Mac Address of serial interface

Endorsed Question
Dec 5th, 2007
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Hi.


Why does a router serial interface don't have a mac address?


Thanks


Cisco Endorsed by Claudiu Cismaru
Kevin Dorrell about 9 years 7 months ago

Let's take a step backwards and ask why a MAC address is not relevant on a 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. Usually. Or does it?


If it is a straight point-to-point connection like HDLC, there is no need for a MAC address 'cos 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.


There are other instance of (theoretically) multi-access serial interfaces, such as SDLC, but that is enough for now.


Kevin Dorrell

Luxembourg


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Jon Marshall Wed, 12/05/2007 - 03:45
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Hi


Mac-addresses are relevant on ethernet interfaces. Serial interfaces are not ethernet interfaces so they don't have one.


Jon

lgijssel Wed, 12/05/2007 - 03:46
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Having a mac adress is only applicable to ethernet interfaces.

Serial interfaces use different protocols and a mac adress is not needed there.


regards,

Leo

Kevin Dorrell Wed, 12/05/2007 - 04:22
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Let's take a step backwards and ask why a MAC address is not relevant on a 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. Usually. Or does it?


If it is a straight point-to-point connection like HDLC, there is no need for a MAC address 'cos 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.


There are other instance of (theoretically) multi-access serial interfaces, such as SDLC, but that is enough for now.


Kevin Dorrell

Luxembourg


bvsnarayana03 Wed, 12/05/2007 - 04:49
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Nice explanation kevin. full points.

afath@live.in Sat, 06/09/2012 - 01:57
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Then what could be content in the data link frame header of EIGRP message format for serial link .

reiman_ph Wed, 09/05/2012 - 11:45
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It doesn't matter. It would depend on what layer 2 technology the EIGRP protocol  is running over.

apratiteZ0r0 Mon, 09/09/2013 - 12:54
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MAC address is assigned to Network adapter, NIC cards, to uniquely identify them in network, in serial interface no NIC cards are required, becoz it is a communication b/w only two devices, eg Router console to PC RS232, or router to router,

hence no mac address is required in serial interfaces. Note that both serial ports and ethernet ports both transfer the bits in serial fashion(1 bit at a time), Unlike parallel fashion(8/16..bits at a time) as in scsi cables. so it does not define the type of transmission, but the type of communication(one to one, or one with many).

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