ip classless question

Answered Question
Apr 13th, 2010
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Hi all,


If Router A  has  no route in the routing table for destination route 10.1.3.0 and we do have default route configured as---


ip route 0.0.0.0 0.0.0.0 serial0/0   on Router A


will the packets be dropped to destination route 10.1.3.0 if no ip classless command is configured on the router A.



or


if we configure the ip classless command in Router A  config then packets will be forwarded to 10.1.3.0 network by serail 0/0?


Please explain


thanks


mahesh

Correct Answer by kevin.hu about 6 years 11 months ago

when you say hosts at remote site you mean to say router at remote site  with config as below?


Yes.


when you say the corporate network  it means that network for example  has  subnet 10.1.3.0 ?


Yes.


and Remote router can not reach this subnet  right?


Yes, because of the no ip classless config.

Correct Answer by Giuseppe Larosa about 6 years 11 months ago

Hello Mahesh,


if the router is connected to major network 10/8, and does not know subnet 10.1.3.0 AND it is working in classful mode (= no ip classless) it will drop packets for the unknown subnet


if routers is NOT connected to 10/8 and does not know subnet 10.1.3.0 AND it is working in classful mode (=  no ip classless) it will use the default route.


So if the router has no interface in network 10/8, it will use the default route both in classful and in classless mode


This was a concept of the past: if the router recevies packets for an unknown subnet of a connected major network, the default route was not used because old classful routing protocols do not support partitioned major networks


the modern behaviour with 12.0 is ip classless and in this case the default route is used also for unknown subnets of a connected major network because the major network could be partitioned


Hope to help

Giuseppe

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kevin.hu Tue, 04/13/2010 - 08:44
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IP classless only affects the operation of the forwarding processes in IOS; it doesn't affect the way the routing table is built. If IP classless isn't configured (using the no ip classless  command), the router won't forward packets to supernets.

interface Serial 0
     ip address 10.1.2.2 255.255.255.0
   !
   interface Ethernet 0
     ip address 10.1.1.1 255.255.255.0
   !
   ip route 0.0.0.0 0.0.0.0 10.1.2.1
   !
   no ip classless

With this configuration, the hosts at the remote site can reach       destinations on the Internet (through the 10.x.x.x cloud), but not  destinations       within the 10.x.x.x cloud, which is the corporate network. Because the  remote       router knows about some part of the 10.0.0.0/8 network, the two  directly       connected subnets, and no other subnet of 10.x.x.x, it assumes these  other       subnets don't exist and drops any packets destined for them. Traffic  destined       to the Internet, however, doesn't ever have a destination in the  10.x.x.x range       of addresses, and is therefore correctly routed through the default  route.

mahesh18 Tue, 04/13/2010 - 09:08
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Hi Kevin


Thanks for reply


when you say hosts at remote site you mean to say router at remote site with config as below ?



interface Serial 0
     ip address 10.1.2.2 255.255.255.0
   !
   interface Ethernet 0
     ip address 10.1.1.1 255.255.255.0
   !
   ip route 0.0.0.0 0.0.0.0 10.1.2.1
   !
   no ip classless



and  when you say the corporate network  it means that network for example has  subnet 10.1.3.0 ? and Remote router can not reach this subnet right?


many thanks

mahesh

Correct Answer
kevin.hu Tue, 04/13/2010 - 09:28
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when you say hosts at remote site you mean to say router at remote site  with config as below?


Yes.


when you say the corporate network  it means that network for example  has  subnet 10.1.3.0 ?


Yes.


and Remote router can not reach this subnet  right?


Yes, because of the no ip classless config.

Correct Answer
Giuseppe Larosa Tue, 04/13/2010 - 08:45
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Hello Mahesh,


if the router is connected to major network 10/8, and does not know subnet 10.1.3.0 AND it is working in classful mode (= no ip classless) it will drop packets for the unknown subnet


if routers is NOT connected to 10/8 and does not know subnet 10.1.3.0 AND it is working in classful mode (=  no ip classless) it will use the default route.


So if the router has no interface in network 10/8, it will use the default route both in classful and in classless mode


This was a concept of the past: if the router recevies packets for an unknown subnet of a connected major network, the default route was not used because old classful routing protocols do not support partitioned major networks


the modern behaviour with 12.0 is ip classless and in this case the default route is used also for unknown subnets of a connected major network because the major network could be partitioned


Hope to help

Giuseppe

mahesh18 Tue, 04/13/2010 - 09:36
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Hi kevin and giuslar,


many thanks for explaining we concept.


regards


mahesh

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