Section 16: Routing Fundamentals

  ICND1 CCENT

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For CCNA, these are sections 94 – 100

94: Routing Fundamentals – Introduction

https://www.udemy.com/course/ccna-complete/learn/lecture/7393474#overview

https://www.udemy.com/course/cisco-icnd1/learn/lecture/8605340#content

 

95: Connected and Local Routes

https://www.udemy.com/course/ccna-complete/learn/lecture/7393476#overview

https://www.udemy.com/cisco-icnd1/learn/lecture/8605342#overview

Router Functions

A router has 2 main functions:

  • Determine the best path to available networks
  • Forward traffic to those networks

The Routing Table

  • The best available path or paths to a destination network are listed in a router’s routing table and will be used to forward traffic.
  • A routing table consists of directly connected networks and routes configured statically by the admin or dynamically learned through a routing protocol.

Connected and Local Routes

  • The Admin configures IP addresses on the router’s interfaces

  • When IP addresses are added to interfaces, it automatically adds those routes to the routing table.
  • show ip route
R1#show run
...
interface FastEthernet0/0
 ip address 10.0.0.1 255.255.255.0
 duplex auto
 speed auto
!
interface FastEthernet0/1
 ip address 10.0.1.1 255.255.255.0
 duplex auto
 speed auto
!
...
R1#show ip route
...
      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.0.0.0/24 is directly connected, FastEthernet0/0
L        10.0.0.1/32 is directly connected, FastEthernet0/0
C        10.0.1.0/24 is directly connected, FastEthernet0/1
L        10.0.1.1/32 is directly connected, FastEthernet0/1

Local Routes

  • From IOS 15, local routes will also be added to the routing table
  • Local routes always have a /32 mask and show the IP address configured on the interface.
    • See above:
      • C = Connected route
      • L = Local route (direct IP address)

96: Connected and Local Routes Lab Demo

https://www.udemy.com/course/ccna-complete/learn/lecture/7393478#overview

https://www.udemy.com/cisco-icnd1/learn/lecture/8605344#overview

R1#show ip route
...
      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.0.0.0/24 is directly connected, FastEthernet0/0
L        10.0.0.1/32 is directly connected, FastEthernet0/0
C        10.0.1.0/24 is directly connected, FastEthernet1/0
L        10.0.1.1/32 is directly connected, FastEthernet1/0
C        10.0.2.0/24 is directly connected, FastEthernet2/0
L        10.0.2.1/32 is directly connected, FastEthernet2/0

97: Static Routes

https://www.udemy.com/cisco-icnd1/learn/lecture/8605346#overview

  • if a router receives traffic for a network which it is not directly attached to, it needs to know how to get there in order to forward the traffic.
  • An Administrator can manually add a static route to the destination, or the router can learn it via a routing protocol.

To add a static route:

  • ip route <destination network> <Destination Subnet Maks> <Router IP>
  • To get to 10.1.0.1 from 10.0.2.1:
    • R1(config)#ip route 10.1.0.0 255.255.255.0 10.0.0.2
  • To get from 10.0.1.x and 10.0.2.x from 10.1.0.2
    • R2(config)#ip route 10.0.1.0 255.255.255.0 10.0.0.1
    • R2(config)#ip route 10.0.2.0 255.255.255.0 10.0.0.1

 

Next Step: Add a middle router


R1

R1(config)#ip route 10.1.1.0 255.255.255.0 10.0.0.2
R1(config)#ip route 10.1.0.0 255.255.255.0 10.0.0.2

R2

R2(config)#ip route 10.1.1.0 255.255.255.0 10.1.0.1
R2(config)#ip route 10.0.1.0 255.255.255.0 10.0.0.1
R2(config)#ip route 10.0.2.0 255.255.255.0 10.0.0.1

