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"Because the exit interface is not an Ethernet network, R2 does not have to resolve the next-hop IPv4 address with a destination MAC address." - this is routing from serial port , can anyone explain this to me please :D

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MAC addresses are addresses for some (IEEE LAN) layer-2 protocols. Other layer-2 protocols use other addressing, or no addressing at all.

If you are using frame relay on your serial port, then you are using a DLCI (Data Link Connection Identifier) instead of a MAC address. Something like PPP only has two possible endpoints, so any frame sent will be received by the intended receiver, hence PPP doesn't really have addressing. HDLC is proprietary to each vendor, but more than likely, there is no real addressing. These are common layer-2 protocols used on serial interfaces.

Of the IEEE LAN protocols, some use 48-bit MAC addresses, and some use 64-bit MAC addresses.

Routers strip off the layer-2 frame, which may contain the MAC addressing, depending on the protocol used. The router will then route the layer-3 packet based on the layer-3 address, and it will build a new layer-2 frame for the layer-2 protocol on the interface used to forward the packet. It is common that the local LAN will use MAC addresses, and the WAN will not. The router will strip off the LAN frame, and it will create a frame for the outbound protocol that does not use MAC addressing.

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Ethernet (and other MAC-based network protocols) require a destination MAC in the frame header in order to reach the next hop router (or the final destination). These protocols use a point-to-multipoint scheme and require an address to find their way. The MAC address is usually determined by using some kind of IP-to-MAC resolution mechanism, like ARP for IPv4 over Ethernet.

Other interface including serial ports use a point-to-point scheme and just connect two points. There's no need for any interface-specific addressing because anything sent on one side is received on the other.

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