Default auto values of path cost in STP

[jely donut]

From looking at the code, I can see that the default formula for RSTP is 20'000'000 / Speed (MBs), which gives the values I would expect. However, I am trying to find the default auto values for STP, but have been unable to find them so far.

I appreciate anyone's input.

Thanks

Ian MacPherson

[Satish Ashok]

Hi,
I had a brief look at 802.1D 2004 Table 17.3 Pg 154 which contains STP and RSTP standards now. There doesnt look to be a difference between STP/RSTP in determining path cost.

There is only a difference when the STP standard is 802.1D 1998 standard, from the 2004 standard, we see this..

"Bridges conformant to IEEE Std 802.1D, 1998 Edition, i.e., that support only 16-bit values for Path Cost, should use 65 535 as the Path Cost for these link speeds when used in conjunction with Bridges that support 32-bit Path Cost values."

Since MSTPD is conformant to 2004 standard, we dont have to worry about the 1998 version...

Thanks,
Satish

[jely donut]

Thanks for the response..

From what I've read though, it seems that the values we are using now is "long mode". I'm still searching, but it looks like you can't mix short and long mode switches together without possibly issues arising.

Do you know if this is the case?

Thanks,

Ian

[Satish Ashok]

Hi Ian,
You can look at the 1998 802.1D standard at the below location, Table 8.5, Page 127

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CDUQFjAB&url=http%3A%2F%2Fylescop.free.fr%2Fmrim%2Fprotocoles%2F802%2F802.1D-1998.pdf&ei=mDjCUbTELMSLjALtzoC4DA&usg=AFQjCNF2OLUMArPuKzqtuY44pjYIqCjrHA&sig2=lPunn3yxNabbWY-cnlo0rw&bvm=bv.48175248,d.cGE

One problem I could foresee is since the same link speed in both those standards would give a different path cost value, this could lead to suboptimal root bridge calculation when both these bridges are inter operating..

802.1D 1998 has Path cost 4 for 1Gb/s
802.1D 2004 has Path cost 20000 for 1Gb/s

So, 802.1Q looks to be suggesting to manually configure 65535 if the bridge is only 802.1D 1998 compliant.

Thanks,
Satish

[Vitalii Demianets]

Hello, Ian!

Here is excerpt from 802.1Q-2011 (chapter 13.16, page 349):

NOTE 2—BPDUs are capable of carrying 32 bits of Root Path Cost information, though IEEE Std 802.1D, 1998 Edition, and its earlier revisions limited the range of the Port Path Cost parameter to a 16-bit unsigned integer value.
Table 13-4 uses the full 32-bit range to extend the range of supported link speeds. Additional recommended values can be calculated as 20 000 000 000/(Link Speed in Kb/s). Limiting the range of the Path Cost parameter to 1–200 000 000 ensures that the accumulated Path Cost cannot exceed 32 bits over a concatenation of 20 hops. Where bridges using the IEEE Std 802.1D, 1998 Edition, recommendations and others using Table 13-4 are mixed in the same Bridged Local Area Network, explicit configuration is likely to be necessary to obtain reasonable CST topologies.

Here is the analogous passage from 802.1D-2004 (chapter 17.14, page 154):

NOTE 3—BPDUs are capable of carrying 32 bits of Root Path Cost information; however, IEEE Std. 802.1D, 1998 Edition and earlier revisions of this standard limited the range of the Port Path Cost parameter to a 16-bit unsigned integer value. The recommended values shown in Table 17-3 make use of the full 32-bit range available in BPDUs in order to extend the range of link speeds supported by the protocol. The recommended values for any intermediate link speed can be calculated as 20 000 000 000/(Link Speed in Kb/s). Limiting the range of the Path Cost parameter to 1–200 000 000 ensures that the accumulated Path Cost cannot exceed 32 bits over a concatenation of 20 hops. In LANs where Bridges that use the recommended values defined in IEEE Std 802.1D, 1998 Edition and Bridges that use the recommended values shown in this table are required to interwork, either the older Bridges will need to be re-configured in order to make use of the Path Cost values shown, or the new Bridges will need to be re-configured to make use of Path Cost values compatible with the values used by the older Bridges. The range of Path Costs that can be configured in an older Bridge is insufficient to accommodate the range of data rates available.

So, in the case of mixing "old" (16-bit PathCosts, 802.1D-1998, plain STP) and "new" (32-bit PathCosts, 802.1D-2004 and 802.1Q, RSTP and MSTP) bridges in one network the standards advice to do manual configuration of the path costs.

MSTPD is the "new" one (32-bit PathCosts, 802.1Q-2005 compliant).

[jely donut]

Thanks guys! Appreciate it! :o)