Pinging between different subnets on L3 LAN switches

Unanswered Question
Oct 30th, 2009

We have an architecture with interconnected LAN switches with multiple VLANs on each. In some cases a VLAN will have an IP interface (SVI) created, but in other cases the switches just have the VLAN, with access ports on the VLAN and the uplinks trunking the VLAN, but no actual L3 SVI. I've noticed that when I ping from one switch to an SVI on another switch, the ping will only be returned successfully if an SVI from the same VLAN exists on the source switch. For example, if I create vlan 10 and interface vlan 10 with IP address 10.0.0.1 on one switch and vlan 10 with interface vlan 10 and address 10.10.10.2 on the other, I can ping between them. But if I only have vlan 10 on the source switch, without the L3 SVI, then try to ping interface vlan 10/10.10.10.2, the ping will not be returned. At the same time, if I ping 10.10.10.2 from my workstation, with my workstation being on a completely different subnet, the ping will return successfully. So why can't the two interconnected switches ping to each other unless they both have SVIs on the same subnet? It seems that if you have "ip routing" configured on the switch, and the VLAN exists on both switches, then the ping to interface VLAN 10 would be returned to one of the other configured SVIs on the source switch.

I hope that makes sense.

Thanks.

I have this problem too.
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thotsaphon Fri, 10/30/2009 - 09:23

Patrick,

Switch needs to have a source IP address to do so. They both have connected interfaces that we call Vlan interfaces. You then have no any route to connect the network between 2 switchs in this case. When you try to ping from one switch(no vlan 10 interface) to the other switches, it has no idea to handle this packet. Why? Firstly you have no any route to get there. Secondary you have no int vlan 10 on the first switch to do arp fuctions to get there.

Hopes I help you some.

Toshi

glen.grant Fri, 10/30/2009 - 09:26

Where are you trying to ping from ? Are you consoled into the switch or do you have a pc plugged into a port in the same vlan . If you have a pc plugged into the same vlan with an ip address and default gateway on your nic you should be able to ping the other switch. If you are consoled into a switch with no layer 3 SVI on it then no you will not be able to ping from the switch because it has no ip address or gateway on it .

pweinhold Fri, 10/30/2009 - 12:47

The switch I'm pinging from has a L3 SVI, just not on the same subnet as the destination switch. For example, the source switch has the following configured:

!

ip routing

!

interface vlan 11

ip address 11.0.0.1 255.255.255.0

!

vlan 11

state active

vlan 10

state active

!

Meanwhile, the destination switch has the following configured:

!

ip routing

!

int vlan 10

ip address 10.0.0.1 255.255.255.0

!

vlan 10

state active

!

vlan 11

state active

!

So shouldn't I be able to ping from 11.0.0.1 to 10.0.0.1?

Thanks.

thotsaphon Fri, 10/30/2009 - 12:51

Patrick,

From the switch itself : No

Let me be the SW1. How can I know about the routing for vlan 10?

HTH,

Toshi

glen.grant Fri, 10/30/2009 - 15:16

Where is your l3 routing being done for those 2 address spaces ? What switches are they ? Is routing even turned on for those switches ?

iyde Sun, 11/01/2009 - 08:37

Hi.

Switch 1 has an IP in VLAN 10 and switch 2 has an IP in VLAN 11. Unless you have another device which has an IP address in both VLAN10 and VLAN 11 and does routing between those interfaces, then nothing makes the two VLAN able to communicate with each other.

Try an IP address on both VLAN 10 and VLAN 11 on one of the switches. Then your connectivity should be there.

HTH, Ingolf

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