Lesson 23 - Introduction to Rapid STP (802.1w)

By

Lesson 23 - Introduction to Rapid STP (802.1w)

Good news is that if you have learned 802.1d protocol (STP standard), only few things change in terms of the terminology and operation in the Rapid Spanning-Tree Protocol. RSTP is clearly an evolution of the regular STP but definitely NOT the revolution. The three phases of operation mentioned in lesson 20 still apply. 

This post is going to be the last in the series on Spanning-Tree Protocol fundamentals. 



Let's focus on the major changes introduced in RSTP by comparing it to the standard STP.

  • STP Port States: "disabled", "blocking", "listening" are renamed to DISCARDING in RSTP. As far as the "forwarding" state, the name remains unchanged.
  • STP Port Roles: designated and root roles are still the same. But the non-designated role has been split in two new ones: ALTERNATE and BACKUP roles in RSTP.
Below picture will give you an indication as to what they describe.

Pic. 1 - Rapid STP Alternate Port.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

An alternate port is a port that is in a blocking state and receives superior (better) BPDU frames from another switch. The port F0/2 of SW2 is the example of alternate port.

Pic. 2 - Rapid STP Backup Port.
Icons designed by: Andrzej Szoblik - http://www.newo.pl

A backup port is a port in the blocking state that receives superior BPDU frames from the same switch like shown in Pic. 2. The port F0/3 of SW3 is the example of backup port.

Cisco STP Enhancement such as: spanning-tree uplinkfast and spanning-tree backbonefast are no longer used (except for spanning-tree portfast which Cisco still maintain in the IOS as an edge port). They have been replaced with negotiations of the port state that is based on the previously unused flags in the BPDU frame and a different mechanism of sending BPDU frames. The flags are shown below (pic. 3).

Pic. 3 - BPDU Flags.

Understanding the nuts and bolts of the RSTP operation is beyond the scope of this tutorial. All you need to know at this stage is the terminology and benefits of using it over a legacy STP. The major advantage of RSTP is that switches use BPDU frames differently than before. Ports, upon first connection or when topology changes, exchange BPDUs with proposal/agreement flags set helping achieve fast transition to the appropriate state. They negotiate whether the port should be the designated, non-designated or root port role. If the port becomes the root port, all other ports begin negotiation by exchanging proposal/agreement BPDUs since the path towards the root bridge is different. It is extremely fast mechanism that speeds up the convergence upon a topology change.

A few other changes implemented in RSTP you must be aware of are as follows.
  • All switches exchange BPDUs instead of relaying them after they have received them from the root bridge. BPDUs have become a true keepalive mechanism. The switches do not have to wait for the root bridge to notify them about topology change that occurred somewhere in the network. A switch that detects change notifies its neighbors about it. And those in turn, notify their neighbors immediately. It is a mechanism similar to OSPF updates in that respect.
  • RSTP is backward compatible with STP (802.1d) speaking switches. Upon receiving BPDU version 1 (802.1d), the port transitions to the legacy STP protocol. All timers (max_age, forward_delay) are used according to the STP specifications (802.1d).
There are also two new concepts introduced in RSTP that are worth mentioning:
  1. Edge Port - which is something you are already familiar with. It is the port that is the candidate for a quick transition to forwarding state as it connects device that cannot create the loop (no BPDUs received on that port). Cisco enables Edge Port capability by using spanning-tree portfast feature described in the previous lesson. The ports must be a point-to-point link type (full duplex).
  2. Link Type - the type of the link is derived from the duplex of the port:
  • Full Duplex = Point-to-Point link
  • Half Duplex = Shared link
All these changes serve one purpose: they help in fast convergence on a detection of the new connection or/and a topology change. The speed of convergence is similar to the one when Cisco STP Enhancements are used in STP. The difference is that now fast convergence is accomplished as an industry standard mechanism, not a proprietary solution.

The configuration of RSTP is pretty straightforward.

SW(config)#spanning-tree mode rapid-pvst

That is it!

Deterministic election of the root bridge and all the jazz related to it explained in the previous lesson are identical.

If you want to learn more on RSTP operation google it by using this query:
understanding rstp. You'll find a plethora of documents out there.

Comments

Post a Comment

Popular posts from this blog

Lesson 27 - IPv4 Address Dissected - Part 1

By
Image
Lesson 27 - IPv4 Address Dissected - Part 1 If you already feel comfortable with binary-to-decimal and decimal-to-binary conversions you're up to this post's challenge: learning more about IP version 4 address (IPv4). If you need to brush upon the numeral conversions, check my  previous post . In one of the earlier posts on  TCP/IP traffic flow , I briefly described the importance of the addresses. In order for devices to communicate they must be uniquely identified in the network after all. There are typically three such identifiers we use to distinguish between devices: Names  - These are used by humans. Applications use addresses not names. Hence, DNS services out there (DNS service is used to resolve names-to-ip, and ip-to-names).  Layer 3 addresses  -  logical  and  hierarchical  unique identifiers of devices in the network. All layer 3 protocols such as IPX, Apple-Talk, IP etc. use different forms of layer 3 addresses.  Layer 2 addresses  -  flat  and most ofte
Read more

Lesson 21 - Spanning-Tree Protocol in Practice

By
Image
Lesson 21 - Spanning-Tree Protocol in Practice Previous post was designed to present in a nutshell the STP operation. However, without some practice it's just academic knowledge. I think it is a good idea to look at the same concepts using real equipment. Here goes... The below topology (pic. 1) uses redundant links which create the loops. Pic. 1 - Network Topology Icons designed by: Andrzej Szoblik -   http://www.newo.pl If there's one thing the administrator should do with such design, that would be configuring the root bridge. Typically, the most powerful switch in the center of the network plays that role. You do not want some access switch to be transmitting the frames between other switches. Access switches are designed to connect your computers to the network, and not to handle the majority of the traffic between the switches which root bridge must deal with. If you do not configure root bridge yourself, the switch with the lowest MAC address becomes th
Read more

Lesson 4 - Introduction to TCP/IP Layers

By
Image
Lesson 4 - Introduction to TCP/IP Layers In this lesson we take a sneak peek at the fundamentals regarding TCP/IP. This is one of the most important aspects to understand in order to follow the upcoming lessons. It is not my ambition to explain everything here as this would turn into a hefty book to read. You can find a lot of them on the market anyway. Instead, I will try to focus on some basic aspects of TCP/IP. They will constitute the minimum knowledge to help us understand how computers communicate. If you feel the urge to learn more on TCP/IP right now please visit the following site:  http://www.tcpipguide.com/free/t_toc.htm NOTICE! Familiarize yourself with ALL terms in red. They will be used throughout the classes. Introduction Computer communication follows some well defined rules and guidelines which we call  protocols . In order for the computers to exchange data they have to agree on using the same rules, otherwise they become incompatible. That was the case in t
Read more