R3

R3(config)#ip route 10.0.1.0 255.255.255.0 10.1.0.2
R3(config)#ip route 10.0.2.0 255.255.255.0 10.1.0.2
R3(config)#ip route 10.0.0.0 255.255.255.0 10.1.0.2

98: Static Routing Fundamentals – Lab

https://www.udemy.com/cisco-icnd1/learn/lecture/8605350#overview

Example same as above

R1

10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.0.0.0/24 is directly connected, FastEthernet0/0
L        10.0.0.1/32 is directly connected, FastEthernet0/0
C        10.0.1.0/24 is directly connected, FastEthernet1/0
L        10.0.1.1/32 is directly connected, FastEthernet1/0
C        10.0.2.0/24 is directly connected, FastEthernet2/0
L        10.0.2.1/32 is directly connected, FastEthernet2/0
S        10.1.0.0/24 [1/0] via 10.0.0.2
S        10.1.1.0/24 [1/0] via 10.0.0.2

R2

R2(config)#ip route 10.1.1.0 255.255.255.0 10.1.0.1
R2(config)#ip route 10.0.1.0 255.255.255.0 10.0.0.1
R2(config)#ip route 10.0.2.0 255.255.255.0 10.0.0.1
R2(config)#ip route 10.1.1.0 255.255.255.0 10.1.0.1
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.0.0/24 is directly connected, FastEthernet1/0
L        10.1.0.1/32 is directly connected, FastEthernet1/0
C        10.0.0.0/24 is directly connected, FastEthernet0/0
L        10.0.0.1/32 is directly connected, FastEthernet0/0
S        10.1.1.0/24 [1/0] via 10.1.0.1
S        10.0.1.0/24 [1/0] via 10.0.0.1
S        10.0.2.0/24 [1/0] via 10.0.0.1

R3

R3(config)#ip route 10.0.1.0 255.255.255.0 10.1.0.2
R3(config)#ip route 10.0.2.0 255.255.255.0 10.1.0.2
R3(config)#ip route 10.0.0.0 255.255.255.0 10.1.0.2
10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
C        10.1.1.0/24 is directly connected, FastEthernet0/0
L        10.1.1.1/32 is directly connected, FastEthernet0/1
C        10.1.0.0/24 is directly connected, FastEthernet1/0
L        10.1.0.1/32 is directly connected, FastEthernet1/0
S        10.0.0.0/24 [1/0] via 10.1.0.2
S        10.0.1.0/24 [1/0] via 10.1.0.2
S        10.0.2.0/24 [1/0] via 10.1.0.2

99: Summarization and Default Routes

https://www.udemy.com/cisco-icnd1/learn/lecture/8605352#overview

Summary Routes

  • These can be summarized into a single routing entry per router
  • Less routes:
    • Less memory consumption
    • Reduced errors
    • This can be very important on much larger networks.

Routes on R1

R1(config)#ip route 10.1.0.0 255.255.0.0 10.0.0.2

To Summarize a tighter route (10.1.0.0 – 10.1.3.0)

R1(config)#ip route 10.1.0.0 255.255.252.0 10.0.0.2

Longest Prefix Match

  • When there are overlapping routes, the longest prefix route will be selected
    • This will be the subnet mask with the most bits
    • This can be viewed as ‘the most specific route will win’.

Load Balancing

  • When equal length routes are added for the same destination
    • The router will add them both to the routing table and load balance between them.
R1(config)#ip route 10.0.0.0 255.255.0.0 10.0.0.2
R1(config)#ip route 10.0.0.0 255.255.0.0 10.0.3.2

Default Route (Gateway of Last Resort)

  • When no specific route is located, use the Default Route
    • IP: 0.0.0.0
    • MASK: 0.0.0.0
    • Routing IP: (Probably the Internet provider)

100: Summary Routes and Longest Prefix Match Lab Demo

https://www.udemy.com/cisco-icnd1/learn/lecture/8605356#overview

  • Summary Routes on R1 to 10.1.0.0 255.255.0.0
  • Specific Route on R1 to 10.1.3.0 255.255.255.0
  • Traceroute to R4 shows hops through R2 and R3
  • Traceroute to R5 shows direct hop.

