Routers Operate in a Mesh –Many possible alternative routes between two stations Only One of Many Possible Alternative Routes Packet

Routers Benefits of Alternative Routes –Can route around a router or trunk line failure –Can route around congestion –Can select an optimal route based on cost, latency, security, or other goal In Contrast –Ethernet deals poorly with failures –ATM can change virtual circuits in case of failure, but this takes time

Routing is Complex For each packet, router must consider all possible alternative routes As noted earlier, it must do complex calculations on each router forwarding table row This complexity requires a complex and expensive router for relatively low packet forwarding rates ?

Switches Versus Routers Switches Fast Inexpensive No benefits of alternative routing Routers Slow Expensive benefits of alternative routing “Switch where you can; route where you must”

Early Site Networks Organization –LANs (subnets) based on hubs –Routers link hubs –Hierarchy of Routers Router Hub

The Switching Revolution Switches Push Routers to the Edge –Switches replace most routers in site networks –Because switches are cheaper than routers External Switch

The Switching Revolution Switches Push Routers to the Edge –Router still needed at the edge of the site network to communicate with outside world because routers handle expensive long-distance links very well External Switch

The Switching Revolution Layer 3 Switches –Traditional switches operate at Layer 2; Switch based on MAC addresses –Layer 3 switches switch based on internet layer IP addresses External Layer 3 Switch

The Switching Revolution Layer 3 Switches –Layer 3 switches are replacing many Layer 2 switches in site networks because of their ability to switch based on IP addresses External Layer 3 Switch

The Switching Revolution Layer 3 Switches versus Routers –Layer 3 switches are much faster than routers –Layer 3 switches cost less than routers External Layer 3 Switch

The Switching Revolution Layer 3 Switches versus Routers –At the internetworking layer, Layer 3 switches normally only support IP and sometimes IPX; Routers route many more internet layer protocols, including those of AppleTalk, SNA, and others –At the data link layer, Layer 3 switches normally support only Ethernet; Rarely ATM

The Switching Revolution Layer 3 Switches versus Routers –Layer 3 switches rarely support Layer 2 WAN protocols –Routers usually are still needed at the edge of the site network, to communicate with external links External Layer 3 Switch

The Switching Revolution Routers –Forward based on IP addresses and other internet layer addresses –Expensive and slow –Handle multiple internet layer protocols –Handle multiple LAN and WAN subnet protocols Layer 3 Switches –Forward based on IP addresses, sometimes IPX addresses –Inexpensive and Fast –Do not handle multiple internet layer protocols –Do not handle multiple LAN and WAN subnet protocols

The Switching Revolution Layer 4 Switches –Examine port fields in TCP and UDP –These fields describe the application –Therefore, can switch based on application (to give priority by application, etc.) Layer 4 Switch