Transcript ccna-RoutingTheory

Ch. 6 – Routing Theory – Part 1
CCNA Semester 2
Originally by Rick Graziani, Instructor
Was modified by Prof. Yousif
Part I
Routing Basics and
Static Routes


Path determination, for traffic going through a network cloud,
occurs at the network layer (Layer 3).
The path determination function enables a router to evaluate the
available paths to a destination and to establish the preferred
handling of a packet.





The network layer provides best-effort end-to-end packet delivery
across interconnected networks.
The network layer uses the IP routing table to send packets from the
source network to the destination network.
After the router determines which path to use, it proceeds with
forwarding the packet.
It takes the packet that it accepted on one interface and forwards it to
another interface or port that reflects the best path to the packet's
destination.
Much more information later in the presentation on “The Routing Table
Structure.”
Very important points:
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
Packet: IP Source and IP Destination (Network Layer) addresses do not
change.
Data Link Source and Data Link Destination addresses do change to
reflect the current and next hop routers.
The routing table (coming) contains the IP address of the next-hop router
– This address is used to find the Data Link Destination address which
is used to encapsulate the original IP packet.
The router’s path determination function looks up the network address
in the routing table and determines which interface it should exit.
The router’s switching function encapsulates it in the proper data link
frame with the proper data link destination address.
Routing Protocols
 Interior Gateway Protocols (IGPs): RIP, IGRP, EIGRP, OSPF,
IS-IS
– IGRP and EIGRP are Cisco Proprietary

Exterior Gateway Protocols (EGPs): EGP, BGP
Routing Protocols: Much more later!
IGPs – Interior Gateway Protocols
 RIP (Routing Information Protocol)
– Distance Vector

IGRP (Interior Gateway Routing Protocol)
– Distance Vector
– Cisco Proprietary

EIGRP (Enhanced Interior Gateway Routing Protocol)
– Advanced Distance Vector or Hybrid
– Cisco Proprietary

OSPF (Open Shortest Path First)
– Link-state

IS-IS (Intermediate-System to Intermediate System)
– Link-state
EGPs – Exterior Gateway Protocols
 EGP (Exterior Gateway Protocol)
– EGP – Path vector

BGP (Border Gateway Protocol)
– EGP – Path vector
Autonomous System (AS) – Networks under
the control of a single organization (within a
single company).
 IGP – Routing protocols used within an AS.
 EGP – Routing protocols used between AS’s

Important Routing Table Principles (Zinin, Cisco IP Routing)
 Every router makes its decision alone, based on the information
it has in its own routing table.
 The fact that one router has certain information in its routing
table does not mean that other routers have the same
information.
 Routing information about a path from one network to another
does not provide routing information about the reverse, or return
path.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.0.0.0/8
RTC
e0
.1
RTA#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
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
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, o - ODR
Gateway of last resort is not set
RTA#
The Routing Table prior to any interface configuration

The command to view the IP Routing table is: (priviledge or user mode)
Router# show ip route

Currently, no routes in the routing table.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.0.0.0/8
RTC
e0
.1
RTA(config)#inter e 0
RTA(config-if)#ip add 192.168.2.1 255.255.255.0
RTA(config-if)#no shutdown
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
192.168.2.0/24 is directly connected, Ethernet0
RTA#
Configuring an interface

