InterVLAN Routing

In this blog we are going to take a look at routing between VLANs. When we want communication between different VLANs we’ll need a device that can do routing. We could use an external router but it’s also possible to use a multilayer switch (aka layer 3 switches).

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CEF (Cisco Express Forwarding)

Perhaps you have heard about the term “wirespeed” before. It’s something the marketing department likes to use when it comes to selling networking equipment. It means that packets can be forwarded without any noticeable delay. Oh btw, for the remaining of this lesson the words “multilayer switch” and “router” are the same thing. Everything that I explain about the multilayer switches from now on also applies to routers.Let’s take a look at the difference between layer 2 and multilayer switches from the switch’s perspective:

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Unicast Flooding due to Asymmetric Routing

Asymmetric routing is not uncommon and it doesn’t always cause issues. There are however a number of scenarios where it could cause problems. For example:

  • Traffic that is translated by a NAT router should also use the same router for return traffic. Otherwise there’s no way to translate the packets back to their original IP address.
  • Firewalls keep track of the state of connections. Traffic should leave your network through the firewall so that return traffic is able to get back in. Otherwise it will be dropped.
  • Unicast flooding can occur when a switch doesn’t know the destination MAC address.

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