Wendell odom

Routed Interfaces
Figure 17-6
Two Links Between Each Distribution and Core Switch
While each individual port in the distribution and core could be treated as a separate routed 
port, it is better to combine each pair of parallel links into a Layer 3 EtherChannel. Without 
using EtherChannel, you can still make each port on each switch in the center of the fig-
ure be a routed port. It works. However, once you enable a routing protocol but don’t use 
EtherChannels, each Layer 3 switch will now learn two IP routes with the same neighboring 
switch as the next hop—one route over one link, another route over the other link.
Using a Layer 3 EtherChannel makes more sense with multiple parallel links between two 
switches. By doing so, each pair of links acts as one Layer 3 link. So, each pair of switches 
has one routing protocol neighbor relationship with the neighbor, and not two. Each switch 
learns one route per destination per pair of links, and not two. IOS then balances the traffic, 
often with better balancing than the balancing that occurs with the use of multiple IP routes 
to the same subnet. Overall, the Layer 3 EtherChannel approach works much better than 
leaving each link as a separate routed port and using Layer 3 balancing.
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17
Chapter 17: IP Routing in the LAN 411
Compared to what you have already learned, configuring a Layer 3 EtherChannel takes only 
a little more work. Chapter 10 already showed you how to configure an EtherChannel. This 
chapter has already shown how to make a port a Layer 3 routed port. Next, you have to 
combine the two ideas by combining both the EtherChannel and routed port configuration. 
The following checklist shows the steps, assuming a static definition .