101: Direct Routes and Load Balancing Lab Demo

https://www.udemy.com/cisco-icnd1/learn/lecture/8605358#overview

  • Setup All routers to push all internet traffic toward R4
  • R1 setup to load balance between R2 and R5
    • R1(config)#ip route 0.0.0.0 0.0.0.0 10.0.0.2
    • R1(config)#ip route 0.0.0.0 0.0.0.0 10.0.3.2
  • R4 setup to load balance back to R1 via R3 and R5
    • R4(config)#ip route 10.0.1.0 255.255.255 10.1.1.2
    • R4(config)#ip route 10.0.1.0 255.255.255 10.1.3.2
    • R4(config)#ip route 10.0.2.0 255.255.255 10.1.1.2
    • R4(config)#ip route 10.0.2.0 255.255.255 10.1.3.2

How Load Balancing Works

  • Traffic from one host will always take the same route.
    • This is to help ensure all packets are received in the correct order.
  • Traffic from a second host might take the alternate route
    • This traffic will continue to take the second route
  • This is shown in the lab by traceroutes from PC1 and PC2, both originating from R1

102: Routing Fundamentals – Lab Exercises

https://www.udemy.com/cisco-icnd1/learn/lecture/8605370#overview

16 Routing Fundamentals Lab Exercise

16 Routing Fundamentals Answer Key

Connected and Local Routes

Configure PC’s with IPs

  1. Console > ip IP.AD.RE.SS /24 DEF.AULT.GATE.WAY
  2. done
  3. show ip route – OK
  4. Yes – done
  5. ping from both, ok.
  6. No, no route. done

Static Routes

7. done
8. done
9.done
10. 10.0.1.1 > 10.0.0.2 > 10.1.0.1 > 10.1.1.1  > 10.1.2.10
11.

      10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
C 10.0.0.0/24 is directly connected, FastEthernet0/0
L 10.0.0.1/32 is directly connected, FastEthernet0/0
C 10.0.1.0/24 is directly connected, FastEthernet1/0
L 10.0.1.1/32 is directly connected, FastEthernet1/0
C 10.0.2.0/24 is directly connected, FastEthernet2/0
L 10.0.2.1/32 is directly connected, FastEthernet2/0
C 10.0.3.0/24 is directly connected, FastEthernet3/0
L 10.0.3.1/32 is directly connected, FastEthernet3/0

12. Ping fails
13. R3(config)#ip route 10.0.0.0 255.0.0.0 10.0.0.2
14. Verified
15. Verified

Longest Prefix Match

16. done
17. no, no route back to PC1 from R5!
18. R5(config)#ip route 10.0.0.0 255.255.0.0 10.0.3.1
– ip route 10.0.0.0 255.255.252.0 10.0.3.1 = tighter
19. long way

20.

R5#traceroute 10.0.1.10
  1 10.0.3.1 ...
  2 10.0.1.10

21.

R1(config)#ip route 10.1.3.0 255.255.255.0 10.0.3.2

22. Verify that traffic between PC1 and R5 fe 2/0 takes the most direct path in both directions.

R5#traceroute 10.0.1.10
 1 10.0.3.1 52 msec 48 msec 24 msec
 2 10.0.1.10 72 msec 68 msec 56 msec
PC1> trace 10.1.3.2
 1   10.0.1.1   30.758 ms  15.639 ms  15.136 ms
 2   10.0.3.2   47.374 ms (ICMP type:3, code:3, Destination port unreachable)

Default Route and Load Balancing

23. done
24. done
25.

R1(config)# ip route 0.0.0.0 0.0.0.0 10.0.0.2
R1(config)# ip route 0.0.0.0 0.0.0.0 10.0.3.2

 

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