From: Peter Van Oene (pvo@xxxxxxxxxxxx)
Date: Fri Apr 27 2001 - 14:52:49 GMT-3
Actually, I would volunteer that your understanding may be a little off. Howev
er, I can attest that classless is a very often misunderstood and almost more o
ften an oddly behaving nob. A router will never make a forwarding decision on
a few bits of address space. It will always look for a best match. How it loo
ks is the domain of ip classless. In classful, a match on the major network is
made, and then a secondary lookup for the longest match subnet is undertaken.
In classless, a straight longest match lookup is performed regardless of major
networks.
This directly impacts networks with discontinuous subnets because as Radha accu
rately described, the router in classful mode will not forward to the default r
oute packets destined to a subnet of a major network for which the router has r
outes. Said another way, if 10/8 is in the routing table with routes for 10.1/
16, 10.2/16 and 10.3/16, packets destined for 10.4 will be dropped. Turning cl
assless on will enable the packets to be forwarded along a less specific path,
like 0.0.0.0 for example.
My original point was that if OSPF or ISIS provides the 0.0.0.0 route, it will
be selected even in classful mode.
Pete
*********** REPLY SEPARATOR ***********
On 4/27/2001 at 5:03 PM daneyon hansen wrote:
>I believe your understanding of ip classless is a little off. By
>default, a router will make IP forwarding decisions based on the first
>few high ordered bits of the first octect in the destination address
>within the IP header. ie...01 class A, 10 class B and 110 class C. By
>turning on IP classless, your telling the router not to make forwarding
>decisions based on the first few bits of the destination address. Your
>telling the router to make the longest match when compaing the
>destination address of the IP packet to the routing table. So it really
>has nothing to do with default routing, IP classless or no IP classless
>the router will still route to it's default.
>
>
>
>Daneyon
>
>>From: "radha rani" >Reply-To: "radha rani" >To: ccielab@groupstudy.com
>>Subject: ip classless >Date: Fri, 27 Apr 2001 10:00:06 -0400 > >I have a
>question re: ip classless. My understanding is that if >there is no
>>specific route for a subnet in the routing table, this command >allows
>the >router to forward packets to unknown destinations using the default
>>route. >This being the case when the destination is a subnet of a
>network >which is >known to the router. > >My question is : since OSPF is
>a classless protocol, why do I need >this >command to have the router
>forward packets using the default route. >I see >this all the time where
>the router will not utilize the default >route until >this command is
>added. > >I can unserstand needing it with IGRP/RIP but why with OSPF? >
>>Can someone expalin. Thanks so much.
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