Message age and max age , STP

[RomanB1005]

Hello

Could you please someone explain me purpose of Message age timer , As I uderstand it , from ROOT bridge to other decrease MAX age value , respectively increase Message age from 0 to 19 , with diameter 7 , but why >? 

Thanks in Advance

[Peter Paluch · Accepted solution]

Hi Roman,

The Message Age field in STP is similar to a TTL field in IP packet and is supposed to

• prevent BPDUs being forwarded between STP-aware switches endlessly
• define a maximum lifetime for a particular BPDU to prevent processing old and possibly outdated information

The Message Age is set to 0 at the root bridge. Every other non-root switch will increase the Message Age field by one in the received BPDU when relaying it via its own ports. Moreover, on each non-root switch, the BPDU itself will age out in (Max Age - Message Age). This makes sure that the more hops this BPDU has traversed from the root, the sooner it will expire.

This document also may be interesting for you:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml

Of course, feel welcome to ask further!

Best regards,

Peter

[RomanB1005]

Oh , thanks ,so that means , if diameter is 7 and I manualy set up maximu age to for example to 25 , last switch in topology could not recieve BPDU ? ...

[Peter Paluch]

Hello Roman,

I am not sure how you arrived to that conclusion - but it is incorrect. Assuming that the root switch is at one end of the 7-switch chain, the very last switch at the other end will receive the BPDUs with whatever Max Age you configured, and with the Message Age of 5. This BPDU will therefore expire in (Max Age - 5) seconds. Unless you configure your Max Age to 7 or less, the BPDU will be accepted and processed. If the Max Age was 7, the BPDU would expire in 7-5=2 seconds which is exactly the Hello time - hence the BPDU would expire at approximately the same time as it is expected to be received, and this could lead to very unpleasant results.

Having the diameter of 7 and the Max Age set to 25 will not produce any ill results - at worst, the BPDU will expire in 25-7=18 seconds at the farthest switch.

Best regards,

Peter

[RomanB1005]

Ok

Sorry , I meant it right , but I have used wrong values ...

Thank you ...

Roman

[JanicFerra]

Hi Peter

it is totally clear for me how Message Age field works and how each Non-root Switches uses and increment the value, my question now is, why does STP need the farther switches expiries its BPDUs stored sooner?

As an example, if I have 5 switches chained and the first one is the root if something happens to it the first that start announcing as new root-bridge is the last one on the opposite direction, why does the protocol do that?

Thanks a lot