Anil Kumar Vishwakarma
The term routing is used for taking a packet from one device and
sending it through the network to another device on a different network.
To be able to route packets, a router must know, at a minimum, the
• Destination address
• Neighbor routers from which it can learn about remote
• Possible routes to all remote networks
• The best route to each remote network
• How to maintain and verify routing information
CONFIGURING STATICIP ROUTING
Static routing has the following benefits:
There is no overhead on the router CPU, which means you could
possibly buy a cheaper router than if you were using dynamic
There is no bandwidth usage between routers, which means you
could possibly save money on WAN links.
It adds security, because the administrator can choose to allow
routing access to certain networks only.
Static routing has the following disadvantages:
The administrator must really understand the internetwork and
how each router is connected in order to configure routes correctly.
If a network is added to the internetwork, the administrator has to
add a route to it on all routers—by hand.
It’s not feasible in large networks because maintaining it would be
a full-time job in itself.
Static Routes built by using IP route command
Default routes are used to route packets with destinations that do
not match any of the other routes in the routing table.
Routers are typically configured with a default route for
Internet-bound traffic, since it is often impractical and
unnecessary to maintain routes to all networks in the Internet.
A default route is actually a special static route that uses this
ip route 0.0.0.0 0.0.0.0 [next-hop-address]
VERIFYING STATIC ROUTE CONFIGURATION
It is important to verify that they are present in the routing table
and that routing is working as expected.
The command show run is used to view the active configuration
in RAM to verify that the static route was entered correctly.
The show ip route command is used to make sure that the
static route is present in the routing table.
Router#sh ip route
TROUBLESHOOTING STATIC ROUTE
If any problem in connection between networks follow this steps:
1. verifying static routes (by using sh ip route) to make sure the
route found in routing table.
2. If the problem exists, run a traceroute command.
DYNAMIC ROUTING OVERVIEW
Purpose of a routing protocol and
Identifying the classes of routing protocols
It is the communication used between routers.
It allows one router to share information with other routers
regarding the networks .
The information a router gets from another router, using a
routing protocol, is used to build and maintain a routing table.
Examples of routing protocols are: RIP, IGRP, EIGRP and OSPF
It is used to direct user traffic.
It provides enough information in its network layer address
to allow a packet to be forwarded from one host to another
based on the addressing scheme.
Examples of routed protocols are: IP and IPX.
An autonomous system (AS) is a collection of networks under a
common administration sharing a common routing strategy.
To the outside world, an AS is viewed as a single entity.
The AS may be run by one or more operators while presenting a
consistent view of routing to the external world.
PURPOSE OF A ROUTING PROTOCOL AND
The goal of a routing protocol is to build and maintain the routing
Routers use routing protocols to manage information received
from other routers .
Autonomous systems (AS) provide the division of the global
internetwork into smaller and more manageable networks.
Each AS has its own set of rules and policies and an AS number
that will uniquely distinguish it from other autonomous systems
throughout the world.
THE TYPES OF ROUTING PROTOCOLS
Most routing algorithms can be classified into one of two categories:
1. Distance Vector
The distance vector routing approach determines the direction
(vector) and distance to any link in the internetwork.
As an example: RIP and IGRP.
2. Link State
The link-state approach, also called shortest path first, recreates
the exact topology of the entire internetwork.
As an example: OSPF.
Mixed between distance vector and link state
As an example: EIGRP.
DISTANCE VECTOR ROUTING
Distance vector routing updates
Distance vector routing loop issues
Defining a maximum count
Elimination routing loops through split
Preventing routing loops with holddown
DISTANCE VECTOR ROUTING UPDATES
Routing table updates occur periodically or when the topology in a
distance vector protocol network changes.
Distance vector algorithms call for each router to send its entire
routing table to each of its adjacent neighbors.
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 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 placed in the routing table.
If both advertised routes to the same network have the same AD,
then routing protocol metrics will be used to find the best path to the
The routing tables include information about the total path cost
as defined by the metrics.
DISTANCE VECTOR ROUTING LOOP ISSUES
Routing loops can occur when inconsistent routing tables are not
updated due to slow convergence in a changing network.
