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Understanding The Routing Table

Understanding The Routing Table. Part 1: The Routing Table Structure Part 2: The Lookup Process Rick Graziani Cabrillo College. The Routing Table on PDF. Instructors and Students: Complete information explanations associated with this PowerPoint can be found on my web site.

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Understanding The Routing Table

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  1. Understanding The Routing Table Part 1: The Routing Table Structure Part 2: The Lookup Process Rick Graziani Cabrillo College

  2. The Routing Table on PDF • Instructors and Students: Complete information explanations associated with this PowerPoint can be found on my web site. Understanding the Routing Table • The Routing Table: Part 1 or 2 - The Routing Table Structure (PDF)  • The Routing Table: Part 2 or 2 - The Routing Table Lookup Process (PDF)  • Discard Routes - Avoiding Routing Loops (PowerPoint) Rick Graziani graziani@cabrillo.edu

  3. Part 1: The Routing Table Structure

  4. Directly Connected Networks RouterB#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, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set RouterB# Rick Graziani graziani@cabrillo.edu

  5. Directly Connected Networks RouterB#debug ip routing RouterB(config)#interface s 1 RouterB(config-if)#ip add 192.168.1.1 255.255.255.0 RouterB(config-if)#no shutdown 00:59:48: %LINK-3-UPDOWN: Interface Serial1, changed state to up 00:59:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to up 00:59:48: RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0] 00:59:48: RT: interface Serial1 added to routing table RouterB(config-if)#end RouterB#undebug all All possible debugging has been turned off RouterB# Rick Graziani graziani@cabrillo.edu

  6. Directly Connected Networks RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  7. Creating a Parent/Child Route RouterB(config)#interface fastethernet 0 RouterB(config-if)#ip add 172.16.3.1 255.255.255.0 RouterB(config-if)#no shutdown 01:21:11: RT: add 172.16.3.0/24 via 0.0.0.0, connected metric [0/0] 01:21:11: RT: interface FastEthernet0 added to routing table RouterB(config-if)#end RouterB#undebug all All possible debugging has been turned off RouterB# RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 1 subnets C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  8. Parent, Child, Ultimate Routes So far on Router B: Parent Route: 172.16.0.0/24 is subnetted, 1 subnets Child Route: C 172.16.3.0 is directly connected, FastEthernet0 Ultimate Route: C 192.168.1.0/24 is directly connected, Serial1 Rick Graziani graziani@cabrillo.edu

  9. Creating Another Child Route RouterB(config)#inter s 0 RouterB(config-if)#ip add 172.16.2.2 255.255.255.0 RouterB(config-if)#end RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  10. Same structure no matter the source of route RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 4 subnets S 172.16.4.0 is directly connected, Serial1 R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 10.0.0.0/16 is subnetted, 1 subnets S 10.1.0.0 is directly connected, Serial1 C 192.168.1.0/24 is directly connected, Serial1 S 192.168.100.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  11. Static Routes and Recursive Lookups RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 RouterA# RouterA(config)#ip route 172.16.3.0 255.255.255.0 172.16.2.2 03:12:45: RT: add 172.16.3.0/24 via 172.16.2.2, static metric [1/0] RouterA(config)# Rick Graziani graziani@cabrillo.edu

  12. Static Routes and Recursive Lookups RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 S 172.16.3.0 [1/0] via 172.16.2.2 RouterA# First lookup of 172.16.3.0: S 172.16.3.0 [1/0] via 172.16.2.2 Second lookup of 172.16.2.0: C 172.16.2.0 is directly connected, Serial0 Rick Graziani graziani@cabrillo.edu

  13. Static Routes without Recursive Lookup RouterB(config)#ip route 172.16.1.0 255.255.255.0 serial 0 05:05:30: RT: add 172.16.1.0/24 via 0.0.0.0, static metric [1/0] RouterB(config)# RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 is directly connected, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  14. Static Routes and Administrative Distance You might have also noticed that the routing table states that this static route is “directly connected.” S 172.16.1.0 is directly connected, Serial0 • In the case of a static route, directly connected means that the static route was configured with an exit interface. • This does not mean that it is a directly connected network like an interface on the router. • Like all static routes, the default administrative distance of this static route is still “1”. • Only directly connected interfaces, (which have a code of “C” in the routing table) can have an administrative distance of “0”. • Although it is possible to modify the administrative distance of static and dynamic routes, they cannot be given the administrative distance of “0”. • Likewise, directly connected interfaces can only have an administrative distance of “0”. Rick Graziani graziani@cabrillo.edu

  15. Static Routes via Ethernet Interfaces RouterB(config)#ip route 172.16.1.0 255.255.255.0 172.16.2.1 RouterB(config)#end RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0/24 [1/0] via 172.16.2.1 C 172.16.2.0 is directly connected, FastEthernet1 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  16. Static Routes via Ethernet Interfaces RouterB(config)#ip route 172.16.1.0 255.255.255.0 fastethernet 1 172.16.2.1 RouterB(config)#end RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0/24 [1/0] via 172.16.2.1 FastEthernet1 C 172.16.2.0 is directly connected, FastEthernet1 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  17. VLSM RouterX#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks C 172.16.1.4/30 is directly connected, Serial0 C 172.16.1.8/30 is directly connected, Serial1 C 172.16.3.0/24 is directly connected, FastEthernet0 RouterX# Rick Graziani graziani@cabrillo.edu

  18. Part 2: The Lookup Process

  19. Subnet Mask = # of bits that must match RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0 R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0 RouterA# Rick Graziani graziani@cabrillo.edu

