Spanning Tree Protokolü

Giriş
Spanning Tree Protokolü broadcast paketlerin Layer 2 Switch'ler arasında sonsuz döngüye girmesini engellemek için kullanılır.
Radio Perlman tarafından geliştirilmiştir.

Elimizde şöyle bir ağ tasarımı olsun

A<-->B<-->C
^                 ^
|--------------|

Ethernet paketlerinde TTL alanı olmadığı için A switch'ine gelen broadcast paket B'ye, oradan C'ye ve oradan yine A'ya gelecektir. Böylece sonsuz döngüye girer. Spanning Tree Protokolü bu sonsuz döngüyü engeller.

Halka şeklindeki bir ağda tek bir SPT Switch olması yeterlidir. Şöyle yaparız.

                             [router]
                                 |
|----------------------------[SPT switch]----------------------|
|                                                              |
|--[s]--[s]--[s]--[s]--[s]--[s]--[s]--[s]--[s]--[s]--[s]--[s]--|

Tüm Cihazlar STP'yi Desteklemez
Açıklaması şöyle.

To oversimplify, devices running STP will broadcast BPDUs and listen for duplicate messages. If duplicate messages are heard, a loop will be detected and the STP domain will converge by blocking the appropriate ports.

Many consumer-grade devices do not call out support for IEEE 802.1D or newer flavors of STP such as this common D-Link switch.

Bridge Protocol Data Units - BDU
Açıklaması şöyle.

STP was originally standardized as IEEE 802.1D. Its purpose is to build loop-free, layer 2 typologies. STP does not detect and then mitigate Broadcast Radiation. STP builds loop-free topologies by sending special frames called Bridge Protocol Data Units.

Açıklaması şöyle.

BPDUs are sent out as multicast frames to which only other layer 2 switches or bridges are listening. If any loops (multiple possible paths between switches) are found in the network topology, the switches will co-operate to disable a port or ports to ensure that there are no loops; that is, from one device to any other device in the layer 2 network, only one path can be taken. - Configuring Spanning Tree Protocol

Root Bridge Seçimi
Seçim şöyledir.

When the switches first come up, they start the root switch selection process. Each switch transmits a BPDU to the directly connected switch on a per-VLAN basis.

The Configuration BPDU has the BID (bridge priority and unique device identifier) and it used during initial setup to find out who will be the root bridge. Lower is preferable.

Root Bridge Seçimi Ne Zaman Biter
Açıklaması şöyle.

When the received root ID is the same as the advertised root ID, the election is done, as far as that bridge is concerned. When all the bridges agree, we say the network has converged. But each bridge doesn't "know" when that happens.

Çok Katlı Switchler
Spanning Tree yine tek bir yol oluşturur. Açıklaması şöyle.

Spanning tree will make sure that all traffic from one switch to another switch only has a single path, and that is through the root switch, which really needs to be the distribution switch.

STP guarantees a loop-free path. The way it does that is to create a single path that is most direct to the root switch. All other paths are placed in blocking. If you had multiple paths that allowed layer-2 loops, then your network would crash with broadcast storms.