DEFINICIÓN DEL ÁREA DE ESTUDIO
EL TRANSPORTE DE CARGA
6.3.4 ASPECTOS ECONÓMICOS
6.3.4.4 MEDIO PERCEPTUAL TURISMO
Classful and Classless Routing
- Routing protocols can be classified into different categories. eg: distance-vector, link-state, and balanced-hybrid. Routing protocols can also be classified as either classful or classless.
- Classful routing protocols must consider class rules when making a routing decision. They were designed to use little memory and processing power; hence they advertise just the basic routing information and do not send subnet mask information along with the subnet number in the routing updates in order to conserve bandwidth. A router running a classful routing protocol assume that all remote networks have the same subnet mask as the exiting interface, and all subnetworks of the same major network use the same subnet mask.
- Classless routing (prefix routing) protocols ignores class rules when making a routing decision.
They transmit the subnet mask information along with the subnet number in the routing updates, which is also known as prefix routing.
- The class rules imply that a routing protocol must consider the Class A, B, or C network number that a subnet resides when performing its tasks.
- Interior IP routing protocols comparison chart:
Routing Protocol
Classful / Classless
Sends Subnet Mask along with Routing Updates
Supports VLSM
Supports Route Summarization
RIPv1 Classful No No No
IGRP Classful No No No
RIPv2 Classless Yes Yes Yes
EIGRP Classless Yes Yes Yes
OSPF Classless Yes Yes Yes
- As classful routing protocols do not advertise subnet mask information along the routing updates, a router will guess the subnet mask when it received a classful routing protocol routing updates.
Classful routing protocols expect a Fixed-Length Subnet Mask (FLSM) throughout a network, and this does not cause any problem as long as all subnets are in the same class.
- Classless routing protocols have more advantages over classful routing protocols as they support some advanced and important features, eg: VLSM and route summarization. Besides that, classless routing protocols also overcome some network design issues that exist only in classful routing protocols, eg: autosummarization.
- Autosummarization happens when a router has interfaces in more than one Class A, B, or C networks. When advertising routes with classful addresses different than the classful address of the interface that sends out the routing updates, the routes will be summarized as a single summary route up to the classful network boundary of the addresses.
Ex: 172.16.1.0/24 and 172.16.2.0/24 (Class C) are summarized as 172.16.0.0/16 (Class B).
- Autosummarization works fine in contiguous networks, but this behavior can causes problems in discontiguous networks – networks that have 2 subnets of a classful network separated by another different classful network. Classful routing protocols do not support discontiguous networks; while classless routing protocols do support discontiguous networks.
- RIPv1 and IGRP perform autosummarization by default, and cannot be disabled – it is a feature of classful routing protocols. For RIPv2 and EIGRP, the autosummarization feature which enabled by default can be disabled with the no auto-summary router subcommand.
Autosummarization allows RIPv2 and EIGRP to be backward-compatible with their predecessors – RIPv1 and IGRP.
- OSPF and IS-IS do no perform autosummarization and do not even have this feature.
Figure 16-1: Autosummarization
- Below shows the routing table on RT1 with just a single route to network 10.0.0.0/8 due to the autosummarization feature of RT2 (RIPv1).
Figure 16-2: The Pitfall of Autosummarization RT1#sh ip route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 2 subnets
C 172.16.2.0 is directly connected, Ethernet0 C 172.16.3.0 is directly connected, Serial1
R 10.0.0.0/8 [120/1] via 172.16.3.2, 00:00:15, Serial1 RT1#
RT1#debug ip rip
RIP protocol debugging is on RT1#
00:08:54: RIP: received v1 update from 172.16.3.2 on Serial1 00:08:54: 10.0.0.0 in 1 hops
RT1#
172.16.2.0
172.16.3.0 10.3.4.0
10.3.5.0 10.3.6.0 10.3.7.0
RT1 RT2
S1
S0
10.0.0.0 / 8
172.16.2.0
172.16.1.0 172.16.3.0
10.2.1.0 10.2.2.0 10.2.3.0 10.2.4.0
10.3.4.0 10.3.5.0 10.3.6.0 10.3.7.0
RT1 RT2 RT3
S0
S1 S0
S0
10.0.0.0 / 8 10.0.0.0 / 8
Which route to 10.0.0.0/8 ?!?
