Ch' 6 Routing Theory Part 1

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• Ch. 6 Routing Theory Part 1
• CCNA Semester 2
• Originally by Rick Graziani, Instructor
• Was modified by Prof. Yousif

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Part I
Routing Basics and Static Routes
3

• 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.

4

• 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.

5

• Very important points
• 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 routers path determination function looks up
  the network address in the routing table and
  determines which interface it should exit.
• The routers switching function encapsulates it
  in the proper data link frame with the proper
  data link destination address.

6

• Routing Protocols
• Interior Gateway Protocols (IGPs) RIP, IGRP,
  EIGRP, OSPF, IS-IS
• IGRP and EIGRP are Cisco Proprietary
• Exterior Gateway Protocols (EGPs) EGP, BGP

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• 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)

8

• 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 ASs

9

• 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.

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Directly Connected Networks and the IP Routing
Table
RTAshow 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.

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Directly Connected Networks and the IP Routing
Table
RTA(config)inter e 0 RTA(config-if)ip add
192.168.2.1 255.255.255.0 RTA(config-if)no
shutdown RTAshow ip route Codes C -
connected,.. ltOther codes and gateway information
omittedgt 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.
• Dont forget the no shutdown
• Dont forget the interface must be in up and
  up

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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 002856 RT add 192.168.2.0/24 via
0.0.0.0, connected metric 0/0 002856 RT
interface Ethernet0 added to routing
table RTAshow ip route Codes C - connected,..
ltOther codes and gateway information omittedgt C
192.168.2.0/24 is directly connected,
Ethernet0 RTA undebug all

• Viewing the Routing Table Process
• 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.

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Directly Connected Networks and the IP Routing
Table
RTA debug ip routing RTA(config)inter e
0 RTA(config-if)shutdown 003438 RT interface
Ethernet0 removed from routing table 003438
RT del 192.168.2.0 via 0.0.0.0, connected metric
0/0 003438 RT delete network route to
192.168.2.0 RTAshow ip route Codes C -
connected,.. ltOther codes and gateway information
omittedgt RTA undebug all

• Viewing the Routing Table Process
• 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, dont 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.

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Directly Connected Networks and the IP Routing
Table
RTAshow ip route Codes C - connected,.. ltOther
codes and gateway information omittedgt C
172.16.0.0/16 is directly connected, Serial0 C
192.168.2.0/24 is directly connected,
Ethernet0 RTBshow ip route Codes C -
connected,.. ltOther codes and gateway information
omittedgt C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.1.0/24 is
directly connected, Serial1 RTCshow ip
route Codes C - connected,.. ltOther codes and
gateway information omittedgt 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.

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Directly Connected Networks and the IP Routing
Table
RTC(config)inter e 0 RTC(config-if)ip add
10.1.0.1 255.255.0.0 RTCshow ip route Codes C
- connected,.. ltOther codes and gateway
information omittedgt 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
• 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.

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Directly Connected Networks and the IP Routing
Table
RTAshow ip route C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.2.0/24 is
directly connected, Ethernet0 RTAping
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 RTAping 172.16.0.2 !!!!! RTAping
192.168.1.1 ..... RTAping 192.168.1.2 ..... RT
Aping 10.1.0.1 .....

• Routing Only directly connected hosts (routers)
• Routers can only reach networks known about in
  its own routing table.

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Directly Connected Networks and the IP Routing
Table
RTAshow ip route C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.2.0/24 is
directly connected, Ethernet0 RTAping
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 RTAping 172.16.0.2 !!!!!

• Routing Routing tables must have the necessary
  network routes
• Question If RTA can ping RTBs 172.16.0.2
  interface why cant it ping RTBs 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?

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Directly Connected Networks and the IP Routing
Table
RTAshow ip route C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.2.0/24 is
directly connected, Ethernet0 RTBshow ip
route Codes C - connected,.. ltOther codes and
gateway information omittedgt C 172.16.0.0/16
is directly connected, Serial0 C
192.168.1.0/24 is directly connected,
Serial1 RTAping 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
• 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.

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Directly Connected Networks and the IP Routing
Table
RTAshow ip route Codes C - connected,.. ltOther
codes and gateway information omittedgt C
172.16.0.0/16 is directly connected, Serial0 C
192.168.2.0/24 is directly connected,
Ethernet0 RTBshow ip route Codes C -
connected,.. ltOther codes and gateway information
omittedgt C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.1.0/24 is
directly connected, Serial1 RTCshow ip
route Codes C - connected,.. ltOther codes and
gateway information omittedgt 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.

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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

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(No Transcript)
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• 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.

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Configuring Static Routes
RTAshow ip route Codes C - connected,.. ltOther
codes and gateway information omittedgt C
172.16.0.0/16 is directly connected, Serial0 C
192.168.2.0/24 is directly connected,
Ethernet0 RTBshow ip route Codes C -
connected,.. ltOther codes and gateway information
omittedgt C 172.16.0.0/16 is directly
connected, Serial0 C 192.168.1.0/24 is
directly connected, Serial1 RTCshow ip
route Codes C - connected,.. ltOther codes and
gateway information omittedgt 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

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Configuring Static Routes

• RTR(config) ip route prefix mask address
  interface distance tag tag permanent
• 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)

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Configuring Static Routes

• 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.

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Configuring Static Routes
RTA(config)ip route 192.168.1.0 255.255.255.0
172.16.0.2 RTAshow 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
Network/subnet route
Intermediate-Address (usually next-hop)

• Basic static route example
• Be sure to use the proper subnet mask!

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Configuring Static Routes
RTA(config)ip route 192.168.1.0 255.255.255.0
172.16.0.2 RTAshow 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
• 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.

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Configuring Static Routes
RTA(config)ip route 192.168.1.0 255.255.255.0
172.16.0.2 RTAshow 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.

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Configuring Static Routes
RTA(config)ip route 192.168.1.0 255.255.255.0
172.16.0.2 RTAshow 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
2
1

• 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.

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Configuring Static Routes
RTAdebug ip routing IP routing debugging is
on RTAconf 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 055348
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 055438 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.

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Configuring Static Routes
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)i
p route 10.1.0.0 255.255.0.0 192.168.1.2 RTC(conf
ig)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

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Configuring Static Routes
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 RTAshow 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 RTAping 10.1.0.1 !!!!! RTAp
ing 192.168.1.2 !!!!! RTAping 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.

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Configuring Static Routes
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 RTBshow 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 RTBping 192.168.2.1 !!!!! RTBping
10.1.0.1 !!!!!

• Examining RouterB

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Configuring Static Routes
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 RTCshow 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 RTCping
192.168.1.1 !!!!! RTCping 172.16.0.1 !!!!!

• Examining RouterC

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Advantages and Disadvantages of Static Routing

• Disadvantages
• High-maintenance configuration
• No adaptability (except for floating static
  routes)

• Advantages
• Low processor overhead
• No bandwidth utilization
• Secure operation dont inadvertently advertise
  networking information to an untrusted source
• Predictability (precise control)

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• 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.

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10.1.1.1/24
ISP
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 VCCs 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 its customers networks, in
  this case VCC College.

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Default
10.1.1.1/24
ISP
ip route 172.16.0.0 255.255.0.0 10.1.1.2
10.1.1.2/24
Cabrillo College
172.16.0.0/16
ip route 0.0.0.0 0.0.0.0 10.1.1.1

• 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.

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Default
10.1.1.1/24
ISP
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
ip route 0.0.0.0 0.0.0.0 10.1.1.1
RTBshow 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.

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• 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.

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• 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.