04-13-2010 08:35 AM - edited 03-06-2019 10:35 AM
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
Solved! Go to Solution.
04-13-2010 08:45 AM
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
04-13-2010 09:28 AM
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.
04-13-2010 08:44 AM
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.
04-13-2010 09:08 AM
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
04-13-2010 09:28 AM
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.
04-13-2010 08:45 AM
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
04-13-2010 09:36 AM
Hi kevin and giuslar,
many thanks for explaining we concept.
regards
mahesh
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