Article ID: 4832
Protocols/Features
Spanning Tree
Standards
802.1d
802.1t
Goals
STA Bridge Priority settings
Change Spanning Tree Bridge Priority
Symptoms
Cannot set bridge priority
Bridge priority can not be changed
Bridge priority not working properly
dot1dStpPriority
Solution
The 802.1t standard permits only a selected set of values for Bridge Priority (MIB dot1dStpPriority=1.3.6.1.2.1.17.2.2).
Similar to 802.1d Spanning Tree, the default Bridge Priority is 32768 (0x8000).
Unique to 802.1t, the acceptable values for Bridge Priority are 0-61440 (0x0000-0xF000), in increments of 4096 (0x1000), giving a total of 16 possible Bridge Priority values. Any attempt to modify Bridge Priority to other values will result in a mib set error.
This can be especially confusing since it is possible to have a customized, non-conforming Bridge Priority value from prior to upgrading to 802.1t-compliant firmware. Just keep in mind that only the high-order (left-most) four bits of the 16-bit Bridge Priority value are used.
Note: As of SmartSwitch 2000/6000 2nd/3rd Gen firmware rev 5.04.19, 802.1d Bridge Priority values are translated in the MIBs as well, upon upgrade to these 802.1t-compliant revs. That is, it is no longer possible to find invalid Bridge Priority values in the MIBs, for that product.
The net effect is that, in 802.1t, Bridge Priority values as present in the mibs are effectively rounded down to the nearest valid value (last 12 bits ignored), before use. For example, values 0-4095 (0x0000-0x0FFF), are interpreted as 0 (0x0000).
See 4723 for more about 802.1t, and applicable Path Costs.
Note that products supporting 802.1t normally implement 802.1t Path Costs as an option, for best compatibility with most existing network equipment. However, for those devices the Bridge Priority settings as stated above will generally prevail no matter what Path Cost option is selected, and thus this Bridge Priority behavior can serve as a tell-tale sign of 802.1t support.
