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Ramping up on RSTP here. One thing that is unclear to me is how RSTP links converge initially on P2P links. Consider the classic 3 switch topo as below:

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  1. For RSTP, won't all the links move to forwarding state immediately since they are all p2p links? In this case, will there be a brief moment where l2 loop will be formed? Initially, I thought that only edge ports are considered p2p, but got to know that any duplex interface can be a p2p link.

Really confused here. Any help is appreciated

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Point-to-point links can transition to the Forwarding state without allowing a time period for convergence (IEEE 802.1Q Clause 13.4). They don't have to.

Your links are not point to point, as there's another link behind each bridge. On the physical layer, all modern (Ethernet) links are point to point. When it comes to RSTP/MSTP and bridges it's about point to point on the data link layer.

As with pretty much all links, it's up to the admin whether the links start forwarding immediately (see the fastport or admin-edge-port option, depending on vendor).

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For RSTP, won't all the links move to forwarding state immediately since they are all p2p links?

No. A (non-edge) RSTP port will only be "rapid" if negotiation (a.k.a. Proposal & Agreement) is happening between the bridges. [*]

If there are no (R)STP hellos incoming, there cannot be a Proposal & Agreement phase, a the port will go through 30sec of LRN phase, mimicking STP behaviour.

See https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#anc17

If a designated discarding port does not receive an agreement after it sends a proposal, it slowly transitions to the forwarding state, and falls back to the traditional 802.1D listening-learning sequence. This can occur if the remote bridge does not understand RSTP BPDUs, or if the port of the remote bridge is blocking.


[*] this is also why - even with a variety of RSTP - setting port type edge [trunk] or spanning-tree portfast [trunk] (syntax may vary by vendor) is still very much needed for end system ports (hosts, routers, firewalls...). Without that setting, these would remain non-edge ports, and if they don't receive BPDUs with the Proposal/Agreement flags, they mimick STP behaviour and stay blocked for 30seconds.

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  • 1. hello packets would be sent and received even in regular STP phase, irrespective of proposal and agreement phase, right? 2. Is proposal and agreement in RSTP only applicable for P2P links? 3. In junos implementation, is edge port different from a p2p link? Edge port refers to link to end host (probably similar to cisco's portfast?), while p2p in rstp context refers to connection between 2 switches: [edit ... protocols rstp interface interface-name] user@host# set mode (p2p | shared) [edit ... protocols rstp interface interface-name] user@host# set edge Sep 14, 2023 at 5:55

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