Where does BridgeID get MACaddress from?

Paul#4

In STP every bridge/switch has a bridgeID which consists of 2byte-priority and 6byte-MACAddress.

Where does the MACaddress come from?

I read that some switches there is a pool of available MACaddresses.

Every switch interface needs a MACAddress correct?

This means that the BridgeID MacAddress must come from somewhere other then the interfaces...



Thanks

Paul#4

Pash

http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm

If all switches are enabled with default settings, the switch with the lowest MAC address in the network becomes the root switch.

It comes from the switch interfaces. Hope that paper helps.

Cheers,

Paul#4

Hey Pash,

That was a great link for STP, but that particular link mentioned absolutely nothing about which MACAddress is used for the BridgeID. hahaha

I also read in BCMSN that certain switches have pools of MACaddresses that are used for different purposes, such as STP BridgeID.

This excerpt was taken from CCNP BCMSN Study guide

---

Cisco Catalyst Switches use a Mac Address from a pool of Mac Addresses assigned to either the backplane or to the supervisor module.

---

Anyways, if you find a link that mentions something about what I'm talking about feel free to post.


Best of luck

Pash

Ohh sorry if that link isnt helpful. This is a good question then, I would of said all interfaces have a say in which mac address is potentially used. But maybe it is model dependent. If I find a link for you ill paste it up. Or if you find information can you let us know?

Cheers,

dtlokee

The base MAC address is buned into the parameter block of the switch and can be modified by breaking the boot sequence and resetting the MAC_ADDRESS parameter and then written to the parameter block (although this should not need to be done). On a layer 3 switch the mac address of any layer 3 interface is based off the base mac address + offset of the port number. Layer 2 interfaces don't have an individual MAC addres, only the VLAN interface used for management.

HTH

Paul#4

Yeah, I just want to know if STP ever uses interface MACaddresses...

Doesn't each interface have its own MACAddress?

I'm guessing maybe Cisco low end models use MACAddresses off designated interfaces....

But nowadays most cisco switches are Catalyst, so I think they all use MacAddresses from a pool.

I did some research and didn't find much...I'm focusing on the ICND test for tomorrow.


I will let you guys know how I do tomorrow.

Thanks

Paul#4

dtlokee wrote:

The base MAC address is buned into the parameter block of the switch and can be modified by breaking the boot sequence and resetting the MAC_ADDRESS parameter and then written to the parameter block (although this should not need to be done). On a layer 3 switch the mac address of any layer 3 interface is based off the base mac address + offset of the port number. Layer 2 interfaces don't have an individual MAC addres, only the VLAN interface used for management.

HTH

Thanks for your reply!

Netstudent

Paul#4 wrote:

Yeah, I just want to know if STP ever uses interface MACaddresses...

Doesn't each interface have its own MACAddress?

I'm guessing maybe Cisco low end models use MACAddresses off designated interfaces....

But nowadays most cisco switches are Catalyst, so I think they all use MacAddresses from a pool.

I did some research and didn't find much...I'm focusing on the ICND test for tomorrow.


I will let you guys know how I do tomorrow.

Thanks

The only time a switchport has a MAC address is when there is a Computer cabled to it. Or if it is cabled to another switch there may be numerous MAC addresses associated with that interface that is connected to the other switch. This is in the CAM table or MAC table. AS far as a switch out of the box having a MAC address assigned to an interface, I would say no. Also you can explicitly define the MAC addresses that are allowed on those ports.

tech-airman

Paul#4 wrote:

In STP every bridge/switch has a bridgeID which consists of 2byte-priority and 6byte-MACAddress.

Where does the MACaddress come from?

The MAC address comes from the MAC address of the switch port connected to the other switch.

So let's say you've got two switches, SwitchA and SwitchB. You connect switch port 0/1 on SwitchA to switch port 0/1 on SwitchB with a crossover cable. Let's say that switch port 0/1 on SwitchA has a MAC address of 00-00-00-00-00-01. I am completely making up this MAC address. Now, let's say switch port 0/1 on SwitchB has a MAC address of 00-00-00-00-00-02. Let's say the priorities for SwitchA and SwitchB are the same. So the root bridge election process between these two switches is basically based a comparison of MAC addresses. Since we know that the root bridge election process winner is the one that sends the Bridge Protocol Data Unit (BPDU) with the lowest "unique bridge ID," in this case, SwitchA would win because 00-00-00-00-00-01 is lower than 00-00-00-00-00-02. So since SwitchA won the root bridge election, that makes SwitchB the "designated bridge."

Source:

1. Configuring STP - Cisco Systems - http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/8aew/configuration/guide/spantree.html#wp1020421

Paul#4 wrote:

I read that some switches there is a pool of available MACaddresses.

Every switch interface needs a MACAddress correct?

The pool of available MAC addresses are the pool of individual switch ports on the switch device. Each individual switch port of a switch was assigned a MAC address by the manufacturer so it no longer needs a MAC address.

However, according to the "Catalyst 4000 Family Switch Cisco IOS Software Configuration Guide, 12.1(8a)EW - Configuring STP" webpage, it states...

Catalyst 4000 Family Switch Cisco IOS Software Configuration Guide, 12.1(8a)EW - Configuring STP wrote:

MAC Address Allocation
The supervisor engine has a pool of 1024 MAC addresses that are used as the bridge IDs for the VLAN spanning trees. You can use the show module command to view the MAC address range (allocation range for the supervisor) that the spanning tree uses for the algorithm.

MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1, the second MAC address in the range assigned to VLAN 2, and so forth. For example, if the MAC address range is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so on.

Paul#4 wrote:

This means that the BridgeID MacAddress must come from somewhere other then the interfaces...

Thanks

Paul#4

In the case of the supervisor engine of a Cisco Catalyst 4000 switch, that seems correct.

Good luck on your 640-811 ICND exam today.

dtlokee

A layer 2 switchport (the only kind you can have on a Cisco 2950) will NEVER have an assigned mac address. The mac-address-table is a table of the mac address of the connected device, not the switch interface itself. This table is used to make forward/filter decisions when a unicast frame is received on an interface of the switch, but it is important to understand the difference. A layer 2 switchport does not respond to arp requests it forwards them to all interfaces in the same vlan and the end device with the ip address in the arp request will reply, not the switch itself. On the other hand, a layer 3 switchport (one that has been configured with the "no switchport" command) will have a mac address and will respond to an arp request for it's ip address. Once an interface has been configured as a layer 3 interface it will function basically the same as an interface on a router, you can assign it an ip address, configure routing protocols, and it will route traffic the same as a router (ie removing the layer 2 header/trailer, routing to the correct egress interfacce and adding a new layer 2 header/trailer to the frame)

As for the pool of addresses, in per vlan spanning tree (PVST) each vlan can have a different root bridge (sometimes used for load balancing across multiple layer 2 links). When using PVST the switch will send BPDUs for each vlan including the Priority, MAC address of the bridge and the VLAN the BPDU is advertising.

Hope this helps