Just before the failure of Network 1, all routers have consistent
knowledge and correct routing tables.
When Network 1 fails, Router E sends an update to Router A.
Router A stops routing packets to Network 1, but Routers B, C, and D
continue to do so because they have not yet been informed of the
When Router A sends out its update, Routers B and D stop routing to
However, Router C has not received an update, so Network 1 is still
reachable via Router B.
Now Router C sends a periodic update to Router D, indicating a path
to Network 1 by way of Router B.
Router D changes its routing table to reflect this good, but incorrect,
information, and propagates the information to Router A.
Router A propagates the information to Routers B and E, and so on.
Any packet destined for Network 1 will now loop from Router C to B
to A to D and back to again to C.
DEFINING A MAXIMUM COUNT
The invalid updates of Network 1 will continue to loop until some other
process stops the looping. This condition, called count to infinity
ELIMINATION LOOPS BY SPLIT HORIZON
Split horizon reduces incorrect routing information and routing overhead
in a distance-vector network by enforcing the rule that routing
information cannot be sent back in the direction from which it was
Route poisoning is used to overcome large routing loops and offer
explicit information when a subnet or network is not accessible.
This is usually accomplished by setting the hop count to one more
than the maximum.
A holddown prevents regular update messages from reinstating a route that
is going up and down (called flapping).
Configuring RIP Routing.
Verifying the RIP Routing Tables.
Holding Down RIP Propagations.
Vector Distance Protocol.
Open standard protocol.
RIP works well in small networks.
Default AD (administrative distance) is 120.
Hop count is (number of routers) used as Metric for path selection.
Maximum Hop count is 15.
RIP v2RIP v1
Classless RoutingClassful Routing
Support for VLSMNo support for VLSM
Support for discontiguous networksNo support for discontiguous
Use broadcast or multicast to
deliver routing updates.
Use broadcast to deliver routing
Route update timer: Sets the interval (typically 30 seconds) between
periodic routing updates, in which the router sends a complete copy of its
routing table out to all neighbors.
Route invalid timer: Determines the length of time that must elapse
(180 seconds) before a router determines that a route has become
Route flush timer: Sets the time between a route becoming invalid and
its removal from the routing table (240 seconds).
Holddown timer: routes will enter into the holddown state when an
update packet is received that indicated the route is unreachable.
The default is 180 seconds.
VERIFYING THE RIP ROUTING
Two of the most common commands are the show ip route command
and the show ip protocols command.
The show ip protocols command shows which routing protocols are
carrying IP traffic on the router.
Another command use to for troubleshooting, debug ip rip. To
cancel this opertaon, use this command undebug all.
HOLDING DOWN RIP
You probably don’t want your RIP network advertised everywhere on
your LAN and WAN.
The easiest way is through the passive-interface command.
This command prevents RIP update broadcasts from being sent out a
defined interface, but that same interface can still receive RIP updates.
Vector Distance Protocol.
IGRP can works well in large networks.
Default AD (administrative distance) is 100.
Uses an autonomous system number.
Uses bandwidth and delay of the line as metric.
Maximum Hop count is 255.
Don’t support VLSM and discontiguous networks.
Update timers These specify how frequently routing-update
messages should be sent. The default is 90 seconds.
Invalid timers These specify how long a router should wait before
declaring a route invalid if it doesn’t receive a specific update about
it. The default is three times the update period (270 sec).
Flush timers The default is seven times the routing update period.
If the update timer is 90 seconds by default, then 7 * 90 = 630
seconds elapse before a route will be flushed from the route table.
Holddown timers These specify the holddown period. The default
is three times the update timer period plus 10 seconds (280 sec).
CONFIGURING IGRP ROUTING
The command used to configure IGRP is the same as the one
used to configure RIP routing with one important difference: you
use an autonomous system (AS) number.
Router(config)#router igrp 10
To Delete routing table built by IGRP, use this command.
Router(config)# no router igrp 10
VERIFYING IGRP ROUTING TABLE
To verify that IGRP has been configured properly, enter the show
ip route command and look for IGRP routes signified by an "I".
show ip protocols
debug ip igrp events
debug ip igrp transactions