Msg Age and Max Age in RSTP

[zarnims]

Hello,

Someone please shed some light on the logic behind the Message Age and Maximum Age fields in RSTP BPDU. I'm having a temporary brain block .

Thanks in advance. Have a good day!!

[Bilal Nawaz · Accepted solution]

Hello,
Each BPDU configuration contains another time-related parameter that is known as the message age. The message age is not a fixed value. The message age contains the length of time that has passed since the root bridge initially originated the BPDU. The root bridge sends all its BPDUs with a message age value of 0, and all subsequent switches add 1 to this value. Effectively, this value contains the information on how far you are from the root bridge when you receive a BPDU.

The maximum age timer specifies the maximum expected arrival time of hello BPDUs. If the maximum age timer expires, the bridge detects that the link to the root bridge has failed and initiates a topology reconvergence. The maximum age timer should be longer than the configured hello timer.

Hope this helps

Sent from Cisco Technical Support iPhone App

[zarnims]

Hi Bilal,

Thanks a lot for your explanation. Now I can continue my journey. Cheers!!

[paul driver]

Hello Bilal,

I have to disagree with you on your post mate- what you have discribed is infact what is related to STP not RSTP.

Please review this document:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

res

Paul

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Thanks.

[Bilal Nawaz]

Hello Paul,

My understanding is that when 3x hello's are missed, this is then the max-age time that has been expired. It's expecting x3 hello's but doesn't get them, then expires. Therefore not able to get to the root (yet).

I have always thought of message age as being the hop count field. Message age is incremented by every passing switch when the BPDU of its parent/upstream switch is received on its Root port.

Please correct my understanding.

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[paul driver · Accepted solution]

Hello Bilal,

In your previous posting you was explaing the process of  802.1d (STP) not 802.1w (rstp)

The maximum age timer specifies the maximum expected arrival time of  hello BPDUs. If the maximum age timer expires, the bridge detects that  the link to the root bridge has failed and initiates a topology  reconvergence. The maximum age timer should be longer than the  configured hello timer.

RSTP doesn't have to rely on the timer values of STP ( forward delay and max age).

instead It uses a proposal/agreement mechanism for rapid convergance of a port.

Please review that this rstp cco document, it gives a very good explantion with topologys how rstp syncs its non desgianted ports and how the proposal/agreement is met.

res

Paul

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Thanks.

[Bilal Nawaz]

Okay I think I understand now after doing some reading. There are two ways to detect failure. The old way with max age timer, which is different from the 3x hello detection time when the protocol information is aged out (this is what I was after and wasn't made apparent and still relies on hello's being sent on time) as far as I understand it, it achieves the same thing via different methods.

Please rate useful posts and remember to mark any solved questions as answered. Thank you.

[zarnims]

Hi Bilal,

I understand the same thing for Message Age. pdriver is correct about the Max Age tho. This is a little bit confusing. Since every switch generates its own BPDU in RSTP they will know when 3 hellos is missed in a row. It seems the Max Age timer is used for backward compatibily with legacy STP.

Thanks a lot.

[paul driver]

Hello Bilal,

Your inital posting regards STP timers related to 802.1d (stp) not 802.1w (rstp).

With regards to your missing hellos in your latest post-Then rstp does use these hellos as a keepalive but it doesn't then use the 802.1d forward delay and max age, in fact these will only be used if a rstp switch is connected to a non rstp switch (802.1d) or the interconnect links or access ports are not properly defined. (ie:spanning-tree link-type point-to-point or portfast not configured)

res

Paul

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Thanks.