Mac address of serial interface of router

[mahesh18]

Hi all,

Here is exam question

Pc is conected to switch ,and switch is connected to router r1.Router r1 has serail connection to ISP router.

We are accessing remote server.when frame leaves router R1 serial interface will it carry any mac address or not.

many thanks

mahesh

[Roberto Salazar · Accepted solution]

Serial interfaces does not have mac addresses, so the answer would no mac-address.

[mahesh18]

Hi bosalaza,

thanks for reply

can you explain me please how packet

will move from one serial int of router to another serial interface with out

MAc

Thanks

mahesh

[badalam_nt · Accepted solution]

MAC address is used only on Ethernet interfaces, not on serial ones.

For your specific case, probably you have used HDLC on your serial interface.

The router just fill in that HDLC frame with the necessary header info, encapsulates the IP packet and passes the frame then on its serial interface.

The router does not need to know any info about the destination L2 address, so the router just sends the frame out the corresponding serial interface.

[Roberto Salazar]

It will depend on what encapsulation the serial interface is using, for example on frame relay, you will map an ip for example to a dlci.

[badalam_nt]

Yes of course, that is true, for Frame Relay you need this mapping IP@ to DLCI.

As the question was not pointing to a frame relay, I assumed it is the default encapsulation (HDLC) and my comment was around HDLC only, not generalizing for all L2 types.

But it is maybe better you pointed this out, it will exclude any possible confusion.

[Richard Burts · Accepted solution]

Mahesh

To helpfully understand this question and its answer we should start by reviewing a few fundamental concepts about layer 3 addressing and about layer 2 addressing.

The layer 3 addressing is IP addressing. And a fundamental concept is that as a packet is forwarded from the source to the destination that the IP addresses (the source address and the destination address) remain the same all the way through. So all the way, over all the different segments that it may travel it will have the same source IP address and the same destination IP address.

The layer 2 addressing depends on the individual segment. If it is Ethernet then the layer 2 address is a MAC. And if the layer 2 is Frame Relay then the layer 2 address is the DLCI. And the fundamental concept is that as a packet is forwarded through the network the layer 2 address is unique to the local segment and will change as the packet is forwarded over different segments. So as a packet is forwarded over a segment its layer 2 source address represents the interface that sends it onto that segment and its layer 2 destination address represents the interface that is its next hop toward the destination.

So for example if a packet is received by a router on an Ethernet interface and is forwarded out another Ethenet interface, then the packet had a source MAC address when recieved on one Ethernet interface and it has a different source MAC when it is forwarded out the next interface.

And if a packet is received by a router on an Ethernet interface it will have source and destination MAC addresses. And when it is forwarded out the outbound interface it will have layer 2 addresses consistent with the outbound interface. If the outbound interface is Frame Relay then the layer 2 address is the DLCI and there is no MAC address. And if the outbound interface is HDLC or PPP it will have the layer 2 addresses of those protocols and there is no MAC address.

HTH

Rick

[mahesh18]

Hi Rburts

thanks for great reply

mahesh

[Mj LUITEL]

Hi Richard,

Last line from you post  - " And if the outbound interface is HDLC or PPP it will have the layer 2 addresses of those protocols and there is no MAC address."

isn't layer 2 address MAC address?   What layer is mac address? 

I want to know, in HLDC address header, for the address field, does it use IP address (L3 address) or MAC (layer 2 address)?

Thank you,

mj

[Richard Burts]

mj

Let me try to clarify. Certainly a mac address is a layer 2 address. But there are layer 2 addresses that are not mac addresses. A mac address is the layer 2 type of address used by Ethernet. But for example serial interfaces (which are not Ethernet) have layer 2 addresses such as the DLCI used for Frame Relay, or the HDLC or PPP address used by those protocols. HDLC and PPP do not use IP addresses as the layer 2 address. Since HDLC and PPP are for point to point connections their addressing is more simple than the mac address used for Ethernet. The addressing used for HDLC was derived from the polling addressing developed for IBM SDLC protocol, from which HDLC was derived . That is the kind of thing I was talking about.

[Mj LUITEL]

Hi Richard,

Thank you for the explanation. I understand it now.

Mj

[mahesh18]

Hi Bosalaza,

Many thanks for your reply

mahesh