Routed Interfaces
Figure 17-5
Using Routed Interfaces for Core and Distribution Layer 3 Links
Implementing Layer 3 EtherChannels
So far, this section has stated that routed interfaces can be used with a single point-to-point
link between pairs of Layer 3 switches, or between a Layer 3 switch and a router. However,
in most designs, the network engineers use at least two links between each pair of distribu-
tion and core switches, as shown in Figure 17-6.
A11
A12
A13
A14
A21
A22
A23
A24
Layer 2 Access: SVIs
D11
D12
D21
Core1
D22
Core2
Layer 2 Access: SVIs
Point-to-Point, Layer 3 Distribution & Core
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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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.
Step 1.
Configure the physical interfaces as follows, in interface configuration mode:
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