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What is the exact difference between these two values in RSTP and what values it can hold. How is the switch's behavior changes as these values are changed.

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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STP-aware switches endlessly —define a maximum lifetime for a particular BPDU to prevent processing old and possibly outdated information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.

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Message age - Root Bridge sends out BPDU with message age as 0. And the receiving subsequent non-root bridges adds the value 1 to it. Effectively, this values says how far a bridge is from the root bridge. It is not a fixed value.

Max age - If 3x hello packets (which is sent out every 2s) are missed, a bridge waits for the configured max age time (default 20s) to conclude link failure to root bridge and initiates topology re-convergence.

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