L2switch and mac address pool

Answered Question
Dec 13th, 2008
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Hi every body!

I was looking at output of " show int" command,I find every port has mac address assigned from the pool.

My question is what is the purpose of these mac-address because hosts connected to ports, do not need to use the mac addresses ,assigned to L2 ports, in order to communicate with each other.

Besides in stp, base mac address is used, again raising the questions about the use of mac addresses on L2 ports of the switch.

thanks a lot !

Correct Answer by scottmac about 8 years 6 months ago

It's not too mysterious; remember that a "switch" (an L2 switch) is logically, and by definition, just a big multi-port bridge.


The MAC of the port identifies the forwarding destination for the internal bridging logic and permits building a table that associates a host MAC with a physical port (or for VLANs, a logical port).


I suppose it comes down to: The original bridges were just re-packaged computers with Ethernet interfaces. Each interface had a MAC. They stayed with that paradigm and now some protocols rely on it.


Good Luck

Scott

Correct Answer by lejoe.thomas about 8 years 6 months ago

Hi,


As Giuseppe said the Bridge ID in STP is formed from a base address but BPDUs, which originates from particular port has the source address of that port's MAC.

This source address is used to build the frame that carries the BPDU, but Bridge ID's MAC is within the actual config BPDU.


A decode of BPDU will verify this.


HTH

Lejoe

Correct Answer by Giuseppe Larosa about 8 years 6 months ago

Hello Sarah,

this is a very good question: sometimes I think that only one MAC address in the box would be enough for these jobs like STP, CDP, VTP, DTP, PAGP, LACP.


However, we need to be sure that if we connect two ports of the same switch with a cross-over cable they are able to see each other and understand the difference with an RJ-45 plug.

STP could be able to support it for the STP port index and port priority but the simple keepalive works by sending a frame with SA=DA = port address.

Not sure but MAC SA of BPDU could be that of the port then I agree the Bridge-id is made using the base address.


In addition, that mac address would be statically associated to all ports and this would mean an exception to the rule that says that one unicast MAC address should be seen on only one port.

It would require a special class of mac addresses distinct from normal unicast MAC addresses.


Hope to help

Giuseppe


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Correct Answer
Giuseppe Larosa Sat, 12/13/2008 - 15:28
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Hello Sarah,

this is a very good question: sometimes I think that only one MAC address in the box would be enough for these jobs like STP, CDP, VTP, DTP, PAGP, LACP.


However, we need to be sure that if we connect two ports of the same switch with a cross-over cable they are able to see each other and understand the difference with an RJ-45 plug.

STP could be able to support it for the STP port index and port priority but the simple keepalive works by sending a frame with SA=DA = port address.

Not sure but MAC SA of BPDU could be that of the port then I agree the Bridge-id is made using the base address.


In addition, that mac address would be statically associated to all ports and this would mean an exception to the rule that says that one unicast MAC address should be seen on only one port.

It would require a special class of mac addresses distinct from normal unicast MAC addresses.


Hope to help

Giuseppe


Correct Answer
lejoe.thomas Sat, 12/13/2008 - 16:32
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Hi,


As Giuseppe said the Bridge ID in STP is formed from a base address but BPDUs, which originates from particular port has the source address of that port's MAC.

This source address is used to build the frame that carries the BPDU, but Bridge ID's MAC is within the actual config BPDU.


A decode of BPDU will verify this.


HTH

Lejoe

sarahr202 Sat, 12/13/2008 - 17:59
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Thanks Lejoe,Giuseppe and Scott !


Correct Answer
scottmac Sat, 12/13/2008 - 16:37
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It's not too mysterious; remember that a "switch" (an L2 switch) is logically, and by definition, just a big multi-port bridge.


The MAC of the port identifies the forwarding destination for the internal bridging logic and permits building a table that associates a host MAC with a physical port (or for VLANs, a logical port).


I suppose it comes down to: The original bridges were just re-packaged computers with Ethernet interfaces. Each interface had a MAC. They stayed with that paradigm and now some protocols rely on it.


Good Luck

Scott

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