- Below shows the routing table of RT2, with RIPv1 as the routing protocol. RT2 has 2 routes to
10.0.0.0/8 network due to autosummarization. RT2 will perform load balancing across the paths since they are equal-cost paths to the same destination, which will definitely cause malfunctioning on applications!
- This problem can be solved by migrating to a classless routing protocol with autosummarization disabled, eg: RIPv2, EIGRP. Remember to disable autosummarization as it is enabled by default.
- As mentioned before, routing protocols are considered either classful or classless. Nevertheless, IP routing can also be considered either classful or classless as well.
- The concepts of classful and classless routing are independent of any routing protocol, as the concepts still applicable to IP routing even if only static routes are being used.
Note: Classlessness can be a characteristic of a routing protocol or a router.
- Classful or classless routing affects how a router uses its default route. A very good topic for the discussion of classful and classless routing is default routing, where packets destined to networks with no specific routes in the routing table will be directed to the default route.
- The [no] ip classless global configuration command enables and disables classless routing respectively. Disabling classless routing is equivalent to enabling classful routing.
- Note: The classless IP behavior is enabled by default in Cisco IOS Release 12.0 and later.
Note: The classless IP behavior has no effect on some later Cisco IOS 12.3 releases.
Figure 16-3: Sample Default Routing Network
RT1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RT1(config)#no ip classless
RT1(config)#ip route 0.0.0.0 0.0.0.0 150.15.1.1 RT1(config)#^Z
RT1#
RT2#sh ip route
Gateway of last resort is not set
172.16.0.0/24 is subnetted, 3 subnets
C 172.16.1.0 is directly connected, Serial0 C 172.16.2.0 is directly connected, Ethernet0 C 172.16.3.0 is directly connected, Serial1
R 10.0.0.0/8 [120/1] via 172.16.1.1, 00:00:20, Serial0 [120/1] via 172.16.3.2, 00:00:15, Serial1 RT2#
150.15.100.0/24
10.1.1.0/24 RT1 150.15.1.0/24 ISP
.1 .2
- As seen from the output above, ping to 150.15.100.1 with classful routing failed, but ping has succeeded with classless routing. This shows even if there is a default route in the routing table, it might not be used!
- Cisco IOS uses either classful or classless routing logic when matching a destination IP address with the routes in the routing table and decide when to use the default route.
- The classful logic – no ip classless:
i) RT1 needs to send a packet to 150.15.100.1.
ii) RT1 matches Class B network – 150.15.0.0, a directly connected network. Due to it is running classful routing, the default route cannot be used!
iii) RT1 does not have a more specific route to 150.15.100.0 from its routing table.
iv) RT1 discards the packet as the default route cannot be used.
- The classless logic – ip classless:
i) RT1 needs to sends a packet to 150.15.100.1.
ii) RT1 does not have a more specific route to 150.15.100.0.
iii) The default route is being used as there is no specific route matched.
- With classful routing, the default route is only being used when a packet’s destination Class A, B, or C major network number is not in the routing table.
Ex: A packet destined to 10.2.1.1 is discarded by a classful router with a routing table that consisting routes to 10.0.0.0/8, 10.1.1.0/24, 10.1.2.0/24, as the major network of 10.2.1.1 –
10.0.0.0
RT1#sh ip route
Gateway of last resort is 150.15.1.1 to network 0.0.0.0
150.15.0.0/24 is subnetted, 1 subnets
C 150.15.1.0 is directly connected, Serial0/0 S* 0.0.0.0/0 [1/0] via 150.15.1.1
RT1#
RT1#ping 10.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/36/40 ms RT1#ping 150.15.100.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.15.100.1, timeout is 2 seconds:
...
Success rate is 0 percent (0/5) RT1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RT1(config)#ip classless RT1(config)#^Z
RT1#ping 150.15.100.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.15.100.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/38/40 ms RT1#
- With classless routing, the default route is used whenever a packet does not match a more specific route in the routing table (unknown subnets of known a classful network).
- If the supernet or default route is learned via a classless routing protocol (eg: OSPF, IS-IS), the classful nature (the no ip classless global configuration command is configured) of a router is ignored – a classful router may use a default route to reach networks that are not listed in the routing table regardless of the ip classless command.