Saturday, June 9, 2012

mac address for serial interface !!

1. Why mac address is not assigned to serial interface?
MAC is Media Access Control.  A MAC address is relevant in Multi-Access networks like Ethernet where you have several hosts on the same network.  In such a network, you have to do an ARP broadcast to find the MAC address of the host, so you can send packets address to it and it only.
 A serial line has only two ends, so it is fairly obvious where to send the packets.  That is why a serial line doesn't have a MAC address.
 If it is a straight point-to-point connection like HDLC, there is no need for a MAC address because you know to send the packets to the guy at the other end of the link.
 But consider a multi-access serial link like Frame Relay.  Does that have a MAC address?  Well yes, it does, sort of ... it is called the DLCI, or channel number.  That is why in Frame Relay, you either have to have a static ARP mapping, or rely on frame-relay inverse-arp, or tie the (sub-)interface to a single MAC address with frame-relay interface-dlci.