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Just before we get to multilayer switching a couple of comments on router behavior when routing classfully versus classlessly.
- In classless routing, if there’s a default route and there’s no specific prefix to match up, then the default route will be used to forward the packet.
- In classful routing, if there’s a default route and there’s no A, B or C network for the destination IP address, then the default route gets used.
- In classful routing, if some part of the classful network is in the routing table, but there’s no specific subnet match for the packet at hand, the packet get sent to the bit bucket.
- Use “ip classless” to enable classless routing.
Multilayer switching is referring to a device that can switch a frame not only because of layer 2 information, but also information from the other layers as well. Layer 3 switching is just a fancy way to say “routing”, although implicitly it’s routing at a very high forwarding rate.
On a Cisco switch performing MLS, you have both layer 2 and layer 3 interfaces on the same box. You define switchports to be a member of a VLAN. To add the routing component, you build a layer 3 VLAN interface that is a logical (not physical) interface. This VLAN interface is termed an “SVI” or switched virtual interface. Switch and router all in one box. Spiffy. It’s worth noting that in order for this magical 2-in-1 box to forward a packet, it’s got to know one additional detail mere routers don’t need: they need to know also what physical interface they are going to forward the packet on, and consequently will hit the bridging table to figure it out.
On layer 3 switches, it’s possible to configure a physical port to be a true router port, not a VLAN port. The interface then is NOT a switchport, it’s not in a VLAN, there’s no switching information stored about the interface, and then you don’t need that extra step of hitting the bridging table to know how to forward.
You can make an etherchannel (portchannel) be a routed interface. You use a “no switchport” command, just like any other. And it’s recommended that you set the load-balancing method for the portchannel to be layer 3 addresses instead of layer 2, since most of the traffic headed across the etherchannel will be back and forth to the same MAC addresses of the routers on either end of the link.
Ethan Banks writes & podcasts about IT, new media, and personal tech.
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