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The Routing Table Lookup Process- Understanding ip classless
CCNA/CCNP
Rick Graziani
Cabrillo College
Rick Graziani [email protected]
The IP Routing Table
• This information and much more is included as two PDF documents.
• Can also be downloaded from my web site:
– 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)
• Note: There is much more in depth information in the documents above and you are encouraged to read them.
Rick Graziani [email protected]
Classful and Classless: Routing Behavior versus Routing Protocol
Notes:
• Classful or classless routing behavior has nothing to do with classful or classless routing protocols.
Routing Protocol
• Classful/Classless routing protocols are only concerned about how the routes get into the table. (Building the routing table.)
Routing Behavior (nothing to do with Routing Protocol)
• Classful/Classless routing behavior has to do with the lookup process within the routing table AFTER the routes have entered the routing table. (Looking up routes in the routing table).
Rick Graziani [email protected] 4
Directly Connected Networks
RouterB#show ip routeCodes: 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#
Router B Router CRouter A
s0 s0 s0s1
fa0 fa0 fa0
172.16.1.0/24
172.16.2.0/24
172.16.3.0/24 172.16.4.0/24
192.168.1.0/24.1
.1 .2
.1
.1
.1
.2
Rick Graziani [email protected] 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 [email protected] 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#
Ultimate Routes
• Routes with masks equal to or less than default classful mask (includes supernet routes.
Rick Graziani [email protected] 7
Creating a Parent/Child Route
RouterB(config)#interface fastethernet 0RouterB(config-if)#ip add 172.16.3.1 255.255.255.0RouterB(config-if)#no shutdown01:21:11: 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 tableRouterB(config-if)#endRouterB#undebug allAll possible debugging has been turned offRouterB#RouterB#show ip routeCodes: 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 subnetsC 172.16.3.0 is directly connected, FastEthernet0C 192.168.1.0/24 is directly connected, Serial1RouterB#
Rick Graziani [email protected] 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
Router B Router CRouter A
s0 s0 s0s1
fa0 fa0 fa0
172.16.1.0/24
172.16.2.0/24
172.16.3.0/24 172.16.4.0/24
192.168.1.0/24.1
.1 .2
.1
.1
.1
.2
Rick Graziani [email protected] 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 [email protected] 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 [email protected] 11
New Topology and Examples
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
Rick Graziani [email protected]
Parent and Child Routes
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
Parent Route
• Created automatically whenever there is a route with a mask greater than the classful mask.
• For non-VLSM routes, contains the mask of the child routes.
Child Routes
• Routes with masks greater than the default classful mask.
Ultimate Routes
• Routes with masks equal to or less than default classful mask (includes supernet routes.
Rick Graziani [email protected]
Lookup what?
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
Routing Table process matches:
• The routing table process compares the left-most bits in the packet’s destination IP address with the left-most bits in the route in the routing table, looking for a longest-bit-match.
• The subnet mask of the route in the routing table specifies the minimum number of left-most bits that must match.
• Before checking child routes, the classful mask of the parent route is used.
• For child routes the parent route’s mask is used.
• For VLSM routes, the mask is contained with the child route.
Rick Graziani [email protected]
Parent and Child Routes
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
DA = 192.168.1.10
• 16 bits of 172.16.0.0 do not match, so child routes are not checked.
• 24 bits of 192.168.1.0/24 do match, so this route is used.
DA = 172.16.2.1
• 16 bits of 172.16.0.0 do match, so child routes are checked.
• 24 bits of 172.16.1.0 do not match, so continue to next child route.
• 24 bits of 172.16.2.0 do match, so this route is used!
Rick Graziani [email protected]
Parent and Child Routes
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
DA = 32.1.1.10
• 16 bits of 172.16.0.0 do not match, so child routes are not checked.
• 24 bits of 192.168.1.0/24 do not match, so this route is not used.
• 8 bits of 172.0.0.0/8 do not match, so this route is not used.
• 4 bits of 160.0.0.0/4 do not match, so this route is not used.
• 0 bits of 0.0.0.0/0 does match, so this route is used!
Rick Graziani [email protected]
Parent and Child Routes
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
DA = 172.16.4.1
• 16 bits of 172.16.0.0 do match, so child routes are checked.
• 24 bits of 172.16.1.0 do not match, so continue to next child route.
• 24 bits of 172.16.2.0 do not match, so continue to next child route.
• 24 bits of 172.16.3.0 do not match, no more child routes.
Now what??? It depends!
Rick Graziani [email protected]
Classful Routing Behavior
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
DA = 172.16.4.1
Router(config)# no ip classless
• With classful routing behavior, if the child routes are checked but there are no matches, the routing lookup process ends and the Packet is dropped. (The packets get in, but they can’t get out!)
• Supernet and default routes are not checked.
• Default with IOS 11.2 and prior
Rick Graziani [email protected]
Classless Routing Behavior
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
DA = 172.16.4.1
Router(config)# ip classless
• With classless routing behavior, if the child routes are checked but there are no matches, the routing lookup process continues with other routes in the routing table, including supernet and default routes.
• 8 bits of 172.0.0.0/8 do match, so this route is used!
• Default with IOS 11.3 and later
Rick Graziani [email protected]
Classful Routing Behavior
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
DA = 172.16.4.1
RouterB(config)# no ip classlessA Graphical Look! The Roach Router Motel• This might help you remember the difference between the no ip classless and the ip
classless command (and then again, maybe not )• no ip classless: “The packets get in, but they can’t get out.” • Once the lookup process enters, because of the parent-bait, they never look any further.
(Looks for a match within the child routes.)• Note: They only enter if there is a match with the parent-bait.
Rick Graziani [email protected]
Classful Routing Behavior
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
DA = 172.16.4.1RouterB(config)# no ip classless
RouterB#ping 172.26.4.1
Sending 5, 100-byte ICMP Echoes to 172.16.4.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
Rick Graziani [email protected]
Classless Routing Behavior
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
DA = 172.16.4.1
RouterB(config)# ip classlessThe Roach Router Motel – There’s a hole!• ip classless: “The packets get in, and they have a way out.” • Once the lookup process enters, because of the parent-bait, they can look
further in the routing table if there is no match. • If there is a match, the process ends, else the process continues looking for a
default or supernet route.• Again, they only enter if there is a match with the parent-bait.
Rick Graziani [email protected]
Classless Routing Behavior
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
DA = 172.16.4.1
RouterB(config)# ip classless
RouterB#ping 172.16.4.1Sending 5, 100-byte ICMP Echoes to 172.16.4.1, timeout is 2
seconds:!!!!!Success rate is 100 percent (5/5),round-trip
min/avg/max=56/57/60 ms
Rick Graziani [email protected]
VLSM and Routing Tables
RouterX#show ip route
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#
Parent Route
• Created automatically whenever there is a route with a mask greater than the classful mask.
• For VLSM routes, the mask is the default classful mask.
Child Routes
• Routes with masks greater than the default classful mask.
• For VLSM routes, each child route has its own mask.
Rick Graziani [email protected]
Classful and Classless: Routing Behavior versus Routing Protocol
Notes:
• Classful or classless routing behavior has nothing to do with classful or classless routing protocols.
Routing Protocol
• Classful/Classless routing protocols are only concerned about how the routes get into the table. (Building the routing table.)
Routing Behavior (nothing to do with Routing Protocol)
• Classful/Classless routing behavior has to do with the lookup process within the routing table AFTER the routes have entered the routing table. (Looking up routes in the routing table).
The Routing Table Lookup Process- Understanding ip classless
CCNA/CCNP
Rick Graziani
Cabrillo College