Adding an ip address/mask to an interface tells the router that it is a
member, “Directly Connected” to that network – just like when a host
computer is configured with an ip address/mask.
 Notice the route is shown with the subnet mask and the “exit-interface.”
 Don’t forget the “no shutdown”
 Don’t forget the interface must be in “up” and “up”
Directly Connected Networks and the IP Routing Table
RTA# debug ip routing
RTA(config)#inter e 0
RTA(config-if)#ip add 192.168.2.1 255.255.255.0
RTA(config-if)#no shutdown
00:28:56: RT: add 192.168.2.0/24 via 0.0.0.0, connected metric [0/0]
00:28:56: RT: interface Ethernet0 added to routing table
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
192.168.2.0/24 is directly connected, Ethernet0
RTA# undebug all
Viewing the Routing Table Process
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Use the “debug ip routing” command to view the Cisco IOS routing
table process of adding a directly connected network to the routing
table.
When finished, be sure to use “undebug all”
Debug commands are used to view detailed information about Cisco
IOS processes – more later.
Directly Connected Networks and the IP Routing Table
RTA# debug ip routing
RTA(config)#inter e 0
RTA(config-if)#shutdown
00:34:38: RT: interface Ethernet0 removed from routing table
00:34:38: RT: del 192.168.2.0 via 0.0.0.0, connected metric [0/0]
00:34:38: RT: delete network route to 192.168.2.0
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
RTA# undebug all
Viewing the Routing Table Process
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Directly connected routes will also be removed if the link goes down.
Directly connected routes will only be in the routing table if, it is not
administratively down, the line is “up” and protocol is “up”
For serial interfaces, don’t forget the “clock rate” command on the
router with the DCE cable – neither interface will be “up” and “up” until
both ends are configured correctly.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.0.0.0/8
RTC
e0
.1
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTB#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.1.0/24 is directly connected, Serial1
RTC#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
10.0.0.0/8 is directly connected, Ethernet0
C
192.168.1.0/24 is directly connected, Serial1
The Routing Tables

Notice that the routers only know about their own directly connected
networks.
 They are not sharing routing information because we have not
configured any static routes or dynamic routing protocols.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTC(config)#inter e 0
RTC(config-if)#ip add 10.1.0.1 255.255.0.0
RTC#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
10.0.0.0/16 is subnetted, 1 subnets
C
10.1.0.0 is directly connected, Ethernet0
C
192.168.1.0/24 is directly connected, Serial1
RTC#
Configuring an interface as part of a subnet
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We will discuss this in much more detail later using the presentation –
“The Routing Table.”
For now, notice that when the subnet mask is not a classful mask, but a
subnetted /16 mask.
The routing table information shows the route to the subnetted network
The mask is shown in the above, “parent” classful network.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
172.16.0.0/16
RTA
s0
s0
.1
.2
.1
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA#show ip route
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTA#ping 172.16.0.1
Sending 5, 100-byte ICMP Echos to 172.16.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
RTA#ping 172.16.0.2
!!!!!
RTA#ping 192.168.1.1
.....
RTA#ping 192.168.1.2
.....
RTA#ping 10.1.0.1
.....
Routing – Only directly connected hosts (routers)

Routers can only reach networks known about in its own routing table.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
172.16.0.0/16
RTA
s0
s0
.1
.2
.1
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA#show ip route
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTA#ping 172.16.0.1
Sending 5, 100-byte ICMP Echos to 172.16.0.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/57/60 ms
RTA#ping 172.16.0.2
!!!!!
Routing– Routing tables must have the necessary network routes
Question: If RTA can ping RTB’s 172.16.0.2 interface why can’t it ping
RTB’s 192.168.1.1 interface? - RTA does not have a route to it in its
routing table.
 Question: Would an extended ping from RTA, using the source IP
address of 192.168.2.1 be able to ping 172.16.0.1 on RTB? Why or
why not? Where does the echo request or echo reply fail?

Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA#show ip route
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTB#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.1.0/24 is directly connected, Serial1
RTA#ping
Protocol [ip]:
Target IP address: 172.16.0.2
Extended commands [n]: y
Source address or interface: 192.168.2.1
Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
.....
Routing– Routing tables must have the necessary network routes
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Question: Would an extended ping from RTA, using the source IP address of
192.168.2.1 be able to ping 172.16.0.1 on RTB? Why or why not?
The echo request from RTA reaches RTB because RTA has a route to
172.16.0.0/16 in its routing table.
However, the echo reply from RTB back to RTA fails, because RTB does not
have a route for 192.168.2.0/24 in its routing table.
Directly Connected Networks and the IP Routing Table
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTB#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.1.0/24 is directly connected, Serial1
RTC#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
10.0.0.0/16 is subnetted, 1 subnets
C
10.1.0.0 is directly connected, Ethernet0
C
192.168.1.0/24 is directly connected, Serial1
Routing Table Principles Revisited (Zinin, Cisco IP Routing)

