update Spanning Tree
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shango-wk
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[[VLAN (Part 2)]]
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[[VLAN (Part 3)]]
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[[DTP - VTP]]
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[[Protocole Spanning Tree]]
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20. Protocole Spanning Tree (part1).md
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20. Protocole Spanning Tree (part1).md
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id: 20. Protocole Spanning Tree
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aliases: []
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tags: []
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---
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# Protocole Spanning Tree
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## Networks Redundancy
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- Redundandy is an essential part of network design.
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- Modern networks are expected to run 24/7/365
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- if one network component fails, you must ensure that other components will take over
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with little or no downtime
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- As much as possible, you must implement redundancy at every possible point in the network
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note: Most PCs only have a single network interface card [[NIC]], so they can only be plugged
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into a single switch. However, important servers typically have multiple NICs, so they can be
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plugged into multiple switches for redundancy
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The [[Ethernet header]] doesn't have a [[TTL]] field. These broadcast frames will loop around
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the network indefinitely, if enough of these looped broadcasts accumulate in the network, the
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network will be too congested for legitimate traffic to use the network. This is called
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[[Broadcast Storm]]
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Network congestion isn't the only problem. each time a frame arrives on a switchport, the switch
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uses the source [[MAC]] address field to learn the MAC address and update its MAC address table.
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When a frames with the same source MAC address repeatedly arrive on different interfaces,
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the switch is continously updating the interface in its MAC address table. this is known as
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[[MAC Address Flapping]]
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## Spanning Tree protocol
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- Classic Spanning Tree Protocol is **IEEE 802.1D**
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- Switches from all vendors run STP by default.
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- STP prevents Layer 2 loops by placing redundant ports in a blocking state, essentially disabling
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the interface.
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- These interfaces act as backups that can enter a forwarding state if an active (=currently forwarding)
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interface fails
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- Interfaces in a frowarding state behave normally. They send and receive all normal traffic.
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- Interfaces in a blocking state only send or receive STP messages (called **[[BPDU]]s**)
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Bridge Protocol Data Units
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note: Spanning Tree Protocol still use the term [[Bridge]]. However, when we use the term
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bridge, we really mean [[Switch]]. Bridges are not used in modern networks.
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- By selecting which ports are forwarding and which ports are blocking, STP creates a single path to/from
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each point in the network. This prevents Layer 2 loops.
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- There us a set process that STP uses to determine which ports should be forwartding and which should
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be blocking
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- STP-enanbled switches send/receive Hello BPDUs out of all interfaces, the default timer is
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2 seconds (the switch will send a Hello BPDU out of every interface, once every 2 secong)
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- If a switch receives a Hello BPDUs on an interface, it knows that interface is connected to another
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switch (routers, PCs, etc. do not use STP, so they do not send Hello BPDUs)
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- Switches use one field in the STP BPDU, the Bridge ID field, to elect a root bridge for the network
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- The switch with the lowest Bridge ID becomes the root bridge.
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- ALL ports on the root bridge are put in a forwarding state,and other switches in the topology
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must have a path to reach the root bridge
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+---------------------------------+
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| Bridge ID |
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|------------------+--------------|
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| Bridge Priority | Mac address |
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| 16 bits | 48 bits |
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+---------------------------------+
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The default bridge priority is 32768 on all switches, so by default the MAC address is used
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as the tie-breaker (lowest MAC address becomes the root bridge)
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**The Bridge Priority is compared first. if they tie, the MAC address is then compared**
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Howerver the Bride ID have been updated
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+---------------------------------+
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| Bridge ID |
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|------------------+--------------|
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| Bridge Priority | Mac address |
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| 16 bits | 48 bits |
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+---------------------------------+
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+----------------------------+
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| Bridge | Extended System ID|
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|Priority| (VLAN ID) |
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| 4bits | 12 bits |
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+----------------------------+
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Cisco switches use a version of STP called PVST (per-VLAN Spanning Tree).
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PVST runs a separate STP instance in each VLAN, so in each VLAN different interfaces
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can be forwarding/blocking
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in the default VLAN of 1, the default bridge priority is actually 32769 (32768 + 1)
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The STP bridge priority can only be changed in units of 4096
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All interface on the root bridge are **designated ports**. designated ports are in a forwarding state
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Whe a switch is powered on, it assumes it is the root bridge.
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it will only give up its position if it receives a superior BPDU (lower bridge ID)
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Once the topology has converged and all switches agree on the root bridge, only the root bridge sends BPDUs
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Other switches in the network will forward these BPDUs, but will not generate their own original BPDUs
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### STP Cost
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| Speed | STP Cost |
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| -------------- | --------------- |
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| 10Mbps | 100 |
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| 100Mbps | 19 |
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| 100Gbps | 4 |
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| 10Gbps | 2 |
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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
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another switch must not block it
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### Port ID
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STP Port ID = port priority (default 128) + port number
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Every collision domain has a single STP designated port
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### Steps
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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).
