Arp problem

Unanswered Question
Aug 15th, 2007

I am trying to ping a IP from a different subnet and I am not getting any answer from it. If I ping it from my C3550 Version 12.1(13)EA1a (my gateway) then it work fine. Any idea were I should start looking ?

I have this problem too.
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ankbhasi Wed, 08/15/2007 - 09:11

Hi Friend,

Is both the subnet directly connected to this switch? Can you paste the output of "sh ip route" and confirm which subnet cannot ping which subnet?

Regards,

Ankur

msaulnier Wed, 08/15/2007 - 09:30

The machines are not directly connected to the switch.

The source is in vlan 1 and the destination is in vlan 6

interface Vlan1

description Administration

ip address 172.30.4.33 255.255.0.0

no ip redirects

ip route-cache flow

standby ip 172.30.4.70

standby priority 150

standby preempt delay sync 0

standby name fensw-cls01

interface Vlan6

description Usine

ip address 10.30.30.33 255.255.224.0

no ip redirects

ip route-cache flow

standby 1 ip 10.30.30.70

standby 1 priority 100

standby 1 preempt

standby 1 name VLAN6-HSRP

This is the output of sh ip route:

C 10.30.0.0/19 is directly connected, Vlan6

C 172.30.0.0/16 is directly connected, Vlan1

Jon Marshall Wed, 08/15/2007 - 09:34

So the source machine has 172.30.4.70 as it's default gateway and the destination has 10.30.30.70 as it's default gateway.

And the ports for the source and destination are allocated into the right vlans ?

Jon

msaulnier Wed, 08/15/2007 - 09:43

Actually the source is using 172.30.4.70 but the destination is using 10.30.30.33.

Yes, the port are on the right vlan.

ankbhasi Wed, 08/15/2007 - 09:47

Hi Friend,

Can you change the destination machines gateway as the standby address you defned?

Ankur

msaulnier Wed, 08/15/2007 - 09:50

I found something interesting.

From the source I can 172.30.4.70 but I can't ping 10.30.30.33.

From The destination I can ping both.

ankbhasi Wed, 08/15/2007 - 09:35

Hi Friend,

What is the gateway address you have assigned to your machines? Also if you have asigned the gateway address as the virtual/stadby ip address can you ping your gateway address from your machines?

Also if it is pinging can you ping other subnet gateway address?

Regards,

Ankur

Richard Burts Wed, 08/15/2007 - 09:44

Marc

I am slightly puzzled. The title given to this thread in the original post indicated ARP problem. But in the description there is no mention of ARP? Is there an aspect of this that is related to ARP?

Given what has been discussed so far, if the 2 end stations are connected to ports in the 2 VLANs it would seem most likely that the problem is that at least 1 of the end stations does not have a correct default gateway configured. Can you verify the default gateway of each end station?

HTH

Rick

msaulnier Wed, 08/15/2007 - 09:54

Because the problem was fixed when I did a ping from the switch to the destination i assumed it was a arp problem.

Right now I can ping the source and the destination both way.

But I cant ping 10.30.30.33 from the source.

Richard Burts Wed, 08/15/2007 - 10:02

Marc

That adds an interesting aspect to the problem. Can you verify that the end stations have the correct subnet mask as well as the correct default gateway address?

HTH

Rick

msaulnier Wed, 08/15/2007 - 10:05

The source is set to:

IP 172.30.66.2

Mask 255.255.0.0

GW 172.30.4.70

The Destination is set to:

IP 10.30.30.15

Mask 255.255.224.0

GW 10.30.30.33

glen.grant Wed, 08/15/2007 - 10:20

Destination default gateway should be 10.30.30.70 though what you have should work also , it should point to the hsrp ip address .

glen.grant Wed, 08/15/2007 - 10:44

In another problem something along these lines , we have a what is called a HAAS milling machine connected to the network and we also cannot reach this off the directly connected network . The people that we have talked to say they cannot find anywhere to put a default gateway on this device so we are at a loss on how to make this work . We had a couple of other devices where we ended up turning on proxy arp against our better judgement and it fixed those but it did not fix the problem of the milling machine . Any ideas on how to get around this ? We can ping it from the directly attached router but if you source the ping from any other subnet it doesn't get there , the arp table is populated with the correct address and mac . Any ideas appreciated.

Jon Marshall Wed, 08/15/2007 - 11:03

Glen

One thing that springs to mind although it depends on the switch is to use NAT. If it isn't a 6500 switch though you can't do it.

Just in case it is you could use policy NAT and any source IP addresses that are destined for your milling machine are Natted to the router interface that connects to the milling machine subnet. That way the milling machine will always be able to send the packet back to an address on it's own subnet.

So if you happen to have this on a 6500 switch or have a L2 switch with a router and if NAT doesn't break it it might just work :)

HTH

Jon

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