Decoding show interfaces fastEthernet

[avilt]

Hello,

When I execute show interfaces fastEthernet0/1 command on a catalyst switch it displays the hardware address which is +1 of base MAC address.

When and where does the switch use this MACaddress? Does it mean that a 48 port switch will have 49 mac addresses?

[garapoglou]

Hi,

There is one base mac address used by the device while there is one mac address for each switchport.

So, to answer your question, yes. A 48 port switch has 49 mac addresses.

Best regards,

Giorgos

[avilt]

When does the switch use each ports mac address?

[garapoglou]

Well, MAC addresses are needed when you're working with VLANs or if the switch is a Layer3 one.

Best regards,

Giorgos

[avilt]

I am talking from a L2 perspective. I have not read anywhere that mac address from each port is used with vlans.

[garapoglou]

Hi,

Here's a very good example:

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml

Best regards,

Giorgos

[Somasundaram Jayaraman · Accepted solution]

Hi,

Let us consider the output of below.

Boris# show spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    0
             Address     0007.4f1c.e847
             Cost        131
             Port        136 (GigabitEthernet3/8)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     00d0.003f.8800

Here the root mac address is 0007.4f1c.e847, and this mac address is the base mac address

The interfaces will have the mac address as  Base mac address + 2 bits change in the last part of
base mac.

mac address of gig 0/1 ---- 0007.4f1c.e801--> Only last 2 part is changed from the base mac address

++ This show interface mac-addres would be used in bpdu packets.

++ If you see the bpdu packet the src mac would be interface mac address.

Hope this helps

Cheers
Somu

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