sent toward its destination. Routing the packet.
•Each router along the path is called Hop.
•Path determination:
•Destination address is obtained
from the packet.
•The mask is applied to the address.
•Network address is compared to
routing table.
•If matched, the packet is forwarded to the
port associated with the routing table entry.
•If there is no match,
Default route is checked.
•If there is no default route:
Packet is discarded and a message of
destination unreachable is sent back.
•Routing tables include:
•protocol type.
•Destination/next-hop associations
•Routing metric
•Outbound interfaces
•IGP and EGP
•Autonomous system (AS):
Is a network or set of networks under
common administration control..
•Two families of routing protocols:
IGP.
EGP.
•IGP route data within AS:
- RIP and RIPv2
- IGRP
- EIGRP
- OSPF
•EGP route data between ASs:
- BGP
•distance vector routing protocols:
•Sends their routing tables periodically to
their neighbors.
•Router relies on its neighbors to get
routing information.
•Examples of distance-vector protocols:
- RIP (hop count).
-IGRP Cisco protocol to overcome RIP
limitations in large networks.
- EIGRP: (Cisco proprietary, hybrid
protocol.
•Link - state routing protocols:
- Routers exchange LSAs.
- Each router builds its own topology data
base.
- Each router runs SPF algorithm and
builds its own routing tables.
- Send link-state refreshes at longer time
intervals.
- Routers send trigger updates only when a
network changes has occured.
(OSPF and IS-IS)
Mechanics of subnetting
•Classes of IP addresses:
-bits are borrowed starting from the leftmost
bit of the host.
- Borrowed bits are reassigned as network
bits.
- Subnet address include class A, B , or C
plus the subnet field.
•Establishing the subnet mask addressing:
- Determine the maximum number of
subnets.
- Determine the maximum number of hosts
per subnet.
- Never assign the last two bits regardless of
the IP class.
- Use of all bits except the last two bits will
result in subnets with only two usable hosts.
(for serial links).
- The subnet mask gives the router the
information required to determine in which
network and subnet a particular host
resides.
- The subnet mask is created by using binary
ones in the positions of borrowed bits.
- If 3 bits were borrowed, the subnet mask
for calss C would be 255.255.255.224
- This mask can be represented as /27.
Usable subnets:
2n - 2
Usable hosts:
m
2
-2
•Example: