Address of a switch port - generic question

[hostettle]

Hi,

I hope my question is not difficult and sufficiently clear.

For management, a switch has 2 kinds of address.

- The address of the whole switch

- The individual address of each of its ports

Would you have pleased a case where the individual address of the switch port is included in the DA address of the MAC frame?

I think it's more common to have this address in the SA of the MAC frame => we have STP, MRP.

Best regards,

Michel          

[Peter Paluch · Accepted solution]

Hello Michel,

You are very much welcome. You pose some very good questions!

What is the relation between the address of the management port and the base MAC address of the switch?

Well, what is a management port? Cisco Catalyst series 2960 up to 3750 usually do not have any dedicated Ethernet port just for management (some of them do, most don't). In any case, let me show you a 3560G:

SW-Dist1#show version | include MAC

Base ethernet MAC Address       : 00:26:0A:54:BB:80

SW-Dist1#show interfaces gi0/1 | include bia

  Hardware is Gigabit Ethernet, address is 0026.0a54.bb81 (bia 0026.0a54.bb81)

SW-Dist1#show interfaces gi0/2 | include bia

  Hardware is Gigabit Ethernet, address is 0026.0a54.bb82 (bia 0026.0a54.bb82)

SW-Dist1#show interfaces gi0/3 | include bia

  Hardware is Gigabit Ethernet, address is 0026.0a54.bb83 (bia 0026.0a54.bb83)

Notice that the MAC addresses of individual switchports form an increasing sequence depending on their position in the switch. As I see it, the switch internally enumerates its ports and assigns MAC addresses to its ports in the ascending order of the internal enumeration.

Best regards,

Peter

[hostettle]

Hello Peter,

Sorry for the late, my week was very busy. Hope you will see me.

> Cisco Catalyst series 2960 up to 3750 usually do not have any dedicated Ethernet

> port just for management (some of them do, most don't).

In fact, it would seem that switches which can be managed remotly by SNMP for example, have a port management, not necessarily physically connected to the LAN.

The access to this management port would be through the forwarding database. We can see the process in 802.1Q (figure 8-7).

(I try to put the figure, but I'm not sure it works with my old PC)

> As I see it, the switch internally enumerates its ports and assigns MAC addresses

> to its ports in the ascending order of the internal enumeration

It' very instructive, thanks for this trace.

Best regards,

Michel

[Peter Paluch · Accepted solution]

Hello Michel,

In fact, it would seem that switches which can be managed remotly by  SNMP for example, have a port management, not necessarily physically  connected to the LAN.

Alright, I get your point. With port, we usually mean a physical connector somewhere on the switch. What you are talking about is a logical interface that provides IP connectivity of the manageable entity inside the switch to the surrounding IP network.

With Catalyst switches, such a management interface is a SVI interface, i.e. any interface Vlan X interface. In fact, every SVI is considered a management interface on Catalysts. Regardless of which IP address configured on SVIs you send packets to, they will all be received and processed by the switch. The MAC addresses of these SVIs are also derived from the base MAC address of the switch:

SW-Dist1#show ver | i MAC
Base ethernet MAC Address       : 00:26:0A:54:BB:80
SW-Dist1#show int Vlan1 | i bia
  Hardware is EtherSVI, address is 0026.0a54.bbc0 (bia 0026.0a54.bbc0)
SW-Dist1#show int Vlan5 | i bia
  Hardware is EtherSVI, address is 0026.0a54.bbc1 (bia 0026.0a54.bbc1)
SW-Dist1#show int Vlan11 | i bia
  Hardware is EtherSVI, address is 0026.0a54.bbc2 (bia 0026.0a54.bbc2)
SW-Dist1#show int Vlan12 | i bia
  Hardware is EtherSVI, address is 0026.0a54.bbc3 (bia 0026.0a54.bbc3)

Even here you can see that the SVI addresses start with certain offset from the base MAC and then proceed sequentially.

Best regards,

Peter

[hostettle]

Hello Peter,

Thanks very much for all your information and all your patience.

I keep this thread in mind.

Best regards,

Michel