Every router makes its decision alone, based on the information it has
in its own routing table.
 The fact that one router has certain information in its routing table does
not mean that other routers have the same information.
 Routing information about a path from one network to another does not
provide routing information about the reverse, or return path.
Topics
Part I. Routing Basics and Static Routing

Basic Concepts:
– Network Layer
– IP Routing Table
– Path Determination
– Routed Protocols versus Routing Protocols
– Network Layer Protocol Operations
– Path Switching (Introduction)
– Multiprotocol Routing
 IP Routing Table and Directly Connected Networks
 Static Routing
– Configuring Static Routes
– Static Routing in the Real-world
– Default Static Routes
– Recursive Lookups
– Static Routes and the Routing Table Process
– Advantages and Disadvantages of Static Routing
Static Routes

In this presentation we will look at how to configure static
routes.
Dynamic Routes

In this presentation we will look at the concepts of dynamic
routing, but will discuss the configuration and more of the
concepts in the Chapter 12 – Routing Protocols.
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
RTA#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.2.0/24 is directly connected, Ethernet0
RTB#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
C
172.16.0.0/16 is directly connected, Serial0
C
192.168.1.0/24 is directly connected, Serial1
RTC#show ip route
Codes: C - connected,.. <Other codes and gateway information omitted>
10.0.0.0/16 is subnetted, 1 subnets
C
10.1.0.0 is directly connected, Ethernet0
C
192.168.1.0/24 is directly connected, Serial1
Current IP Routing Tables
e0
.1
Configuring Static Routes
RTR(config)# ip route prefix mask {address
| interface} [distance] [tag tag]
[permanent]


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
prefix
IP route prefix for the destination.
mask
Prefix mask for the destination.
address IP address of the “next hop” that can be
used to reach that network.
interface Network interface to use (exit-interface)
distance (Optional) An administrative distance.
tag tag
(Optional) Tag value that can be used as a
"match" value for controlling redistribution via route
maps. (CCNP Advanced Routing)
Permanent (Optional) Specifies that the route will not
be removed, even if the interface shuts down. (CCNP
Advanced Routing)
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
Configuring static routes
 Routers do not need to configure static routes for their own
directly connected networks.
 We need to configure static routes for networks this router
needs to reach.
 We will need to configure static routes for the other routers as
well, as “routing information about a path from one network to
another does not provide routing information about the reverse,
or return path.”
 Convergence – When all the routers in the network (AS) have
accurate and consistent information, so that proper routing and
packet forwarding can take place.
 Convergence will not happen until all the routers have complete
and accurate routing information, meaning we must configure
static routes on all the routers before packets will be correctly
delivered.
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
Network/subnet route
Intermediate-Address
(usually “next-hop”)
RTA#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
S
192.168.1.0/24 [1/0] via 172.16.0.2
C
192.168.2.0/24 is directly connected, Ethernet0
Basic static route example

Be sure to use the proper subnet mask!
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
RTA#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
S
192.168.1.0/24 [1/0] via 172.16.0.2
C
192.168.2.0/24 is directly connected, Ethernet0
Basic static route example (continued)
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[1/0] – [ Administrative Distance / Metric ]
Administrative Distance – This is the “trustworthiness” of the routing
information. The default administrative distance of static routes is 1.
The Administrative Distance of a directly connected route is 0.
Lower the AD the more trustworthy.
If the router learns about a route to a network from more than one
source, it will install the route with the lower administrative distance in
the routing table. – More later.
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
RTA#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
S
192.168.1.0/24 [1/0] via 172.16.0.2
C
192.168.2.0/24 is directly connected, Ethernet0
Basic static route example (continued)

[1/0] – [ Administrative Distance / Metric ]
Metric – This is the “cost” of getting to this route, I.e. how far away this
network is.
 The lower the cost, the closer the network.
 Static routes always show a cost of “0” even if it was configured with
the intermediate address is multiple-hops away.
 Much more later.

Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
2
1
RTA#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
S
192.168.1.0/24 [1/0] via 172.16.0.2
C
192.168.2.0/24 is directly connected, Ethernet0
Recursive Lookup

The router knows it can get to 192.168.1.0/24 network by forwarding
the packets to the router at the ip address of 172.16.0.2
 How does the router know how to get to the ip address 172.16.0.2?
 It does a recursive lookup – first (1) by looking up the 192.168.1.0/24
network and finding it needs to forward the packet to 172.16.0.2 – the
router then (2) looks up the 172.16.0.0 network and sees it can forward
it out the interface Serial 0.
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA#debug ip routing
IP routing debugging is on
RTA#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
05:53:48: RT: add 192.168.1.0/24 via 172.16.0.2, static metric [1/0]
RTA(config)#ip route 10.1.0.0 255.255.0.0 172.16.0.2
05:54:38: RT: add 10.1.0.0/16 via 172.16.0.2, static metric [1/0]
RTA(config)#undebug all
Static Routes and the Routing Table Process

Notice that the static route is entered into the routing table by the
routing table process (debug ip routing) with a metric of 0.
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
RTA(config)#ip route 10.1.0.0 255.255.0.0 172.16.0.2
RTB(config)#ip route 192.168.2.0 255.255.255.0 172.16.0.1
RTB(config)#ip route 10.1.0.0 255.255.0.0 192.168.1.2
RTC(config)#ip route 192.168.2.0 255.255.255.0 192.168.1.1
RTC(config)#ip route 172.16.0.0 255.255.0.0 192.168.1.1
Configuring all of the static routes

Notice that the intermediate-address is always the next-hop ip address.
 This does not always have to be the case, and we will look at other
options in the presentation on Static Routes- Additional Information
 Good idea to do a “copy running-config startup-config” if everything is
working right.
 To verify the routes are in there, you can do a:
Router# show running-config
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
e0
.1
RTA(config)#ip route 192.168.1.0 255.255.255.0 172.16.0.2
RTA(config)#ip route 10.1.0.0 255.255.0.0 172.16.0.2
RTA#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
10.0.0.0/16 is subnetted, 1 subnets
S
10.1.0.0 [1/0] via 172.16.0.2
S
192.168.1.0/24 [1/0] via 172.16.0.2
C
192.168.2.0/24 is directly connected, Ethernet0
RTA#ping 10.1.0.1
!!!!!
RTA#ping 192.168.1.2
!!!!!
RTA#ping 192.168.1.1
!!!!!
Examining RouterA
Notice the 10.0.0.0 parent – classful information
 Again, we will look at why in the presentation on The Routing Table.

Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
RTB(config)#ip route 192.168.2.0 255.255.255.0 172.16.0.1
RTB(config)#ip route 10.1.0.0 255.255.0.0 192.168.1.2
RTB#show ip route
Codes: C - connected, S - static,
C
172.16.0.0/16 is directly connected, Serial0
10.0.0.0/16 is subnetted, 1 subnets
S
10.1.0.0 [1/0] via 192.168.1.2
C
192.168.1.0/24 is directly connected, Serial1
S
192.168.2.0/24 [1/0] via 172.16.0.1
RTB#ping 192.168.2.1
!!!!!
RTB#ping 10.1.0.1
!!!!!
Examining RouterB
e0
.1
Configuring Static Routes
192.168.2.0/24
e0
.1
172.16.0.0/16
RTA
s0
s0
.1
.2
192.168.1.0/24
RTB
s1
s1
.1
.2
10.1.0.0/16
RTC
RTC(config)#ip route 192.168.2.0 255.255.255.0 192.168.1.1
RTC(config)#ip route 172.16.0.0 255.255.0.0 192.168.1.1
RTC#show ip route
Codes: C - connected, S - static,
S
172.16.0.0/16 [1/0] via 192.168.1.1
10.0.0.0/16 is subnetted, 1 subnets
C
10.1.0.0 is directly connected, Ethernet0
C
192.168.1.0/24 is directly connected, Serial1
S
192.168.2.0/24 [1/0] via 192.168.1.1
RTC#ping 192.168.1.1
!!!!!
RTC#ping 172.16.0.1
!!!!!
Examining RouterC
e0
.1
Advantages and Disadvantages of Static Routing
Advantages
 Low processor
overhead
 No bandwidth
utilization
– Secure operation
don’t inadvertently
advertise networking
information to an
untrusted source

