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IP Routing Technologies
Lecture

Basic Routing Concepts
 What is routing?
◦ The term routing refers to taking a packet from one
device and sending it through the network to
another device on a different network.
 List of the minimum factors a router must
know to be able to affectively route packets:
◦ Destination address
◦ Neighbor routers from which it can learn about
remote networks
◦ Possible routes to all remote networks
◦ The best route to each remote network
◦ How to maintain and verify routing information

Routing types
 There are several ways to configure the routing
tables to include all the networks in your
internetwork so that packets will be properly
forwarded. Understanding the different types of
routing will be really helpful when choosing the
best solution for your specific environment and
business requirements.
 Static routing
 Default routing
 Dynamic routing

Static Routing
 Static routing is the process that ensures when
you manually add routes in each router’s routing
table. Predictably, there are pros and cons to static
routing, but that’s true for all routing approaches.
◦ There is no overhead on the router CPU, which means
you could probably make do with a cheaper router than
you would need for dynamic routing.
◦ There is no bandwidth usage between routers, saving
you money on WAN links as well as minimizing overhead
on the router since you’re not using a routing protocol.
◦ It adds security because you, the administrator, can be
very exclusive and choose to allow routing access to
certain networks only.

Static Routing
And here are the cons:
◦ Whoever the administrator is must have a vault-tight
knowledge of the internetwork and how each router is
connected in order to configure routes correctly. If you
don’t have a good, accurate map of your internetwork,
things will get very messy quickly!
◦ If you add a network to the internetwork, you have to
tediously add a route to it on all routers by hand, which
only gets increasingly insane as the network grows.
◦ Due to the last point, it’s just not feasible to use it in
most large networks because maintaining it would be a
full-time job in and of itself.

Static Routing
 Here’s the command syntax you use to add a static route to
a routing table from global config:
 ip route [ destination_network ] [ mask ] [ next-hop_address or
exitinterface ] [ administrative_distance ] [permanent]
 This list describes each command in the string:
◦ ip route The command used to create the static route.
◦ destination_network The network you’re placing in the routing
table.
◦ mask The subnet mask being used on the network.
◦ next-hop_address This is the IP address of the next-hop router that
will receive packets and forward them to the remote network, which
must signify a router interface that’s on a directly connected
network. You must be able to successfully ping the router interface
before you can add the route. Important note to self is that if you
type in the wrong next-hop address or the interface to the correct
router is down, the static route will show up in the router’s
configuration but not in the routing table.

Static Routing
 Here’s the command syntax you use to add a static route
to a routing table from global config:
 ip route [ destination_network ] [ mask ] [ next-hop_address
or exitinterface ] [ administrative_distance ] [permanent]
 This list describes each command in the string:
◦ exitinterface Used in place of the next-hop address if you want
and shows up as a directly connected route.
◦ administrative_distance By default, static routes have an
administrative distance of 1 or 0 if you use an exit interface
instead of a next-hop address. You can change the default value
by adding an administrative weight at the end of the command.
◦ permanent If the interface is shut down or the router can’t
communicate to the next-hop router, the route will automatically
be discarded from the routing table by default. Choosing the
permanent option keeps the entry in the routing table no matter
what happens.

Default Routing
 A stub network indicates that the networks in this design have
only one way out to reach all other networks, which means that
instead of creating multiple static routes, we can just use a
single default route. This default route is used by IP to forward
any packet with a destination not found in the routing table,
which is why it is also called a gateway of last resort.
 ip route 0.0.0.0 0.0.0.0 s0/0
 Configuring a default route is a lot easier than typing a bunch of
static routes! Everything the router receives with a destination
not found in the routing table will be forwarded to 172.16.10.5.
 You need to be careful where you place default routes because
you can easily create a network loop!

Dynamic Routing
 With dynamic routing, protocols are used to
find networks and update routing tables on
routers. This is a whole lot easier than using
static or default routing, but it will cost you in
terms of router CPU processing and bandwidth
on network links.
 A routing protocol defines the set of rules used
by a router when it communicates routing
information between neighboring routers.
Common routing protocols include RIP, EIGRP,
and OSPF.

Types of Routing Protocols
 There are two basic types of routing protocols:
 Distance-vector protocols
 Link-state protocols

Distance Vector
 The distance-vector protocols in use today find
the best path to a remote network by judging
distance.
 In RIP routing, each instance where a packet
goes through a router is called a hop, and the
route with the least number of hops to the
network will be chosen as the best one. The
vector indicates the direction to the remote
network.
 RIP is a distance-vector routing protocol and
periodically sends out the entire routing table
to directly connected neighbors.

(a) A network.
(b) Input from A, I, H, K, and the new routing table for J.
Distance Vector Routing

Distance Vector
 Concerns:
◦ Low speed links
◦ Slow Convergence
◦ Count to infinity
 The simplicity and maturity of distance vector
protocols has led to their popularity. The
primary drawback of traditional
implementation of distance vector protocols is
slow convergence

The Count-to-Infinity Problem
The count-to-infinity problem

Link State
 In link-state protocols, also called shortest-path-first
protocols, the routers each create three separate
tables.
◦ One of these tables keeps track of directly attached
neighbors,
◦ One determines the topology of the entire internetwork
◦ And one is used as the routing table.
 Routers running link-state protocols do not exchange
routing tables as distance vector protocols do. Rather,
they exchange information about adjacent neighbors
and networks and include metric information
associated with the connection.

Administrative Distance
 The administrative distance (AD) is used to rate the trustworthiness
of routing information received on a router from a neighbor router.
 An administrative distance is represented by an integer from 0 to
255, where 0 is the most trusted and 255 means no traffic will be
passed via this route.
 If a router receives two updates listing the same remote network, the
first thing the router checks is the AD. If one of the advertised routes
has a lower AD than the other, then the route with the lowest AD will
be chosen and placed in the routing table.
 If both advertised routes to the same network have the same AD,
then routing protocol metrics like hop count and/or bandwidth of the
lines will be used to find the best path to the remote network. The
advertised route with the lowest metric will be placed in the routing
table.
 But if both advertised routes have the same AD as well as the same
metrics, then the routing protocol will load-balance to the remote
network, meaning the protocol will send data down each link.

Administrative Distance
 Table shows the default administrative distances that a Cisco
router uses to decide which route to take to a remote
network.

Split Horizon
 Split horizon is a type of distance-vector
routing rule used to prevent routing loops in
distant-vector routing protocols. When a
router learns a route update from a neighbor,
the router will not advertise the update back
out the interface on which it was received.

Metric
 Often referred to as routing metric, a metric
is a measurement used by a routing protocol
to calculate the best path to a remote
network. Each routing protocol uses a
different metric to calculate the best path.

Next Hop
 Next hop refers to the next closest router a
packet will go through on its way to its
destination.

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7-ROUTING IN COMPUTER NETWORKS .pptx

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