Armed and Dangerous Three generally accepted router deployment strategies exist, and a couple others are waiting in the wings. The traditional, and predominant, approach is to connect individual subnets to discrete physical router ports. We'll refer to routers configured in this manner as being "multiarmed." As we will see, multiarmed routers suffer from a number of shortcomings relative to the other approaches, and they offer network administrators no compelling advantages.

A second, more elegant approach, dubbed by the industry as the "one-armed router" (OAR) or "router on a stick," is installing a single physical interface in the router and provisioning it with multiple "virtual interfaces."

Third, Multiprotocol Over ATM (MPOA), a recently approved ATM Fo rum standard, uses a route server in conjunction with enhanced Layer 2 switches to bypass routers entirely for long-lived intersubnet traffic flows.

An OAR typically is provisioned with one or two physical interfaces. These physical interfaces multiplex traffic from multiple subnets by means of software-defined virtual interfaces. In the case of ATM, you would configure a LAN Emulation Client (LEC) on each virtual interface. The LEC is then configured to join a given subnet's Emulated LAN (ELAN).

Because the cost of the physical interface can be amortized over more subnets than traditional router implementations, the per-subnet cost for router interfaces is reduced. Given the relatively high cost of router ports, your budget will be trimmed considerably with the one-armed approach.

On our campus, subnet ingress and egress traffic ranges from 600 packets per second (pps) average for busy subnets to 10 pps for others. Given our traffic levels, we've been able to accommodate up to 80 subnets on a single router interface while maintaining about a 20 percent to 30 percent CPU load on the router, and an .08 percent packe t-drop rate. These 80 subnets are connected to the ATM backbone through $300 to $500 switched Ethernet ports instead of $2,000 to $4,000 router ports.

You'll be able to implement an OAR with 100/1,000-Mbps Ethernet once the 802.1Q VLAN standard is finalized in early 1998 and vendors are able to provide stable implementations. However, 802.1Q will not use addressing or an end-to-end signaling protocol, so trunks will have to be provisioned on a hop-by-hop basis in a manner similar to ATM permanent virtual circuits (PVCs).