Cat6K and Server failover

[tvanderwerken]

How does the C6K handle an IP address moving from one port to another, i.e. Server1 fail's, server2 assumes server1's IP address, but has a different mac address. How does the switch decide where to send the packets too? And what if only the workload failsover from one server to another, but the NIC card from server1 stays online? How does that work????

[glen.grant]

I believe it will not fail over until the mac address if flushed from the tables , I believe the default is 5 minutes . I believe it can be made shorter if need be .

[tvanderwerken]

So even if the new server #2 comes up with the same MAC/IP address as server #1, and starts sending packets out will that not update the table?

[csthomas]

The packets received by the switch have a mac address. That's where the switch sends them. It doesn't know or care about IP addresses. In order to get the packets sent to the new mac address, either the sender has to re-arp the IP (same vlan) or the router has to re-arp (cross vlan). Router arp table time outs are typically quite long (20 min is typical). You can manually flush the router's arp cache, or set its timeout time down, but the latter can cause a lot of overhead.

[tvanderwerken]

I understand that. But my questions is what happens when the same MAC address appears on 2 different ports on the same switch or same VLAN? I guess this is almost a question of ARP caching on the router. The ARP table timeout is default to 4hrs, that I know. So what besides an ARP will populate the ARP cache? Is the new server coming up and sending packets out enough to change the ARP cache in the router? I'm thinking no.

[bsivasub]

The switch will not learn the same mac-address on two ports. The switch will immediatedly learn the new port for the mac-address. It will stop forwarding on the old port. The default timer for arp is 4 hrs. The router doesn't need to re-arp as long as the ip/mac combo is same.

Your initial question seems to different from the latest statement.

To answer your initial question, the old mac address would be aged out after 5 minutes, the router would still use the old mac address for the arp but fortunately the new server would arp for router and then the new mac info would get updated on the router as well.

So basically you should not experience major problems. let me know if you have additional questions

[michael_tong]

If you are using some servers' clustering feature from OS vander and NIC vander, there is not any concern because they did everything for you. 2nd server will send out ARP broadcast to clear the cache in the networking device.

If you want to load balancing two servers, you should pay attention on the IOS load balancing feature (it provided in C6K + MSFC2 + PFC).

Please also pay attention on MLS when you use C6K + MSFC2. Since the MSFC2 is not using the routing table as the 1st choice to find the path, Sometimes, you have to clear the routing table for clear the cache in the MLS table if your topology changed.