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Spanning Tree Protocol Question
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| I understand the concept of the spanning tree protocol and how to determine the designated ports. The question is: where does the default priority come from, and what exactly is the "priority" anyways?
Tks
JerryL | |
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| The switch with the lowest bridge priority becomes the root switch for that instance of spanning tree.The dafault bridge priority value is 32768. If the bridge priority for the specified VLANS is set to 12768 this value will cause the switch to become the root for this specified VLANs. We can change the root switch both the bridge ID(MAC address) and bridge priority. | |
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| Sorry, need to modify:
Root switch will be changed depend on both bridge ID(MAC address) and Bridge priority.If all the switchs are configured with the default priority(32768). The switch with the lowest MAC address in the layer 2 networks becomes the root switch. | |
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| Hi Hugh;
Tks for the reply. I understand the method in which the root switch is determined. What I don't understand, and it may be very minor, is where does the default value of 32768 come from. This is probably nitpicking anyway. I think for me, if I understood that, it would sink into me that much more.
Tks
JerryL | |
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| the unique bridge ID consist of 8 byte.
2 byte for priority and 6 byte for mac-address.
2 byte = 16 bit -> 2**16 = 65.536 max. value
if you use 32.768 (midrange) as default,
you are able to increase the priority by max. 32.768 -> up to 65.536
you are able to decrease the priority by max. 32.768 -> down to 0
hope that helps for reinforcing the means to your brain.
siegi | |
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| where does the default value of 32768 come from?
It came from <b>IEEE 802.1d(RFC-1493)</b>
"dot1dStpPriority (integer)
This read-write MIB object contains the write-able portion of the bridge ID, that is, the first two octets of the 8-octet bridge ID. The other 6 octets of the Bridge ID are given by the value of dot1dBaseBridgeAddress.
Valid Values: 0 to 65535
Default Value: 32768 "
Source from as follows the web: http://www.ieng.com/univercd/cc/td/...m#xtocid1686113
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| I think the question is how does one actual set the priority on a switch. The MAC address is normally a burned-in value so that is not changeable; but the priority is changeable. So how do you change the value?
1. DIP switches ?
2. Configuration item ?
3. Jumpers on the switch ?
4. Thumb-wheel dials ?
5. Other ?
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| I would say, that any mac-address can be changed or at least overriden.
siegi
[This message has been edited by edlinger (edited 10-28-2000).] | |
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| Thanks guys;
The light bulb came on. Lamle's book doesn't explain that at all (at least that I know of). I didn't realize that it was a mid-range value, or how it was derived.
That's one less question to be concerned with. Unfortunately there will be more on the way as I get deeper into this (ugh!)
By the way, I think you can change the MAC address with an EEPROM burner. The question is: why would you want to do it anyway?
Tks
JerryL
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| It's not as complex as you think, Jerry :-)
This example shows, how to assign a nondefault MAC address to an RSFC VLAN interface and verify the configuration:
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface vlan 200
Router(config-if)#mac-address 1234.5678.9ABC
Router(config-if)#^Z
Router#show interface vlan 200
Vlan200 is up, line protocol is up
Hardware is Cat5k Virtual Ethernet, address is 1234.5678.9abc (bia 0010.0d3e.)
<...output truncated...>
Router#
VLAN interfaces on the RSFC are assigned the base MAC address from a MAC address serial EEPROM on the RSFC that contains 64 MAC addresses. All user-configurable routing interfaces use the base MAC address by default. If desired, you can override the default MAC address assignment for an interface. Normally, there is no need to override the default MAC address, but
although a MAC-address should be unique on the world, it isn't really always...
siegi |
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