Val:~$ whoami

I am Val Glinskiy, network engineer specializing in data center networks. TIME magazine selected me as Person of the Year in 2006.

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Saturday, April 20, 2013

PACL and MAC address learning

I have unenviable task to drag legacy application to 21st century. I am talking about 80-the legacy and some of its functions do not even use IP protocols. One of the proposed solution included 2 servers with the same
IP and MAC addresses (I know, but splitting network in 2 separate VLANs was not an option) connected to different switches, but in the same VLAN. ClientA and ClientB should be able to talk to each other and server connected to the same switch as client. ServerA and ServerB should not even know about each other's existence, so they won't complain about duplicate IP address. The switches are Cisco 6500s. One of the obvious  solutions is to put Port Access Control List on either side of the inter-switch link. PACL successfully blocked the traffic between servers, but switches still learned MAC address of the blocked traffic source and placed it MAC address table. This would cause MAC address flapping on the switch every time clients send ARP query for server's MAC or when both servers need to send traffic to their clients. Why would switch need to keep MAC address of the discarded traffic? Oh, well. Another network mystery.

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