Scott: This makes for more difficult troubleshooting when trying to map a specific IP address in a packet to a particular physical segment.

Bill: Seems like instead of the classic "bait and switch" technique, we instead have a case of "switch and bait."

Scott: We knew it all along: A switch is installed with the "bait" waiting for the poor sucker to buy into a "better" technology, such as:

Bill: ATM (trying hard), Layer 3 switching (up and coming), or a move back to routers (high-performance versions of the proven classic).

Scott: Let's examine our client's situation a bit more.

Bill: We noted that for stations within the same subnet, there was no problem, since stations would first use the Address Resolution Protocol (ARP) for the destination.

Scott: Nor was it a problem for stations located on subnets on the "other side" of the router.

Bill: So we needed to specifically address the needs of IP nodes that had to communicate with other IP nodes on the switch or backbone with a different subnet.

Scott: There are several potential solutions and trade-offs to this problem, a few of which we'll consider here.

Bill: We could have rearchitected the entire address space, changing the subnet mask to accommodate all the stations on the flattened subnet by having more hosts and fewer subnets.

Scott: This idea was rejected because we still needed some segments to have small populations.

Bill: A no ther idea was to always have a station ARP first, even if the target IP address isn't on the same subnet. Now this would've been a great solution!

Scott: Unfortunately, not all the employed operating systems and protocol stacks would allow us to do this. How about static routes in the local workstation's route table?

Bill: Again, a logistical and maintenance nightmare. How about using another router between the switch and backbone?

Scott: What? Back to routing!

Bill: Yes. Believe it or not, this turned out to be the best short-term solution to a problem turned critical.

Scott: The local routing chore was redistributed to another router and the backbone was relieved of doubled traffic.

Bill: Although no IP addresses or subnet masks had to be changed and thing s worked better, it was unfortunate that switching benefits were lost for cross-subnet communications.

Scott: Long term, if our investment in switching was to pay off, we would need to configure stations to ARP to find a station first, and if it fails, send the packet to the default router.

Bill: Good thinking. By ARPing for a station first, we would know the Media Access Control (MAC) address of our destination and be able to send the packet directly to it, without having to go through the router!

Scott: If we did not receive a response to the ARP, then we would know that we needed to go to the default router in order to be routed to the destination.

Bill: In mom-and-pop networks, plug-and-play solutions seem to work well, but in a corporate network, solutions must be carefully architected into the existing network.

Scott: Stack developers also need to accommodate ARP and default router options for the corporate environment or switching benefits will continue to be undermined.

Bill: Meanwhile, "You can pay me now or..."

Bill and Scott can be reached at otw@pmg.com. Selected trace files related to some columns can be found at www.pmg.com.