mac address for switch ports

[haytham elsamadony]

Hi Guys

i want to talk about mac address of switch ports

if each port in switch have a unique mac address or all switch as a device have a one mac address

because i found that show mac address table appear mac address of devices that connected to switch ports ..... then ports don`t have mac address or not i don`t know

so, i write this topic to open a discussion about it between us

wait your replies guys

[Georg Pauwen]

Hello Haytham,

the switch itself has a unique MAC address, and each switchport has one, too. Do a 'show version', the switches MAC address is called the Base ethernet MAC address:

Switch#sh ver
Cisco Internetwork Operating System Software
IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA4, RELEASE SOFTWARE(fc1)
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Wed 18-May-05 22:31 by jharirba
Image text-base: 0x80010000, data-base: 0x80562000

ROM: Bootstrap program is is C2950 boot loader
Switch uptime is 3 hours, 26 minutes, 2 seconds
System returned to ROM by power-on

Cisco WS-C2950T-24 (RC32300) processor (revision C0) with 21039K bytes of memory.
Processor board ID FHK0610Z0WC
Last reset from system-reset
Running Standard Image
24 FastEthernet/IEEE 802.3 interface(s)
2 Gigabit Ethernet/IEEE 802.3 interface(s)

63488K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address: 0060.5CCE.4712
Motherboard assembly number: 73-5781-09
Power supply part number: 34-0965-01
Motherboard serial number: FOC061004SZ
Power supply serial number: DAB0609127D
Model revision number: C0
Motherboard revision number: A0

To see the MAC address of each switchport, do a 'show interfaces' and check the second line in the output for each interface:

Switch#sh interfaces
FastEthernet0/1 is down, line protocol is down (disabled)
Hardware is Lance, address is 0050.0f16.4d01 (bia 0050.0f16.4d01)
BW 100000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 100Mb/s
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:08, output 00:00:05, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue :0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
956 packets input, 193351 bytes, 0 no buffer
Received 956 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
2357 packets output, 263570 bytes, 0 underruns
0 output errors, 0 collisions, 10 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

[Richard Burts]

Perhaps the issue is about what is contained in the output of show mac address table. The original poster seems to believe that it would contain all mac addresses including the switch mac addresses. But the output only contains external mac addresses that the switch has learned.

HTH

Rick

[haytham elsamadony]

ok, Guys

but, this will make me ask another question about benefit of mac address of ports

the mac address of switch or " Base ethernet MAC address " use for STP

and the data forward progress depend on mac table who depend on mac address of external devices connected to ports

then why each port have his unique mac address ..... at any scenario we will need these mac address ??

[Georg Pauwen]

Haytham,

there is a really good explanation what the internal switchport MAC addresses are used for. Check this post and the answer from Peter Paluch:

https://supportforums.cisco.com/discussion/12386956/what-switches-do-their-mac-address

The Base mac address of the switch is indeed used for a.o. spanning tree, where the mac address is part of the bridge ID; but also for identifying a switch in a stack, and in Link Aggregation Control Protocol (LACP) configurations:

--> LACP uses the system priority with the router MAC address to form the system ID and also during negotiation with other systems.

The LACP system ID is the combination of the LACP system priority value and the MAC address of the router.<--