LAYER 3 SWITCH AND A ROUTER

Is Layer-3 Switch More than a Router?

You might think that layer-3 switches perform bridging and routing, while routers do only routing. That hasn’t been the case at least since Cisco introduced Integrated Routing and Bridging in IOS release 11.2 more than 15 years ago. However, Simon Gordon raised an interesting point in a tweet: “I thought IP L3 switching includes switching within subnet based on IP address, routing is between subnets only.”

Layer-3 switches and routers definitely have to perform some intra-subnet layer-3 functions, but they’re usually not performing any intra-subnet L3 forwarding.

Let’s start with the intra-subnet functions the layer-3 forwarding devices (whether you call them switches or routers) do:

• Dynamic neighbor discovery through ARP/ND for packets sent to hosts in directly attached subnets (glean adjacencies in CEF terminology);

• Generation of host routes based on ARP/ND results (cached adjacencies in CEF terminology);

• Forwarding of IP packet to directly attached IP hosts based on ARP/ND-generated host routes.

However, if a layer-3 forwarding device performs MAC-based forwarding in combination with IP-based forwarding, it usually uses the destination MAC address to figure out which forwarding method to use:

• Layer-2 frames sent to router’s own MAC address are passed up the protocol stack into the IP forwarding code (and if the IP packet is sent to router’s IP address, the packet is sent to the control plane for further processing);

• Layer-2 frames sent to other destination MAC addresses are passed to MAC forwarding code, which performs MAC address table (or TCAM) lookup and forwards, floods or drops the packet.

I haven’t seen a device (yet) that would use IP protocol type (0x0800) in Ethernet header to decide whether to use MAC-based or IP-based forwarding. If you’ve seen one, please write a comment (overzealous DSL concentrators that violate layering by peeking inside PPPoE sessions don’t count).

Summary: There’s no difference in intra-subnet (intra-VLAN) forwarding between a router (layer-3 switch) and a simple bridge (layer-2 switch). However, an IP-aware device (even a more sophisticated layer-2 switch) might support IP-based port access lists or DSCP- or ACL-based QoS.