VLAN interfaces no pinging

[AZKhan]

Hello,

In our setup, Cisco 3750X has been deployed as the Gateway for all access layer switches i.e. 2960G.  Another 4506 is also setup on layer 2 mode. VTP is configured with 3750 as the master switch. All VLANs are properly propagated to all switches and switches are successfully pinging to VLAN interfaces but 4506 is not pinging any VLAN interface on 3750. Only one VLAN interface which is also configured on 4506 is pinging.  What's the issue? how to troubleshoot this problem?

[Seb Rupik · Accepted solution]

Hi there,

This sounds like a routing problem. Esentially the 4506 does not have routes installed for the subnets which are locally attached to the 3750X.

I guess the VLAN that the 4506 can ping is the 3750x SVI on the subnet between the two switches.

Either run an IGP between the two switches or simply add static routes between the two switches.

If the above is unclear, please share the configs from both switches and we can give the details on static routes.

cheers,

Seb.

[AZKhan]

@Seb Rupik , Thanks

I have just added default route towards interface VLAN of 3750 and it worked.

But what's the purpose of default-gateway? as i have first tried default-gateway and assumed it would work but it did'nt. so then i tried the default route.

I am also still unclear about the nature of this switch as why it needs routing as we are using this like any other switch Layer-2 switch. 

[Richard Burts]

We do not have enough details about the configuration to be sure of the issue. The symptoms suggest that ip routing is enabled on the 4506. You might intend to use the switch only as layer 2 but it is possible that ip routing did get enabled. There are several things that might be done to shed light on this issue:

- on 4506 show run | include rout

- on 4506 show ip route

- on 4506 show ip protocol

HTH

Rick

[AZKhan]

@Richard Burts , 

Yes u r right, this switch is enabled with Layer-3 globally that's y ping to other SVI's failed as this switch supposed to look up for route and failed to find one in the routing table. The moment i configured default route towards 3750 which served as gateway for all switches, ping becomes successful. I think to treat this as layer-2 switch, i need to turn-OFF routing globally, then it would forward all the frames towards 3750.

[Richard Burts]

I am glad that we are making progress on understanding this issue. There are several points I would like to make about it.

Sometimes there is confusion about switches being layer 2 or layer 3. A layer 2 switch forwards traffic based only on layer 2 addressing (mac address for an Ethernet frame). A layer 3 switch forwards traffic based on both layer 2 and layer 3 (IP) addressing. Some switches are capable of only layer 2 switching. Most switches (such as your 3750 and 4506) are capable of both layer 2 and layer 3. So we need to be careful to understand correctly whether these switches are operating only as layer 2 or as both layer 2 and layer 3. The key to this is whether ip routing is enabled or not enabled. If you do no ip routing on the 4506 then it will operate as layer 2 switch only. It will forward traffic within its vlans. But any routing of traffic between vlans will need to be done on the 3750.

In one of your posts you asked about default-gateway and default route. I would like to explain this. default-gateway is used on layer 2 switches. A layer 2 switch may be configured with an IP address on one of its vlans. This IP address is used for management purposes on the switch (to allow SSH or telnet, to be able to send syslog messages to a collector, to respond to SNMP, and things like that). The switch would be able to communicate with any other device that was in the same subnet (same vlan) as the management IP by using arp to find the destination address. But what if the destination address was in a different subnet? For that the switch would need a gateway. And default-gateway provides that.

A switch that is configured to operate at layer 3 could have configured a default-gateway, but it is not used. default-gateway is used only by switches operating as layer 2 only. One way to understand this is that a layer 2 only switch is operating like a host device in the IP network. And as such it needs a gateway. But when a switch is configured with ip routing enabled then it begins to operate like a router. And a switch operating at layer 3 will ignore the configured default-gateway and will need a default route.

HTH

Rick

[AZKhan]

@Richard Burts , Thanks for ur detail answer.

Is there any command to turn-OFF the routing globally on 4506e switch?

[Richard Burts]

In global config mode use the command no ip routing

HTH

Rick

[balaji.bandi]

You have mentioned 4506 is only Layer2, i believe there is no SVI, other than Manangement SVI configured.

On the Port-channel or trunk between 3750 and 4506, have you allowed required VLAN ?

Do you configured the port to respected VLAN Access port and device configured and tested with Ping.

we also need more information your port config and your Ping from where to where ? that means what is the IP you pinging , what is the destination IP, where (it located).