Download Class Extra Routing for Chapter #6

Chapter 8
Routing
Introduction
• Look at:
– Routing Basics (8.1)
– Address Resolution (8.2)
– Routing Protocols (8.3)
– Administrative Classification (8.4)
– Hierarchical Routing (8.5)
Introduction
• When networking was first introduced,
only a small number of devices were
interconnected
• As networks grew, broadcasts and
collisions caused significant issues
• In order to reduce broadcasts, Layer 3
devices, protocols, and addresses are
used to segment the network
Routing Basics
• Reasons to segment a network at Layer
3 include:
– Creation of small troubleshooting areas
– Creation of small administrator
management areas
– Interconnection of remote offices using
WAN technologies
– Grouping clients together with similar
network resources
Routing Basics
• A router is required to properly forward
data between clients on different
segments.
• A router is a device that forwards data
based on a logical Layer 3 address
• Many routers support the use of
different protocols
Routing Basics
• The routing process usually occurs between
physical network interfaces but can also be
accomplished between logical network
interfaces
• When a single physical network interface has
more than one address assigned, it is said to
have a logical interface
• A router can be any network device with the
proper software to make routing decisions
Routing Basics
• Two key pieces of information are
required for any device to route packets:
– A route to one or more networks
– A destination Layer 3 address
• The routes to a given network are
stored in the memory of the router and
are referred to as a routing table
Routing Basics
• On a Router there is an entry for each
network and its corresponding network
interface kept in memory
• The table allows the router to properly
forward frames out the correct interface
• This occurs once the frame is received
and the destination address has been
processed
Routing Basics
• In standard routing, the route table is
consulted every time a frame is
received and so it plays a fundamental
role in the proper delivery of data
• A routing table only maintains the best
possible route to a destination, not all
possible routes
Routing Basics
• Routing table entries have the following
functions:
– Network destination
– Netmask
– Gateway
– Interface
– Metric
Routing Basics
• The task of populating the routing table is
accomplished by using either dynamic routing
or static routing
• Dynamic routing uses routing protocols to
build route tables automatically
• Static routing requires manual route table
entries and updates to different networks
Routing Basics
• Layer 3 addressing is critical for end-to-end
reachability and does not change throughout
the routing process
• The Layer 2 address moves the packet from
one hand-off point or hop to the next
• The router’s Layer 2 address is the frame’s
destination
• The Layer 3 address remains constant
through each hop
Address Resolution
• Address resolution is the mapping of
one address to another
• It is generally a mapping between a
Layer 3 network address (logical) and a
Layer 2 hardware address (physical)
• The reverse process is also address
resolution
Address Resolution
• Address resolution is accomplished in
one of the following ways:
– Table lookup is a rarely used method of
address resolution
– Closed-form computation is only used in
very specific networks and is time
consuming to configure
– Dynamic message exchange is the most
common and involves an exchange of
information between two hosts
Address Resolution
• Address Resolution Protocol (ARP) is used
when an IP host has a known destination IP
address (Layer 3) and it needs to retrieve the
corresponding Layer 2 MAC address from the
destination host
• The ARP cache is used to further reduce the
need for broadcasts by storing the IP-to- MAC
mapping in memory for a specified duration
Address Resolution
• The ARP Process:
– Client A sends out an ARP broadcast
– All clients receive and process the
broadcast frame but only Machine B
responds
– Client A receives the response and places
Machine B’s MAC address in its ARP
cache
Address Resolution
• ARP locates the Layer 2 address when the
Layer 3 address is known
• Reverse Address Resolution Protocol (RARP)
finds the Layer 3 address when the Layer 2
address is known
• A good example of RARP is found in TCP/IP
address reservations and the Boot Protocol
(BootP)
• Using BootP, IP hosts are automatically
assigned their configuration information
through a BootP server
Routing Protocols
• Dynamic routing uses routing protocols
• Purpose of routing protocols is to build
a routing table with the best routes
• Routing protocols are categorized into
two types:
– Distance Vector
– Link State
Routing Protocols
• Distance vector routing protocols are simple
• Generally they are easy to configure
• They use simple logic (algorithms) to
determine the “best path” to a given destination
• The term “metric” refers to the method or
measurement used by the routing protocol
logic to determine the best path to a given
network – e.g., hops, bandwidth, latency, etc.
Routing Protocols
• A distance vector routing protocol usually uses hop
count as its metric (RIP and RIPv.2). [IGRP – Cisco
proprietary – on the other hand, uses 4 metrics and
MTU, Maximum Transmission Unit, as a tie-breaker.
The four metrics are Bandwidth, Distance, Latency
and Reliability].
• A distance vector routing protocol is characterized by
how it communicates with other routing devices
• Distance vector routing protocols use broadcasts to
advertise their entire routing table to directly
connected peer routers. (With RIP, the broadcasts
are every 30 seconds; with IGRP it’s every 90
seconds. This is very bandwidth-intensive and one
reason that link-state routing protocols are preferred
in large networks with many devices. The more
devices there are, the more broadcasts will be
clogging the network.)
Routing Protocols
• “Convergence” is the time it takes for a given
set of routers to learn routes to all networks
in a give area known as the internetwork.
