Question to "spanning-tree extend system-id"

[kernst]

We have on different locations 650x switches. All of them can handle 1024 MAC-addresses. Most of them have the command "spanning-tree extend system-id", but on a new location Spanning-Tree was implemented without using "spanning-tree extend system-id". - I try to make the configs as similiar as possible. - But I do not really understand what is the advantage or disadvantage to use the command "spanning-tree extend system-id" on these 650x with supporting 1024 MAC-addresses (we are using Rapid PVST). Could somebody describe it in a simple way (and why we should use it or not)?

[Giuseppe Larosa]

Hello,

the space of possible vlans is 4096 per 802.1Q standard so the use of extend system-id allows you to support also the extended vlan range.

It may be easier to troubleshoot STP because only one MAC address per switch is used for all instances with different priorities:

STP priority = base_value + vlan#

where base_value has to be a multiple of 4096 with spanning-tree extend system-id.

without it a different MAC address is picked up for each STP instance.

Hope to help

Giuseppe

[ansalaza · Accepted solution]

This originates with the HW limitation of some switches, which use the same MAC address as part of the Bridge ID for all VLANs:

...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

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

This will explain better:

Catalyst 6500 series switch chassis have either 64 or 1024 MAC addresses available to support software features such as STP. To view the MAC address range on your chassis, enter the show catalyst6000 chassis-mac-address command.

For chassis with 64 MAC addresses, STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.

When the extended system ID is not enabled, STP uses one MAC address per VLAN to make the bridge ID unique for each VLAN.

If you have a network device in your network with the extended system ID enabled, you should also enable the extended system ID on all other Layer 2 connected network devices to avoid undesirable root bridge election and spanning tree topology issues.

When the extended system ID is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN ID. With the extended system ID enabled, a switch bridge ID (used by the spanning tree algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.

If another bridge in the same spanning tree domain does not have the extended system ID enabled, it could win root bridge ownership because of the finer granularity in the selection of its bridge ID.

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/spantree.html#wpxref44429

The Root Bridge election should consider the same parameters in Traditional STP and RSTP.

[kernst]

Thanks very much for that comprehensive and good understandable response.