Layer2 & 3

[netbeginner]

Hello Friends,

Question regarding communications offrames/packets from Layer 2 to Layer 3.

Which protocol is used while handing over the frames from LAYER2 to LAYER3. As know very bit about this that there is some Ethernet Header with frames which are moving or handing over to Layer3 from Layer2 , is this true for this typr of communication.

[gatlin007]

I believe it's more a question of what header information is evaluated in order for a network device to make a forwarding decision.

Let's use the following Ethernet topology as an example:

Server Farm ---- L2-Switch01 ---- L3-Router ---- L2-Switch02 ---- LAN Host

For the LAN Host to communicate with a Server it will ARP for the MAC address of it's default gateway.  For this example we'll assume the default gateway is the L3-Router.  When a frame is forwarded out of the LAN host destined to a server it will posses an IP header and an Ethernet header.  The Ethernet header destination MAC address will be the MAC address of the L3-Router interface(default gateway)  The destination IP address will be the Server IP address.

L2-Switch02 will only evaluate the Ethernet header in order to forward the frame to the L3-Router.  L2-Switch02 will forward this frame to the L3-Router based on an entry in it's MAC address table; in most cases it will have learned this dynamically by 'noticing' the frames coming in with a source MAC address.

When L3-Router receives the frame it will strip off the Ethernet header and evaluate the IP header for a forwarding decision.  It will do a route look-up (actually a CEF table look-up but that's another story) and determine which interface to forward it out of.  L3-Router will install a new Ethernet header with a destination MAC address of the server and a source MAC address of the L3-Router interface facing the Server.


Chris