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2) Each remaining switch will select ONE of its interfaces to be its root port. The interface with the lowest root cost will
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be the root port. Root ports are also in a forwarding state
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Root port selection:
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- lowest root cost
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- lowest neighbor bridge ID
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- lowest neighbor port ID
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3) Each remaining collision domain will select ONE interface to be a designated port (forwarding state). THe other port in the
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collision domain will be non-designated (blocking)
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Designated port selection:
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- Interface on switch wih lowest root cost
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- Interface on switch wih lowest bridge ID
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## Review
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- Redundancy in networks
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- STP (Spanning Tree Protocol)
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21. Protocole Spanning Tree (part2).md
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21. Protocole Spanning Tree (part2).md
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---
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id: 1778913634-WEMK
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aliases:
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- Protocole Spanning Tree (part2)
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tags: []
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---
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# Protocole Spanning Tree (part2)
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## Spanning Tree Port State
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| STP Port State | Stable/Transitional |
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| -------------- | --------------- |
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| Blocking | Statble |
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| Listening | Transitional |
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| Learning | Transitional |
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| Forwarding | Stable |
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| Disabled | Stable |
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- Root/Designated ports remain stab;e in Forwarding state.
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- Non-designated ports remain stabl in a Blocking state.
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- Listening and Learning are transitional states which are passed through when an interface is
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activated, or when a *Blocking* port must transition to a Forwarding state due to a change in the
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network topology
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### Blocking state
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- Non designated ports are in a Blocking state
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- Interfaces in a Blocking state are effectively disabed to prevent loops.
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- Interfaces in a Blocking stae do not send/receive regular naetwork traffic.
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- Interfaces in a Blocking state receive STP BPDUs.
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- Interfaces in a Blocing state do NOT forward STP BPDUs.
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- Interfaces in a Blocking state do NOT learn [[MAC]] addresses.
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### Listening state
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- After the Blocking state interfaces with the Designated or Root role enter Listening state.
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- Only Designated or Root ports enter the Listening state (Non-designated ports are always Blocking)
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- The Listening state is 15 seconds long by default. This is determined by the *orward delay* timer.
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- An interface in the listening state ONLY forwards/receives STP BPDUs.
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- An interface in the Listening state does NOT send/receive regular traffic
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- An interface in the Listening state does NOT Learn MAC addresses from regular traffic that
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arrives on the interface
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### Learning state
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- After the Listeing state, a Designated or ROOT port will enter Learning state.
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- The Learning state is 15 seconds long by default. This is determined by the Forward delay
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timer ( the same timer is used for both the Listening and Learning states).
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- An interface in the Learning state ONLY sends/receives STP BPDUs.
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- An interface in the Learning state learns MAC addresses from regular traffic that arrives on
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the interface.
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### Forwarding state
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- Root and designated ports are in a Forwarding state.
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- A port in the Forwarding state operate as normal
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- A port in the Forwarding state sends/receives BPDUs.
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- A port in the Forwarding state sends/receives normal traffic.
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- A port in the Forwarding state learns MAC addresses.
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### summary
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| STP Port State |Send/Receive BPDUs | Frame forwarding | Mac address learning| Stable/Transitional |
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| -------------- |----|----|----| --------------- |
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| Blocking | No/Yes| NO | NO | Statble |
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| Listening | Yes/Yes| NO | NO | Transitional |
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| Learning | Yes/Yes| NO | Yes | Transitional |
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| Forwarding | Yes/Yes| Yes | Yes | Stable |
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| Disabled | NO/NO| NO | NO | Stable |
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note: Switches do not forward the BPDUs out of their root ports and non-designated ports,
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only their designated ports
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## STP timers
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### Hello
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How often the root bridge sends hello BPDUs
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duration: 2 sec
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### Forward delay
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How long the switch will stay in the Listening and Learning states (each stae is 15 seconds
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= total 30 seconds)
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duration: 15sec
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### Max Age
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How long an interface will wait after ceasing to receive Hello BDPUs to change the STP topology
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duration: 20 sec (10*hello)
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- If another BPDU i received before the max age timer counts down to 0, the time will reset to 20
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seconds and no changes will ocur.
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- If another BPDU is not received the max age timer counts down to 0 and the switch will
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reevaluate its STP choices, including root bridge, and local root, designated, and non-designated ports
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- if a non-designated port is selected to become a designated or root port, it will transition
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from the blocking state to the listening state (15 seconds), learning state (15 seconds),
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and then finally the forwarding state. So it can take a total of 50 seconds for a blocking
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interface to transition to forwarding
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## Review
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- STP state/timers
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- STP BPDU
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- STP optional features
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- STP configuration
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