Beware The Man Of One Platform

I vow to never be one of those guys that expects my word to be law once I am a CCIE.  This is not because I am humble (I’m not) or because the ‘Argument From Authority’ is a logical fallacy; it’s because I am wrong more often than I care to be and I will continue to be wrong more often than I care to be regardless of any digits or abbreviations after my name.  🙂

Case in point: I was troubleshooting an issue last week and was surprised to find that the VLAN interfaces (SVIs) on a 6500 series switch (an old piece of shit 6500 switch running DECNet….but I digress) each shared a single (virtual) MAC address.  I pointed this out to one of my co-workers and he said that this was normal.  I disagreed.  I jumped on a 3750 and showed him that each SVI had a unique MAC address.  I even labbed it up quickly on my 3560 to prove my point.

We noted that this might be an interesting anomaly, but it most likely was not our issue as we were troubleshooting a duplicate IP/HSRP/DECNet/STP loop issue.

Well it turns out that we were both right (and both wrong).  Depending on the platform (and IOS version?) Cisco switches may use the System MAC Address for each SVI or they may use a unique MAC Address (derived from the System MAC Address).  CCIE candidates can see this in their labs by noting the differences between the 3560s and 3550s:

3560 uses a unique MAC for each SVI:
sw1#sh ver | i IOS|emo
Cisco IOS Software, C3560 Software (C3560-IPSERVICESK9-M), Version 12.2(25)SEE2, RELEASE SOFTWARE (fc1)
cisco WS-C3560-48PS (PowerPC405) processor (revision G0) with 118784K/12280K bytes of memory.
512K bytes of flash-simulated non-volatile configuration memory.

sw1(config-if)#do sh int | i Vlan|bia
Vlan1 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c0(bia 0012.018f.d5c0)
Vlan2 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c5(bia 0012.018f.d5c5)
Vlan3 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c6(bia 0012.018f.d5c6)
Vlan4 is up, line protocol is up
  Hardware is EtherSVI, address is 0012.018f.d5c7(bia 0012.018f.d5c7)

3550 uses the same MAC for each SVI:
sw3#sh ver | i IOS|emo
Cisco IOS Software, C3550 Software (C3550-IPSERVICESK9-M), Version 12.2(25)SEE2, RELEASE SOFTWARE (fc1)
Cisco WS-C3550-24(PowerPC) processor (revision D0) with 65526K/8192K bytes of memory.

Vlan1 is administratively down, line protocol is down
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan2 is up, line protocol is up
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan3 is up, line protocol is up
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)
Vlan4 is up, line protocol is down
  Hardware is EtherSVI, address is 000a.410e.0600(bia 000a.410e.0600)

Scott Morris has an article on this issue.

Now you’ll notice that all of the VLAN interfaces have the same MAC address. This is the System MAC address. The reason this is OK has to do with where MAC addresses are used.

A MAC address must be unique within a Layer2 network, a broadcast domain or subnet. Each VLAN is a separate L2 network, broadcast domain and subnet. So there should be no possibility for overlap here and nothing to worry about.

If your configuration is creating some strange bridging or other cross-VLAN behavior, there may be the possibility of odd behavior, but that isn’t the normal issue at all!

So, in the grand scheme of things, you shouldn’t see any duplicate MAC addresses in any place that makes a difference.