Determining IP Routes

© 2002, Cisco Systems, Inc. All rights reserved.
Determining IP Routes
Module 5
Objectives
Upon completing this lesson, you will be able to:
Describe the features and operation of static routing
Describe the features and operation of dynamic routing protocols, including RIP, IGRP, EIGRP,
and OSPF
Build a functional router configuration to support the specified network operational requirements, given a network design
Use show commands to identify anomalies in routing operation, given an operational router
Use debug commands to identify events and anomalies in routing operation, given an operational router
© 2002, Cisco Systems, Inc. All rights reserved.
4
Routing Overview
Objectives
Upon completing this lesson, you will be able to:
Explain the differences between static routing and dynamic routing
Identify the classes of routing protocols
Use Cisco IOS commands to configure static routes and default route forwarding, given a functioning router
Use show commands to identify anomalies in static routing operation, given an operational router
Describe the operation of “router on a stick”
Configure router on a stick for inter-VLAN routing using ISL and 802.1Q trunking, given an operational switch and router
To route, a router needs to do the following:
Know the destination address
Identify the sources it can learn from
Discover possible routes
Select the best route
Maintain and verify routing information
What Is Routing?
Routers must learn destinations that are not directly connected.
What Is Routing? (Cont.)
Static Route
Uses a route that a network administrator enters into the router manually
Dynamic Route
Uses a route that a network routing protocol adjusts automatically for topology or traffic changes
Identifying Static and Dynamic Routes
Static Routes
Configure unidirectional static routes to and from a stub network to allow communications to occur.
Defines a path to an IP destination network or subnet or host
Router(config)#ip route network [mask]
{address | interface}[distance] [permanent]
Static Route Configuration
Static Route Example
This is a unidirectional route. You must have a route configured in the opposite direction.
Default Routes
This route allows the stub network to reach all known networks beyond router A.
Verifying the Static
Route Configuration
router#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
U - per-user static route
 
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
 
10.0.0.0/8 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Serial0
S* 0.0.0.0/0 is directly connected, Serial0

Routing protocols are
used between
routers to determine paths and maintain
routing tables.
Once the path is determined, a router can route a routed protocol.
What Is a Routing Protocol?
An autonomous system is a collection of networks under a common administrative domain.
IGPs operate within an autonomous system.
EGPs connect different autonomous systems.
Autonomous Systems: Interior or Exterior Routing Protocols
Administrative Distance:
Ranking Routes
Classes of Routing Protocols
Classful Routing Overview
Classful routing protocols do not include the subnet mask with the route advertisement.
Within the same network, consistency of the subnet masks is assumed.
Summary routes are exchanged between foreign networks.
Examples of classful routing protocols:
RIP Version 1 (RIPv1)
IGRP
Classless Routing Overview
Classless routing protocols include the subnet mask with the route advertisement.
Classless routing protocols support variable-length subnet masking (VLSM).
Summary routes can be manually controlled within the network.
Examples of classless routing protocols:
RIP Version 2 (RIPv2)
EIGRP
OSPF
IS-IS
Routing Protocol
Comparison Chart
Using the ip classless Command
VLAN-to-VLAN Overview
Network layer devices combine multiple
broadcast domains.
Dividing a Physical Interface into Subinterfaces
Physical interfaces can be divided into multiple subinterfaces.
Routing Between VLANs
with ISL Trunks
Routing Between VLANs
with 802.1Q Trunks
Summary
Routing is the process by which an item gets from one location to another. In networking, a router is the device used to route traffic.
Routers can forward packets over static routes or dynamic routes, based on the router configuration.
Static routes can be important if the Cisco IOS software cannot build a route to a particular destination. Static routes are also useful for specifying a “gateway of last resort” to which all unroutable packets will be sent.
A default route is a special type of static route used for situations when the route from a source to a destination is not known or when it is unfeasible for the routing table to store sufficient information about the route.
Summary (Cont.)
When the static routing configuration is complete, use the show ip route command to verify the configuration.
Dynamic routing relies on a routing protocol to disseminate knowledge. A routing protocol defines the set of rules used by a router when it communicates with neighboring routers.
The ip classless command prevents a router from dropping a packet destined for an unknown subnet.
In a VLAN environment, frames are only switched between ports within the same broadcast domain so a Layer 3 device is required to enable inter-VLAN communication. Use ISL or 802.1q to enable trunking on a router’s subinterface.