Routing and Switching – Communication Between a Switch and Router

routerroutingswitchswitching

When a computer, Computer A, wants to communicate with Computer B, it (Computer A) will use MAC and IP addresses. It will send the packet of information to Switch, which, based upon the MAC destination address contained in the packet of information, will look on its lookup table if the computer that owns the mentioned MAC Destination Address is available on the LAN network, right?

If it is unable to locate the computer on the LAN (suppose here that the lookup table is full), will it send it to Router, or does Computer A decide to send directly to the MAC address of the Network Interface Card of Router (the link with the WAN or the Internet for my LAN)?

I am a little bit confused related to the way that switches communicate with routers. When and how is this done?

Best Answer

A host will first determine if the destination IP address is on the same network. If not, it will send the IP packet to its configured gateway.

A switch doesn't decide this, and, to a switch, a router is just another host. The MAC address to which a frame is delivered is determined by the host before the host encapsulates the packet in a frame.

Related Solutions

Switch Lookup Table

It remembers the MAC of the computer attached to that port.

if you have four computers connected to a switch:

PC1 with MAC1 connected to PORT1
PC2 with MAC2 connected to PORT2
PC3 with MAC3 connected to PORT3
PC4 with MAC4 connected to PORT4

Imagine you just turned the switch on, and the MAC table is empty.

When you send a packet from PC1 to PC2, the switch doesnt yet know where the PC with MAC2 is (on which port), but it knows that it recieved a packet from MAC1 on PORT1 (and it adds this entry to the MAC table). Since it doesnt know where PC2 is, it sends the packet to all ports (except PORT1, since the packet came from there.

All three other PCs get the packet, PC3 and PC4 ignore it, but PC2 probably replies. Since the switch just recieved a packet from a PC with MAC2 on PORT2, it adds this entry to the MAC table. Since it also has an entry for MAC1 on PORT1, it sends the packet there (and only there).

After such a packet exchange (eg. ping), the state of the MAC table would be:

a device with MAC1 is located on PORT1
a device with MAC2 is located on PORT2

Switch – Understanding ARP and Routers

Your reasoning is the wrong way. TCP/IP handles the layers top down, not bottom up.

PC A will first do a lookup in the routing table and decide that the only way to reach PC B is via the router specified in that routing table.

If the MAC address of the router is unknown it will send an ARP request to figure out which MAC address corresponds to the gateway IP.

Then the frame (containing the IP packet) will be sent to the MAC address of the router which will do a lookup in its routing table to see on which interface PC B is connected or via which next hop it should be routed. Assuming PC B is connected to another interface of the router the router will do a lookup (and if needed an ARP request) to figure out the MAC address of PC B and then forward the frame.

Best Answer

Related Solutions

Switch Lookup Table

Switch – Understanding ARP and Routers

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