So this is a really basic question but to be honest I do not come to a conclusion (shame on me).

Given is the following scenario:

Scenario Setup

The router is hooked up with a 100 Mbit/s link to a switch. The switch has three interfaces (1x 100 Mbit/s and 2x 10 Gbit/s). There is one PC hooked up to the switch with a 10 Gbit/s link. There is one server hooked up to the switch with a 10 Gbit/s link. There are three networks (network for router and switch, servers for the server and computers for the PC). The router is configured to route the computers network to the servers network.

There are two possible results:

  1. The bandwidth from PC to server is 50 Mbit/s due to the traffic have to be routed using the only 100 Mbit/s link from switch to router (uplink and downlink sharing the same link).
  2. The bandwidth from PC to server is 10 GBit/s due to the switch asks the router (default route) where to direct the traffic.

In my mind I tend to the first one, but some colleague and I are unsure.

I also can think of some sort of dynamic routing within the switch (as in learning like MAC-to-port assignments) but this is not my specialty. When this is the meaning of dynamic routing what does the number of dynamic routes refer to (single IP addresses or (sub)networks)?

  • 1
    When the PC and server are on two different subnets, the router needs to route between them and therefore the speed will be capped to the 100 Mbit link. If the PC and server were addressed within the same private subnet they could communicate directly with each other at 10 Gbit's and 100 Mbit's to the internet, if the router performed NAT to the public adresses. – user36472 Oct 27 '17 at 8:32

The traffic will go through the router unless your switch is a Layer 3 switch, and you configure it to make a direct routing from PC to Server. However to achieve that you will need to put the PC and the Server in different VLANs.

Keep in mind this to understand the situation:

  • Every device in an ethernet network has a MAC address.
  • MAC addresses are identifiers for the local environment.
  • Every device in an IP network must have and IP address.
  • Every device in an IP network should have defined an default gateway.
  • The default gateway is the IP address of the device that connects to other networks (usually a router).
  • The lack of a default gateway isolates that device of other networks.

So, each time the PC needs to communicate to the server, it will send the IP packet to the default gateway MAC address.

If the switch is a layer 2 switch it will check the destination MAC address and redirect the packet to the port where that MAC resides.

If the switch is a layer 3 switch it can check the MAC but also the destination IP address and take the decision to route the packet directly to the server.

  • 1
    @ToddWilcox Corrected!!!! – jcbermu Oct 27 '17 at 12:00

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