CCNA RnS Article #34: RSTP Concepts

In this article, we are going to talk about RSTP concepts. Some of the concepts in RSTP are same as STP and the list is below –

RSTP & STP both elect the root switch and both use the same logic.
After selecting the root switch, both select the root port and use the same logic.
Designated ports on each LAN segment are selected using the same logic.
Both RSTP & STP put each port either in a forwarding or blocking state.

If you missed the STP Under the Hood article, please review the same here it’s a good reference to understand the STP concepts.

What makes RSTP different from STP is the mechanisms that allow RSTP to converge faster than STP. In this article, we are going to talk about these mechanisms.

In RSTP each switch can generate its own Hello message which is not true in STP where the Root switch controls and forwards the Hello message for all switches. In addition to this, there is a query mechanism added where switches can query rather than wait for the timer to expire when no hello is received.

Figure 1 : RSTP

RSTP does not have a listening state. The blocking state of STP is renamed as a discarding state in RSTP. The discarding state combines the blocking state and disabling state. RSTP vs STP Port details –

Ports that are administratively shutdown – are called Discarding in RSTP but in STP it is called Disabled.
A port that ignores the incoming data frames and not used for data transmission – is called a Discarding state in RSTP but in STP it is called Blocking (it is a kind of stable state).
A temporary state of the port where it listens without learning and frame forwarding – this is not used in RSTP but in STP it is called a listening state.
A temporary state of the port where it learns MAC addresses but doesn’t forward the data frames – is called Learning in both RSTP and STP.
A stable state of the port that allows MAC address learning and data frame forwarding – is called the Forwarding state in both RSTP and STP.

As discussed, RSTP elects the root port the same way as STP does. However, it moves further and selects other ports that can be candidates for root port as Alternate Port (for Root). Alternate Port concepts was not in STP.

Figure 2: Alternate Port

In Figure 2, Root Ports & designated ports are selected using the same rules as STP. However, in RSTP, Switch 2 picks G0/1 as an alternate port. An alternate port is the second-best option for the root port. This alternate port rapidly transitions to the root port in case there is an issue with the current root port without waiting for any timer to expire or going to multiple transient states.

For instance, let's say there is a link failure between switch 2 and switch 1 (root switch). Switch 3 and Switch 2 exchange RSTP messages to confirm that Switch 2 will move its alternate port to the root port. This action caused Switch 3 to clear all its MAC address entries related to Switch 2.

Switch 2 transitions its Gig0/2 port to the disabled state and Gig0/1 to the root port. Switch 2 transitions to the forwarding state on Gig0/1 without going through the learning state.

RSTP new port types

Consider a direct connection between two switches, in RSTP it is called the Point to Point Port.
The Connection between a Switch and the host is called the point-to-point Edge port or simply the edge port.
The connection between a switch and a hub is called the shared port.

Figure 2: RSTP Port States

With this let’s conclude this and we’ll be back with few more topics on STP. Hope you find it informative.

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CCNA RnS Article #34: RSTP Concepts