Predictability
(precise control)
Disadvantages
 High-maintenance
configuration
 No adaptability
(except for floating
static routes)
Static routes in the real-world

Soon we will learn about dynamic routing protocols (RIP, etc.), where
routers can learn automatically about networks, without the manual
configuration of static routes.
 Does this mean that static routes are never used in the real-world?
 No! Static routes are used in conjunction with dynamic routing
protocols.
 It is common to use a static route where using a dynamic routing
protocols would have disadvantages or where it just not needed.
ISP
10.1.1.1/24
ip route 172.16.0.0 255.255.0.0 10.1.1.2
10.1.1.2/24
VCC
College
172.16.0.0/16
Static routes in the real-world (continued)
In the example above, there is only one route, link, between VCC’s
network and the ISP.
 When there is only a single route to a network, this is known as a stub
network.
 It is very common for the ISP to have a static route pointing to it’s
customers’ networks, in this case VCC College.

Default
ISP
10.1.1.1/24
ip route 172.16.0.0 255.255.0.0 10.1.1.2
10.1.1.2/24
Cabrillo
College
ip route 0.0.0.0 0.0.0.0 10.1.1.1
172.16.0.0/16
Static routes in the real-world (continued)
What about VCC College and sending packets to the ISP – packets
going to the Internet?
 It is also common for customer networks to use a special kind of static
route, known as a default static route.
 Of course we will examine this later throughout the rest of this course,
but for now we specify the network and mask as “0.0.0.0 0.0.0.0”
(pronounced “quad-zero”).
 This tells the router to forward all packets to this next-hop address (or
exit interface) that do not have an explicit route in the routing table.

Default
ISP
10.1.1.1/24
ip route 172.16.0.0 255.255.0.0 10.1.1.2
10.1.1.2/24
VCC
College
ip route 0.0.0.0 0.0.0.0 10.1.1.1
172.16.0.0/16
RTB#show ip route
Gateway of last resort is 10.1.1.1 to network 0.0.0.0
C
172.16.0.0/16 is directly connected, Ethernet0
10.0.0.0/24 is subnetted, 1 subnets
C
10.1.1.0 is directly connected, Serial1
S*
0.0.0.0/0 [1/0] via 10.1.1.1
Static routes in the real-world (continued)

Any packets not matching the routes 172.16.0.0/16 or 10.1.1.0/24 are
sent to the router 10.1.1.1 – where it is now their “problem.”
ip default-network command

The curriculum shows another command:
ip default-network

We will look at this command after we have discussed dynamic routing
protocols, specifically IGRP.
Note: This command is used when needing to propagate a default
route with the IGRP or EIGRP routing protocols. It is not commonly
used with static routes, RIP, or OSPF.


Static routes do not lend themselves well to topology changes, and by
themselves will not adjust to network changes (new network, down
network, change in network characteristics – I.e. link bandwidth).
 Although, backup static routes can be configured (later), it is better to
use a dynamic routing protocol which can automatically detect and
adjust to changes in the network topology.
 In many cases with complex network topologies, static routes and
backup-static routes, can not provide complete redundancy and
backup, and can even lead to routing loops. – Later when in the
presentation Static Routes – Additional Information.