  20. Subnet Mask = # of bits that must match RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0 R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0 RouterA# Rick Graziani graziani@cabrillo.edu

  21. Parent then Children RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0 R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0 RouterA# Rick Graziani graziani@cabrillo.edu

  22. RouterA#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, FastEthernet0 C 172.16.2.0 is directly connected, Serial0 R 172.16.3.0 [120/1] via 172.16.2.2, 00:00:00, Serial0 R 192.168.1.0/24 [120/1] via 172.16.2.2, 00:00:00, Serial0 RouterA# Rick Graziani graziani@cabrillo.edu

  23. The Default Route RouterB(config)#ip route 0.0.0.0 0.0.0.0 serial1 RouterB(config)#end RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is 0.0.0.0 to network 0.0.0.0 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  24. Classful Routing Behavior RouterB#show running-config Building configuration... Current configuration: ! version 11.2 ! <text omitted> ! no ip classless ! <text omitted> Rick Graziani graziani@cabrillo.edu

  25. Classful Routing Behavior RouterB#show ip route Gateway of last resort is 0.0.0.0 to network 0.0.0.0 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  26. Classful Routing Behavior Routing Behavior: classful (no ip classless) Packet’s Destination IP Address:172.16.4.10 RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, <text omitted> Gateway of last resort is 0.0.0.0 to network 0.0.0.0 1. Match 172.16.0.0/24 is subnetted, 3 subnets 2. No Match R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0 3. No Match C 172.16.2.0 is directly connected, Serial0 4. No Match C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 5. Drop Packet S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  27. Classless Routing Behavior RouterB#show running-config Building configuration... Current configuration: ! version 11.3 ! <text omitted> ! ip classless ! <text omitted> Rick Graziani graziani@cabrillo.edu

  28. Classless Routing Behavior RouterB#show ip route Gateway of last resort is 0.0.0.0 to network 0.0.0.0 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  29. Classless Routing Behavior Routing Behavior: classless (ip classless) Packet’s Destination IP Address:172.16.4.10 RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, <text omitted> Gateway of last resort is 0.0.0.0 to network 0.0.0.0 1. Match 172.16.0.0/24 is subnetted, 3 subnets 2. No Match R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:13, Serial0 3. No Match C 172.16.2.0 is directly connected, Serial0 4. No Match C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 5. Match! S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  30. Classless and Classful Routing Behavior Changing the default routing behavior, whether classful or classless, is simple. The command to enable classless routing behavior is: RouterB(config)#ip classless RouterB(config)# The command to disable classless routing behavior, in other words, enable classful routing behavior is: RouterB(config)#noip classless RouterB(config)# Use show running-config to verify which command is in affect. Rick Graziani graziani@cabrillo.edu

  31. /16 Classful Routes RouterC(config)#interface loopback1 RouterC(config-if)#ip add 172.20.0.1 255.255.0.0 RouterC(config-if)#exit RouterC(config)#interface loopback2 RouterC(config-if)#ip add 172.30.0.1 255.255.0.0 RouterC(config-if)#exit RouterC#show ip inter brief Interface IP-Address OK? Method Status Protocol FastEthernet0 172.16.4.1 YES NVRAM up up Loopback1 172.20.0.1 YES manual up up Loopback2 172.30.0.1 YES manual up up Serial0 192.168.1.2 YES NVRAM up up RouterC# Rick Graziani graziani@cabrillo.edu

  32. /16 Classful Routes Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is not set 172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks C 172.16.4.0/24 is directly connected, FastEthernet0 S 172.16.0.0/16 is directly connected, Serial0 C 172.20.0.0/16 is directly connected, Loopback1 C 172.30.0.0/16 is directly connected, Loopback2 C 192.168.1.0/24 is directly connected, Serial0 RouterC# Rick Graziani graziani@cabrillo.edu

  33. Static Routes to Supernets RouterB(config)#ip route 172.0.0.0 255.0.0.0 serial1 RouterB(config)#ip route 160.0.0.0 240.0.0.0 serial1 RouterB(config)#end RouterB#show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <text omitted> Gateway of last resort is 0.0.0.0 to network 0.0.0.0 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S 172.0.0.0/8 is directly connected, Serial1 S 160.0.0.0/4 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  34. Supernets RouterB#show ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S 172.0.0.0/8 is directly connected, Serial1 S 160.0.0.0/4 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  35. Supernets – 172.0.0.0 best match RouterB#show ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S 172.0.0.0/8 is directly connected, Serial1 S 160.0.0.0/4 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  36. Supernets – 160.0.0.0 best match RouterB#show ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S 160.0.0.0/4 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  37. The last of the Supernets – 0.0.0.0/0 RouterB#show ip route 172.16.0.0/24 is subnetted, 3 subnets R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:20, Serial0 C 172.16.2.0 is directly connected, Serial0 C 172.16.3.0 is directly connected, FastEthernet0 C 192.168.1.0/24 is directly connected, Serial1 S* 0.0.0.0/0 is directly connected, Serial1 RouterB# Rick Graziani graziani@cabrillo.edu

  38. The Routing Table on PDF • Complete information explanations associated with this PowerPoint can be found on my web site. Understanding the Routing Table • The Routing Table: Part 1 or 2 - The Routing Table Structure (PDF)  • The Routing Table: Part 2 or 2 - The Routing Table Lookup Process (PDF)  • Discard Routes - Avoiding Routing Loops (PowerPoint) Rick Graziani graziani@cabrillo.edu

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