• Convergence describes the time it takes a set
of routers to learn of a change in the network –
devices added, or being turned off, or going
down by malfunction, etc.
• Distance vector routing protocols generally
take longer to converge than link state
protocols because they use a periodic route
advertisement schedule.
Routing Protocols
• A routing loop occurs when routers get
confused during update operations, causing
frames to bounce back and forth between a
set of interfaces
• Two easy methods to identify routing loops:
– Tracert or traceroute TCP/IP utilities
– View the routing table and the metric
associated with the network
Routing Protocols
• Prevent routing loops by using the
following software-based methods:
– Split horizon
– Hold-down timers
– Triggered updates
– Hop count limits
– Poisoning
Routing Protocols
• Link state routing protocols are more
intelligent than distance vector protocols
• The metric used by most link state protocols is
bandwidth allowing more complex routing
configurations
• Routing protocols capable of making complex
decisions use a mathematical formula or
algorithm for deriving the best path or route to a
given network
Routing Protocols
• Some link state protocols are
capable of determining the best
route to a destination network
based on the following:
– Delay
– Load
– Reliability
– MTU
Routing Protocols
• When more than one metric is used it is
referred to as a composite metric
• Link state protocols only send updates when
changes occur, and they only send the
changes, not the entire route table
• Link state protocols use multicast and unicast
traffic instead of broadcast traffic
• Link state routers also develop an overall
picture of the networks available by
establishing neighbor relationships
Administrative Classification
• Routing protocols are also separated by
an administrative classification based
on where they are used in the
networking environment:
– Interior routing protocols or interior
gateway protocols
– Exterior routing protocols or exterior
gateway protocols
Administrative Classification
• Interior gateway protocols (IGPs)
are used within a company’s
network infrastructure to maintain
routing tables and policies set by
the network administrators
• The two industry standard IGPs are:
– Routing Information Protocol
– Open Shortest Path First
Administrative Classification
• RIP is a distance vector protocol that
uses hop count for its metric when
determining the best route to a given
network
• In most implementations, RIP uses split
horizon, hop count limit, and poisoning
for routing loop prevention
• RIP is a classful routing protocol
Administrative Classification
• The shortest path as measured by
Open Shortest Path First (OSPF) is
actually the fastest path based on
bandwidth
• Shortest refers to the shortest time
• OSPF is used in large networks and
ones requiring more intelligence than
distance vector routing protocols
Administrative Classification
• OSPF communicates using unicast and
multicast packets
• It only transmits changes or updates to the
routing table when they occur
• It uses hello packets to determine the current
state of a link between itself and its neighbors
• It utilizes a link state database to maintain a
local view of the entire routing environment
Administrative Classification
• The configuration possibilities using
OSPF
– Areas
– Autonomous system (AS)
– Backbone router
– Area border router (ABR)
– Autonomous system boundary router
(ASBR)
Administrative Classification
• The decision making process of EGPs is far
more complex than that of internal protocols
• The power and routing flexibility associated
with EGPs requires knowledge and
understanding of the complex nature of your
network and its traffic
• EGPs can let you influence and manage
traffic only as it enters or leaves your AS
Administrative Classification
• One member of EGPs is the Exterior
Gateway Protocol (EGP)
• EGP was the first protocol developed
that allowed isolation of autonomous
systems
• EPG is not used today and is replaced
by the Border Gateway Protocol
Administrative Classification
• Border Gateway Protocol (BGP) version 4 is
the most widely used exterior protocol in the
world
• BGP is a well established standard and
commonly used by ISPs and in very large
companies
• there are actually two different classifications
of BGP
– internal
– external
Administrative Classification
• iBGP is used for internal routing
• eBGP is used for external routing
• iBGP functions under different rules
than eBGP
• If two routers running BGP are in the
same AS, they are running an iBGP
connection
Administrative Classification
• BGP communication starts by establishing
peers
• Once the peers have been established, BGP
routing information is exchanged and updated
as necessary
• BGP is an advanced distance vector protocol
that uses triggered updates for
communicating changes in the routing
environment
• Routing loops in BGP are avoided by using
the AS-path attribute
Administrative Classification
• BGP uses active TCP sessions that are
setup and continuously maintained.
Convergence in the routing environment
is very fast
• BGP has features that you can use to
help speed the convergence of the
network routes under your control
Hierarchical Routing
• Hierarchical routing depends on hierarchical
addressing
• It is a routing technique originally designed to
help reduce the size of the routing tables on
the Internet as well as speed up the overall
routing process
• The concept uses an address block or blocks
to represent different sections of a network
Hierarchical Routing
• Summarizing routes is often referred to as
supernetting networks
• The process of summarization is built around
the binary bit patterns just as in subnetting
• The difference is that rather than extending
the subnet mask by adding bits, we remove
bits
Hierarchical Routing
• By using summarization, you reduce the
routing tables on each router
• To accomplish the summarization, you need
to determine how many bits to unmask or unsubnet in order to make the networks appear
as one big address block
• The routing protocol must transmit the
network prefix along with the network address
during route advertisements