MAC Address on Cisco Serial Port

Answered Question

Hi All

Just wondering how we are able to get serial ports on a Cisco router to communicate via TCP/IP when there appears to be no MAC address on the serial port of a Cisco Router?

R_ACCESS_1#sh int

FastEthernet0/0 is up, line protocol is up

Hardware is AmdFE, address is 000d.2999.2320 (bia 000d.2999.2320)

Serial0 is up, line protocol is up

Hardware is HD64570

Description: LINK TO MSC2

MTU 1500 bytes, BW 128000 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation X25, loopback not set

Where does the Serial MAC address come from?

Best Regards,

Michael

I have this problem too.
0 votes
Correct Answer by Pravin Phadte about 8 years 8 months ago

Anyone can correct me if i am wrong.

There is no MAC used in the WAN network.

As MAC works on layer 2 the same way you need to configure the encapsualtion which would work on layer 2.

It can be HDLC or PPP thats the reason how the latyer 2 communcation takes place. If you do not configure the Encapsulation this would not work.

The best example would be the technology Frame-relay its a layer 2 but works on wan without any MAC address.

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Pravin Phadte Thu, 05/08/2008 - 06:30

Serial link does not use MAC address.

Encapsulation X25 is the layer 2 protocol which is used in this setup to communicate.

Hi There

The snippet of Serial interface configuration I used above was probably a bad axample as this link is indeed configured for XOT. However even if I configure a serial interface with an IP address and connect it to another routers serial interface,

Here is a print out form just such a set up

R1-7206VXR#sh int s1/0

Serial1/0 is up, line protocol is up

Hardware is M8T-X.21

Internet address is 172.16.4.1/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, crc 16, loopback not set

R2-7206VXR#sh int s1/0

Serial1/0 is up, line protocol is up

Hardware is M8T-X.21

Internet address is 172.16.4.2/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, crc 16, loopback not set

And still there is no MAC address on the interface. So I am wondering how IP connectivity is working?

Even capturing ping traffic between the two routers does not show a MAC address and I am confused as to how this works.

Best Regards,

Michael

Correct Answer
Pravin Phadte Thu, 05/08/2008 - 07:29

Anyone can correct me if i am wrong.

There is no MAC used in the WAN network.

As MAC works on layer 2 the same way you need to configure the encapsualtion which would work on layer 2.

It can be HDLC or PPP thats the reason how the latyer 2 communcation takes place. If you do not configure the Encapsulation this would not work.

The best example would be the technology Frame-relay its a layer 2 but works on wan without any MAC address.

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