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what is the typical purpose of 3 LAN ports on a router ? What kind of usage require more than 3 ports ?

Are LAN ports on routers similar to the capability of a managed switch ?

  • Routers route packets between networks, and routers can have many interfaces, each connecting a different network. If you have three or more different LANs then you need a router to route packets between those LANs. Switches switch frames on the same LAN, but cannot route packets between LANs. – Ron Maupin May 24 '18 at 14:12
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A router with "LAN ports" (and probably one "WAN port") sounds like a router designed for a specific use case: providing NATted internet access to multiple devices over a connection with one public IP address. These are often consumer or SOHO products.

In this case, the LAN ports will most likely behave as an unmanaged switch. There are of course exceptions. Please be specific if you have a particular device in mind. (Note: consumer grade products are explicitly off-topic here)

A more generic router would just have interfaces, which can be configured in many ways and connected to multiple LANs, WANs and other networks.

  • I am trying to understand the use case of some SMB network&security owner which are buying industrial routers with system boards such as the PC Engines APU, often coming with OPNsense or pfSense software, APU are dedicated system boards for networking with 3 or more GbE ports, i am wondering if these routers can be replaced by a combo of a switch (unmanaged or managed) + one board with small ports count (2 at least i presume), what is the pro and cons of these over the all-in-one solution. – Onirom May 24 '18 at 11:38
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Simple. To connect 3 wired devices. Like if you want to connect three PCs via an ethernet cable to your router, you'd use those ports.

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Multiple LAN ports on a router can mean two different things:

  1. the router integrates a switch, enabling you to connect a small number of local devices directly to the router, without using an external switch - this is very common for consumer-grade devices, off-topic here
  2. the router can use the port separately and has the ability to physically connect multiple zones from your network - this is essential for non-trivial setups; usually the "LAN" ports can also be used for WAN connectivity, DMZ, or similar, depending on configuration
  • SMB/SOHO devices, too. Like a Cisco 1811, etc. (and there are NM/NME/etc switch blades) It's a convenient way to consolidate infrastructure. – Ricky Beam May 24 '18 at 19:08
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At my employer's, when we connect spoke sites with dual WAN connectivity (i.e. one Metro Ethernet service by carrier "A", one MPLS based L3 VPN by carrier "B"), we usually deploy routers with at least three ports, to connect...

  • one port to the WAN service
  • one to the LAN switch(es)
  • one as a cross-connect link from (ours, not the carrier's) router A to router B, if at all possible as a direct patch cord

That A-B cross-link becomes part of the dynamic routing environment of the WAN overlay service (mostly MPLS-o-GRE-o-IPsec) we provide.

That design brought us advantages like:

  • the demarcation point between service provider and customer becomes very clear: the LAN port.
  • the customer's LAN remains completely free of unrelated traffic, even in case of WAN failure (like network or device management traffic to/from our own routers) which might be unwanted there.
  • both routers remain fully reachable and manageable when one WAN service or one LAN switch goes down. That helps both the NMS and the operators to get a very clear and quick assessment of the failure situation.
  • in cases of directly attached customer subnets, the customer subnet on the routers' LAN ports will only "see" FHRP PDUs (in our case: HSRPv2) - which, by service design, the customer will have to accept as part of the service. None of "our" dynamic routing protocols will be exposed on the customer's LAN.
  • FHRP on the LAN can be used in it's "pure" form and doesn't have to be abused as poor man's routing protocol.
    • no need to fiddle with interface/route tracking, holddown timers, floating static routes and the likes
    • keeps the configuration tidy, making operations and troubleshooting easier.
    • if the FHRP-active router's WAN link fails, there's no need to failover to the standby. The active router can just forward and receive traffic through the crosslink.

There's a few catches to avoid (like making sure that the crosslink is never used as transit by other sites or even considered as backbone link), but overall, we've seen improvements in resilience and failover times over our previous FHRP-only based designs.

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