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New Member

IP classless

Hi,

I have a scenario of Rip V1 on three routers connected in row. IOS is 12.0 .

The IP address scheme used is of /24 sunbets of major network 10.0.0.0

Do I have to put this command <<no ip classless>> to enable classless routing behaviour, as bydefault IOS has <<ip classless>>.

I think I need to have classless routing behovior to have updates of all subnets known to all routers.

PLease comment ...

Thx in advcn

Gaurav

6 REPLIES
New Member

Re: IP classless

Hi Gaurav,

If command is enabled that means your router will have a classless behaviour.

Re: IP classless

RIPv1 is classfull protocol and so ip classless will not work regardless so you better disabled it.

I hope it helps .. please rate it if it does !!!

New Member

Re: IP classless

so enabling or disabling <> will not make any difference ?

Hall of Fame Super Silver

Re: IP classless

It is absolutely not true that ip classless will not work regardless ! ip classless works and it matters very little which routing protocol we are talking about.

It is a fundamental fact that it is the responsibility of the routing protocol to build the IP forwarding table (or the routing table if you prefer that way of describing it). And the routing protocol builds the table based on how the routing protocol is designed and ip classless does not impact how the routing protocol works.

ip classless is used when the router makes a decision about how to forward a data packet. (and the router does not use a routing protocol to forward the packet)

It may help to have a small example: lets assume that we have a router and it has this ip forwarding table

10.10.10.0/24 FastEthernet0/0

10.10.20.0/24 FastEthernet0/0

10.1.1.0/30 serial1/0

172.16.0.0/16 serial1/0

0.0.0.0/0 serial1/0

this table could be built by a classful protocol like RIPv1 or by a classless protocol like OSPF (no impact on how the table is built by ip classless or no ip classless)

what would happen when the router gets a packet with destination 10.10.20.50?

the router forwards the packet through interface FastEthernet0/0. (do not need to consider classless in this decision - the router knows where to go)

what would happen when the router gets a packet with destination 192.168.6.19?

clearly it goes to the default route - forwarded out interface serial1/0 (do not need to consider classless in this decision - clearly the router should use the default route)

what would happen when the router gets a packet with destination 10.50.60.33?

this is the point where the router must consider whether it is acting classless or no classless. The router knows some subnets of this network but does not have a route to this particular subnet. If the router is configured with ip classless then the router will forward the packet using the default route (going out serial1/0). If the router is configured with no ip classless then the router will drop the packet (it has nowhere to forward the packet to).

So ip classless or no ip classless affects how the router makes forwarding decisions but does not affect how the routing protocol works.

HTH

Rick

Re: IP classless

Dear,

IP classless need to stop the routing tabel (FIB) from convert the address to its classful mask, the routing protocols responsable about its updates only so if your routing protocol is classless (and you are not using ip classless command)it will send and receive classless address in its updates but when the router select this routes as best path (Based on AD) the router gone reset the address subnet to its default mask

Please rate helpful posts

Best Regards,

Mounir Mohamed

New Member

Re: IP classless

>>packets last resort is default gateway configured

>>packets has not last resort, then will be dropped if not in routing table

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