Chapter 23. Basic Ip Routing; Ip Routing Benefits; Routing Between Ip Subnets - Lenovo Flex System Fabric CN4093 Application Manual

Chapter 23. Basic IP Routing

IP Routing Benefits

Routing Between IP Subnets

© Copyright Lenovo 2017
This chapter provides configuration background and examples for using the
CN4093 10 Gb Converged Scalable Switch (CN4093) to perform IP routing
functions. The following topics are addressed in this chapter:
 "IP Routing Benefits" on page 393
 "Routing Between IP Subnets" on page 393
 "Subnet Routing Example" on page 395
 "Dynamic Host Configuration Protocol" on page 401
The CN4093 uses a combination of configurable IP switch interfaces and IP routing
options. The switch IP routing capabilities provide the following benefits:
 Connects the server IP subnets to the rest of the backbone network.
 Provides the ability to route IP traffic between multiple Virtual Local Area
Networks (VLANs) configured on the switch.
The physical layout of most corporate networks has evolved over time. Classic
hub/router topologies have given way to faster switched topologies, particularly
now that switches are increasingly intelligent. The CN4093 is intelligent and fast
enough to perform routing functions on par with wire‐speed Layer 2 switching.
The combination of faster routing and switching in a single device provides
another service—it allows you to build versatile topologies that account for legacy
configurations.
Consider an example in which 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 con‐
siderably.
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 CN4093s 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.
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