RSTP Intro

Cico's summary:

"RSTP is not a timer-based spannig tree algorithm like 802.1D. Therefores, RSTP offers an improvement over the 30 seconds or more that 802.1D takes to move a link to forwarding. The heart of the protocol is a new bridge-bridge handshake mechanism, which allows ports to move directly to forwarding."

STP and RSTP

Similarities

• RSTP serves the same purpose as STP, blocking specific ports to prevent Layer 2 loops.

• RSTO elects...

  • ... a root bridge with the same rules as STP.

  • ... root ports with the same rules as STP.

  • ... designated ports with the same rules as STP.

Speed	STP Cost	RSTP Cost
10 Mbps	100	2,000,000
100 Mbps	19	200,000
1 Gbps	4	20,000
10 Gbps	2	2000
100 Gbps	X	200
1 Tbps	X	20

Port States

STP Port State	BPDUs	Frame forwarding (regular traffic)	MAC address learning	Stable/Transitional
Discarding	Only receives	No	No	Stale
Learning	Sends and receives	No	Yes	Transtional
Forwarding	Sends and receives	Yes	Yes	Stable

• If a port is administratively disabled (shutdown command) = discarding state.

• If a port is enabled but blocking traffic to prevent Layer 2 loops = discarding state.

Port Roles

The root port role remains unchanged in RSTP.

• The port that is closest to the root bridge becomes the root port for the switch.

• The root bridge is the only switch that doesn't have a root port.

The designated port role remains unchanged in RSTP.

• The port on a segment (collision domain) that sends the best BPDU is that segment's designated port (only one per segment).

The non-designated port role is split into two separate roles in RSTP.

1. alternate port.

2. backup port.

Alternate port

The RSTP alternate port role is a discarding port that receives a superior BPDU from another switch.

This is the same as what you've learned about blocking ports in classic STP.

Functions as a backup to the root port.

If the root port fails, the switch can immediately move its best alternate port to forwarding.

This immediate move to forwarding state functions like a classic STP optional feature called UplinkFast. Because it is built into RSTP, you do not need to activate UplinkFast when using RSTP/Rapid PVST+.

BackboneFast

One more STP optional feature that was built into RSTP is BackboneFast.

BackboneFast allows SW3 to expire the made age timers on its interface and rapidly forward the superior BPDUs to SW2.

This functionality is built into RSTP, so it does not need to be configured.

UplinkFast and BackboneFast

• UplinkFast and BackboneFast are two optional features in classic STP. They must be configured to operate on the switch (not necessary to know for CCNA).

• Both features are built into RSTP, so you do not have to configure them. They operate by default.

Backup port

The RSTP backup port role is a discarding port that receives a superior BPDU from another interface on the same switch.

This only happens when two interfaces are connected to the same collision domain (via a hub).

Hubs are not used in modern networks, so you will probably not encounter an RSTP backup port.

Function as a backup for a designated port.

The interface with the lowest port ID will be selected as the designated port, and the other will be the backup port.