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Home router usually have two interfaces. But they have several ports on the back. Is this different from a canonical router? Do they attach a swrich to consumer routers?

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There are many brands of routers. Can you supply the manufacturer and model # –  James A Mohler Dec 2 '12 at 7:27
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closed as off topic by MadHatter, Iain Dec 2 '12 at 8:16


2 Answers

Typical home routers are often routers, switches, access points, DHCP servers, DNS proxies, firewalls, web servers, file servers, and probably quite a few other things too.

In the most common internal arrangement, all the Ethernet ports are connected to an internal switch. An additional internal port on the switch connects to the CPU's Ethernet interface. The switch is a smart switch with VLAN support, allowing the LAN ports to be separated from the WAN/Internet port (if the router has one).

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You can take a logical view of the home router or a physical view. The picture would be different. On the physical side home routers are often implemented using a System-on-chip (SoC) design. For example a Broadcom BCM95352E that is found in Linksys WRT54G v4.0 and v5.0 is a processor that implements MIPS instruction set, FastEthernet switching and routing logic in one chip.

Logically as David Schwartz explains a Linksys home router is organized as a switch and a router. The CPU is the router it has three interfaces: eth0 connects to the internal switch, eth1 looks into the WAN and eth2 points to the wireless LAN interface. Here is a link to a sketch of the internal organization of a WRT54G v2.0 with a discussion.

On other home routers all physical ports are connected to the switch and the router CPU has a logical port on that same switch. The physical ports can be attached to different VLANs to separate the home LAN and the WAN. For example here is an OpenWRT switch configuration for Broadcom SoC from OpenWRT documentation.

  config switch       "eth0"
      option vlan0    "1 2 3 5*"
      option vlan1    "0 5"
      option vlan2    "4 5"

Here the switch is connected to CPU port eth0. The switch has 5 physical ports 0-4 and a logical port 5 connected to the CPU port. The ports 1-3 on vlan0 are the LAN ports. Two separate VLANs on ports 0 and 4 may be reserved for DMZ and WAN depending on the router configuration.

Also check out the Default configuration diagram for WRT54G v2.0 in DD-WRT documentation.

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