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extended vs traditional priority in STP?

Hello, I am confused with this statement:

According to Cisco Press book, CCNP SWITCH: Offical Certification Guide,  by David Hucaby, if a switch can't support 1024 unique MAC addresses  for its own use, the extended system ID is always enabled by default.   Otherwise, the traditional method is enabled by defaulth

Does this mean that if the switch does not suppoort 1024 unic MAC addresses (it supports less than 1024) it will use Extended System ID, but if it does support 1024 it will use the Traditional method?

I am confused because I read these somewhere else: ...Chassis that support only 64 MAC addresses always use the 12-bit extended system ID. On chassis that  support 1024 MAC addresses, you can enable use of the extended system  ID. STP uses the VLAN ID as the extended system ID.

I just need a clearer explanation please!!!!

1 REPLY
Cisco Employee

Re: extended vs traditional priority in STP?

Historically, Cisco Catalyst switches have supported only up to 1024 VLANs, with this number being derived from the 10-bit VLAN ID field included in Cisco's original trunking protocol, ISL. 802.1Q is the standards-based trunking protocol defined by the IEEE and is now the recommended trunking protocol for Cisco switched networks. 802.1Q includes a 12-bit VLAN ID field, which means that 802.1Q supports up to 4096 VLANs. Cisco refers to the VLANs between 1025 and 4096 as extended-range VLANs.

There may be some chassis on which only 64 mac-addresses are supported. On those chassis extended system-id are enabled by default. If I am not wrong you cannot disable the extended-system id on those.

But on others which has got 1024 mac-addresses, the extended system id is not enabled by default. Now when you exceed the number of mac-address, you will get an error message saying:

Failed to create VLANs xxxx. Spanning-tree extend system-id need to be enabled.

So at this time you will have to enable extended system-id and this might change your root bridge for other vlans since now all spanning-tree instances would use extended system-id.

http://www.ciscotaccc.com/kaidara-advisor/lanswitching/showcase?case=K41893403

Hope this helps

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