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New Member

L3 routing problem between 2 subnets

I have a 3550 as core switch for my network

I have 2 subnets: 192.168.42.0/24 & 192.168.43.0/24

my 3550 is configured as D\G for both subnets

from 42.x machines I can pign any 42.x or 43.x IP

from 43.x I can ping any server on the 42.x subnet BUT not any of the 42.x routers that connect directly to the 3550 switch...

I have no idea why and how can it happen

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Silver

Re: L3 routing problem between 2 subnets

Ofir

Thank you for posting the additional information that I asked for. I believe that it is very hellpful.

From this information I notice that the router has a static route for the 192.168.43.0 subnet. I am going to guess that the static route is of the form:

ip route 192.168.43.0 255.255.255.0 fastethernet0/0

rather than of the form which specifies the next hop address. Can you verify whether this is true?

If this is true then the router knows that it must go through interface Fastethernet0/0 to get to the 43 subnet. And the router must ARP for the destination address in the 43 subnet. And I believe that the switch is receiving the ARP request and not responding. If you change the static route on the router so that it specifies the next hop address (is it 192.168.42.254?) then I believe that the problem will be solved.

HTH

Rick

12 REPLIES
Silver

Re: L3 routing problem between 2 subnets

Hi ,

how is the connectivity between switch and routers.

can I have the network diagram .

Thanks,

satish

Hall of Fame Super Silver

Re: L3 routing problem between 2 subnets

Ofir

The symptoms that you describe sound like the 42.x routers do not have a route in their routing tables for 43.x. Can you check the routers and verify what is in their routing table?

HTH

Rick

Purple

Re: L3 routing problem between 2 subnets

If you are not running a routing protocol it sounds like you have no static route on the .42X routers pointing on how to get to the .43 subnet . On the .42 subnet you are directly attached so it knows how to get back to the .42 address but unless there is a specific static pointing back at the .43 it has no idea on what to do with that packet.

New Member

Re: L3 routing problem between 2 subnets

I have a route on the routers to go ethernet for 43.x

I also checked trace and it is getting to the D\G of 43.x (which is teh same 3550 switch as 42.x D\G)

Hall of Fame Super Silver

Re: L3 routing problem between 2 subnets

Ofir

Can you post the output of show ip route from one of the 42.x routers? If that is a lot of output then perhaps just post the output of show ip route 192.168.42.0 and show ip route 192.168.43.0?

Have you checked on the possibility that the routers might have an access list configured that does not permit access to 43.x or that does not permit ping?

HTH

Rick

New Member

Re: L3 routing problem between 2 subnets

*************************************

this is from one of the 42.x routers:

*************************************

Gateway of last resort is 212.x.y.117 to network 0.0.0.0

S 192.168.46.0/24 [1/0] via 212.x.y.117

C 192.168.42.0/24 is directly connected, Ethernet0/0

S 192.168.43.0/24 is directly connected, Ethernet0/0

212.x.y.0/30 is subnetted, 1 subnets

C 212.x.y.116 is directly connected, Serial0/0

S* 0.0.0.0/0 [1/0] via 212.x.y.117

no Access List on this router (cisco 2611)

**********************

this is from the 3550:

**********************

CORE#sh ip route 192.168.42.0

Routing entry for 192.168.42.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Redistributing via rip

Advertised by rip

Routing Descriptor Blocks:

* directly connected, via Vlan42

Route metric is 0, traffic share count is 1

CORE#sh ip route 192.168.43.0

Routing entry for 192.168.43.0/24

Known via "connected", distance 0, metric 0 (connected, via interface)

Redistributing via rip

Advertised by rip

Routing Descriptor Blocks:

* directly connected, via Vlan43

Route metric is 0, traffic share count is 1

no access-lists on this switch

*************************************

I also remind tyou that I can ping from 43.x to all my servers on 42.x (which also go via same core switch)

H:\>tracert 192.168.42.133

Tracing route to 192.168.42.133 over a maximum of 30 hops

1 <1 ms <1 ms <1 ms 192.168.43.254

2 <1 ms <1 ms <1 ms 192.168.42.133

Trace complete.

Hall of Fame Super Silver

Re: L3 routing problem between 2 subnets

Ofir

Thank you for posting the additional information that I asked for. I believe that it is very hellpful.

From this information I notice that the router has a static route for the 192.168.43.0 subnet. I am going to guess that the static route is of the form:

ip route 192.168.43.0 255.255.255.0 fastethernet0/0

rather than of the form which specifies the next hop address. Can you verify whether this is true?

If this is true then the router knows that it must go through interface Fastethernet0/0 to get to the 43 subnet. And the router must ARP for the destination address in the 43 subnet. And I believe that the switch is receiving the ARP request and not responding. If you change the static route on the router so that it specifies the next hop address (is it 192.168.42.254?) then I believe that the problem will be solved.

HTH

Rick

New Member

Re: L3 routing problem between 2 subnets

WOW! you're the man!!!

I changed ip route 192.168.43.0 255.255.255.0 fastethernet0/0

to be ip route 192.168.43.0 255.255.255.0 192.168.42.254

thanks!!!

Hall of Fame Super Silver

Re: L3 routing problem between 2 subnets

Ofir

I am glad that my suggestion did resolve your problem. Thanks for the rating.

This has been a very interesting problem and helps to illustrate the differences in the different forms of static route.

HTH

Rick

New Member

Re: L3 routing problem between 2 subnets

Run a traceroute command on your 43x machine to

a 42x router and see were the routes are getting lost.

New Member

Re: L3 routing problem between 2 subnets

Tracing route to 192.168.42.3 over a maximum of 30 hops

1 <1 ms <1 ms <1 ms 192.168.43.254

2 * * * Request timed out.

43.254 is the D\G for 43.x network on the 3550 switch

Silver

Re: L3 routing problem between 2 subnets

Hi ,

Can you paste config of 3550 switch

Thanks,

satish

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