Giuseppe, thanks for your response.

Your solution should work if the server is simply connected to a physical switchport running on a single VLAN. What if the connection is a trunk, server is hosting multiple services running on different IPs across multiple VLANs? I cannot simply put a swithport to a desired VLAN. A real world example is a load balancer device which has VIPs on different VLANs.

Hello Jack,

the same reasoning can be used ;

the different vlans will be on a l2 trunk or distributed across multiple NICs simply the procedure needs to be performed for each ip address

at the step clear ip arp x.x.x.x

you need to do it for each ip address in each involved vlan

notice that multiple ip addresses can be associated to the same MAC address in different vlans but you need to clear all entries to avoid out of service when doing the switchover.

Hope to help

Giuseppe

Hi Giuseppe.

I was following the discussion between you and Jack which i found quite interesting.

I have few questions based on Jack's statement,quoted below:

"What if the connection is a trunk, server is hosting multiple services running on different IPs across multiple VLANs?"

My understanding is ip address is associated with physical nic of server. But jack mentioned his server is running different applications and each using different ip addresses. How is it possible?

Thanks and have a nice weekend.

Hello Sarah,

>> My understanding is ip address is associated with physical nic of server. But jack mentioned his server is running different applications and each using different ip addresses. How is it possible?

this is possible and common in our networks the vlan-id is used to multiplex and de-multiplex.

think of a router on a stick with vlan subinterfaces if you look at the cam table from the switch point of view you can see the same MAC address associated with multiple vlans on the same port.

You could use this to detect if the port is a trunk or there is an hub connected to it: if you see multiple mac addresses in the same vlan there is an hub or a non cisco switch: if there are multiple mac addresses in different vlans the port is a trunk.

A CAM entry is made of :

vlan-id, port-id, and MAC address

the ARP table is a different object and that is the place where each ip address is associated with a MAC address but this is not a one-to-one association:

that is to a specific MAC address multiple ip addresses in different vlans can be associated without any problem.

The same happens on a C6500: all SVI interfaces that are the L3 logical interfaces for each vlan shares the same source MAC address and this not a problem.

Modern servers can use also virtual mac addresses to act as a cluster and this makes the real scenario a little more complex.

To give you an idea in our customer network server people is installing servers with up to 14 NICs and with some of them used as trunk ports.

Hope to help

Giuseppe