I have a question about MAC address in the router.

Unanswered Question
Jun 23rd, 2009
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I have known that each IP have only another MAC address number.


But actuality, another result appears on the router when I enter commend " show arp | includ 001a.2f85.4abf ".

Please refer to below logs.



Router#show arp | include 001a.2f85.4abf

Internet 12.53.117.1 - 001a.2f85.4abf ARPA GigabitEthernet3/15

Internet 12.53.117.17 - 001a.2f85.4abf ARPA GigabitEthernet3/16

Internet 12.53.180.2 - 001a.2f85.4abf ARPA Vlan180

Internet 12.52.227.1 - 001a.2f85.4abf ARPA GigabitEthernet6/1

Internet 12.52.228.2 - 001a.2f85.4abf ARPA GigabitEthernet6/2

Internet 12.53.198.2 - 001a.2f85.4abf ARPA Vlan198

LN13P_SW2#sh mod

Chassis Type : WS-C4507R


Power consumed by backplane : 40 Watts


Mod Ports Card Type Model Serial No.

---+-----+--------------------------------------+------------------+-----------

1 2 Supervisor IV 1000BaseX (GBIC) WS-X4515 JAE1052J01A

2 2 Supervisor IV 1000BaseX (GBIC) WS-X4515 JAE1137WQW8

3 18 1000BaseX (GBIC) WS-X4418-GB JAE11054626

4 6 1000BaseX (GBIC) WS-X4306-GB JAE1135V6DK

5 6 1000BaseX (GBIC) WS-X4306-GB JAE0918BF1H

6 6 1000BaseX (GBIC) WS-X4306-GB JAE1046F7P8

7 48 10/100BaseTX (RJ45) WS-X4148-RJ JAE1044EE6C


M MAC addresses Hw Fw Sw Status

--+--------------------------------+---+------------+----------------+---------

1 001a.2f85.4a80 to 001a.2f85.4a81 5.2 12.2(20r)EW1 12.2(31)SGA8 Ok

2 001a.2f85.4a82 to 001a.2f85.4a83 5.4 12.2(20r)EW1 12.2(31)SGA8 Ok

3 0014.1cbe.0d8e to 0014.1cbe.0d9f 1.3 Ok

4 0018.7380.46ea to 0018.7380.46ef 4.3 Ok

5 0013.1ac9.1ff0 to 0013.1ac9.1ff5 4.0 Ok

6 0019.e8dc.4250 to 0019.e8dc.4255 4.2 Ok

7 0019.e788.9630 to 0019.e788.965f 3.3 Ok


Mod Redundancy role Operating mode Redundancy status

----+-------------------+-------------------+----------------------------------

1 Active Supervisor SSO Active

2 Standby Supervisor SSO Standby hot


What is the "001a.2f85.4abf" ?



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Giuseppe Larosa Tue, 06/23/2009 - 23:17
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Hello Kwangwon,

what you see is normal this is a multilayer switch 4500.


in this kind of devices all L3 interfaces use the same MAC address unless explicitly configured.

001a.2f85.4abf is a MAC address in the MAC address range of the active Supervisor as you can see from this line:


1 001a.2f85.4a80 to 001a.2f85.4a81 5.2 12.2(20r)EW1 12.2(31)SGA8 Ok


a different source MAC address taken from the range is used on L2 port for sending CDP messages , STP BDPUs and so on


Hope to help

Giuseppe


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