Handbook

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In this example, a corporate campus has migrated from a router-centric topology to a faster, more
powerful, switch-based topology. As is often the case, the legacy of network growth and redesign has
left the system with a mix of illogically distributed subnets.
This is a situation that switching alone cannot cure. Instead, the router is flooded with cross-subnet
communication. This compromises efficiency in two ways:
Routers can be slower than switches. The cross-subnet side trip from the switch to the router and
back again adds two hops for the data, slowing throughput considerably.
Traffic to the router increases, increasing congestion.
Even if every end-station could be moved to better logical subnets (a daunting task), competition for
access to common server pools on different subnets still burdens the routers.
This problem is solved by using the switches with built-in IP routing capabilities. Cross-subnet LAN traffic
can now be routed within the switches with wire speed Layer 2 switching performance. This not only
eases the load on the router but saves the network administrators from reconfiguring each and every
end-station with new IP addresses.
Take a closer look at the switch in the following configuration example:
Figure 15 Switch-based routing topology
The switch connects the Gigabit Ethernet and Fast Ethernet trunks from various switched subnets
throughout one building. Common servers are placed on another subnet attached to the switch. Primary
and backup routers are attached to the switch on yet another subnet.
Without Layer 3 IP routing on the switch, cross-subnet communication is relayed to the default gateway
(in this case, the router) for the next level of routing intelligence. The router fills in the necessary address
information and sends the data back to the switch, which then relays the packet to the proper
destination subnet using Layer 2 switching.
With Layer 3 IP routing in place on the switch, routing between different IP subnets can be accomplished
entirely within the switch. This leaves the routers free to handle inbound and outbound traffic for this
group of subnets.
To make implementation even easier, UDP Jumbo frame traffic is automatically fragmented to regular
Ethernet frame sizes when routing to non-Jumbo frame VLANS or subnets. This automatic frame
conversion allows servers to communicate using Jumbo frames, all transparently to the user.
Switch
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