---
id: 20. Protocole Spanning Tree
aliases: []
tags: []
---

# Protocole Spanning Tree

## Networks Redundancy

- Redundandy is an essential part of network design.
- Modern networks are expected to run 24/7/365
- if one network component fails, you must ensure that other components will take over
with little or no downtime
- As much as possible, you must implement redundancy at every possible point in the network

      note: Most PCs only have a single network interface card [[NIC]], so they can only be plugged
      into a single switch. However, important servers typically have multiple NICs, so they can be
      plugged into multiple switches for redundancy

The [[Ethernet header]] doesn't have a [[TTL]] field. These broadcast frames will loop around
the network indefinitely, if enough of these looped broadcasts accumulate in the network, the 
network will be too congested for legitimate traffic to use the network. This is called 
[[Broadcast Storm]]

Network congestion isn't the only problem. each time a frame arrives on a switchport, the switch
uses the source [[MAC]] address field to learn the MAC address and update its MAC address table.
When a frames with the same source MAC address repeatedly arrive on different interfaces,
the switch is continously updating the interface in its MAC address table. this is known as 
[[MAC Address Flapping]]

## Spanning Tree protocol

- Classic Spanning Tree Protocol is **IEEE 802.1D**
- Switches from all vendors run STP by default.
- STP prevents Layer 2 loops by placing redundant ports in a blocking state, essentially disabling
the interface.
- These interfaces act as backups that can enter a forwarding state if an active (=currently forwarding)
interface fails
- Interfaces in a frowarding state behave normally. They send and receive all normal traffic.
- Interfaces in a blocking state only send or receive STP messages (called **[[BPDU]]s**)
Bridge Protocol Data Units

      note: Spanning Tree Protocol still use the term [[Bridge]]. However, when we use the term 
      bridge, we really mean [[Switch]]. Bridges are not used in modern networks.

- By selecting which ports are forwarding and which ports are blocking, STP creates a single path to/from
each point in the network. This prevents Layer 2 loops.
- There us a set process that STP uses to determine which ports should be forwartding and which should 
be blocking
- STP-enanbled switches send/receive Hello BPDUs out of all interfaces, the default timer is 
2 seconds (the switch will send a Hello BPDU out of every interface, once every 2 secong)
- If a switch receives a Hello BPDUs on an interface, it knows that interface is connected to another 
switch (routers, PCs, etc. do not use STP, so they do not send Hello BPDUs)

- Switches use one field in the STP BPDU, the Bridge ID field, to elect a root bridge for the network
- The switch with the lowest Bridge ID becomes the root bridge.
- ALL ports on the root bridge are put in a forwarding state,and other switches in the topology
must have a path to reach the root bridge

+---------------------------------+
|            Bridge ID            |
|------------------+--------------|
| Bridge Priority  |  Mac address |
|     16 bits      |  48 bits     |
+---------------------------------+

The default bridge priority is 32768 on all switches, so by default the MAC address is used 
as the tie-breaker (lowest MAC address becomes the root bridge)

**The Bridge Priority is compared first. if they tie, the MAC address is then compared**

Howerver the Bride ID have been updated

+---------------------------------+
|            Bridge ID            |
|------------------+--------------|
| Bridge Priority  |  Mac address |
|     16 bits      |  48 bits     |
+---------------------------------+
         |
+----------------------------+
| Bridge | Extended System ID|
|Priority|    (VLAN ID)      |
|  4bits |     12 bits       |
+----------------------------+

Cisco switches use a version of STP called PVST (per-VLAN Spanning Tree).
PVST runs a separate STP instance in each VLAN, so in each VLAN different interfaces
can be forwarding/blocking

in the default VLAN of 1, the default bridge priority is actually 32769 (32768 + 1)

The STP bridge priority can only be changed in units of 4096

All interface on the root bridge are **designated ports**. designated ports are in a forwarding state

Whe a switch is powered on, it assumes it is the root bridge.
it will only give up its position if it receives a superior BPDU (lower bridge ID)

Once the topology has converged and all switches agree on the root bridge, only the root bridge sends BPDUs
Other switches in the network will forward these BPDUs, but will not generate their own original BPDUs

### STP Cost

| Speed | STP Cost |
| -------------- | --------------- |
| 10Mbps | 100 |
| 100Mbps | 19 |
| 100Gbps | 4 |
| 10Gbps | 2 |

The ports connected to another switch's root port MUST be designated. Because the root port is the switch's path to the root bridge
another switch must not block it

### Port ID

STP Port ID = port priority (default 128) + port number

Every collision domain has a single STP designated port

### Steps

1) The switch with the lowest bridge ID is elected as the root bridge. All ports on the root bridge are designated ports (forwarding state).
2) Each remaining switch will select ONE of its interfaces to be its root port. The interface with the lowest root cost will
be the root port. Root ports are also in a forwarding state
Root port selection:
  - lowest root cost
  - lowest neighbor bridge ID
  - lowest neighbor port ID
3) Each remaining collision domain will select ONE interface to be a designated port (forwarding state). THe other port in the
collision domain will be non-designated (blocking)
Designated port selection:
  - Interface on switch wih lowest root cost
  - Interface on switch wih lowest bridge ID
  
## Review

- Redundancy in networks
- STP (Spanning Tree Protocol)