Spanning Tree Review
Why do we need Spanning Tree
- Good network design provides redundancy in devices and network links
- The simplest solution involves adding a second link between switches to overcome a network link failure or ensuring that a switch is connected to at least two other switches in a topology.
- This can cause problems when a switch must forward broadcasts or when unknown unicast flooding occurs.
- Network broadcasts forward in a continuous loop until the link becomes saturated, and the switch is forced to drop packets.
- Also the MAC address table must constantly change ports as the packets make loops. The packets continue to loop around the topology because there is not a time-to-live (TTL) mechanism for Layer 2 forwarding. The switch CPU utilization increases, as does memory consumption, which could result in the crashing of the switch.
Common Causes of Layer 2 Loops
- STP disabled on a switch
- A misconfigured load balancer that transmits traffic out multiple ports with the same MAC address
- A misconfigured virtual switch that bridges two physical ports (Virtual switches typically do not participate in STP.)
- End users using a dumb network switch or hub
How Spanning Tree Works
- Spanning Tree Protocol (STP) enables switches to become aware of other switches through the advertisement and receipt of bridge protocol data units (BPDUs).
- STP builds a Layer 2 loop-free topology in an environment by temporarily blocking traffic on redundant ports.
- STP operates by selecting a specific switch as the master switch and running a tree-based algorithm to identify which redundant ports should not forward traffic.
Root bridge: The root bridge is the most important switch in the Layer 2 topology.
Bridge protocol data unit (BPDU): This network packet is used for network switches to identify a hierarchy and notify of changes in the topology.
Two Types of BPDUs:
- Configuration
- TCN
Max age: This is the maximum length of time that passes before a bridge port saves its BPDU information.
- default is 20 seconds command spanning-tree vlan vlan-id max-age maxage. If a switch loses contact with the BPDU’s source, it assumes that the BPDU information is still valid for the duration of the Max Age timer
Hello time: time that a BPDU is advertised out of a port.
- default is 2 seconds can be configured to 1 to 10 seconds with the command spanning-tree vlan vlan-id hellotime hello-time.
Forward delay: time that a port stays in a listening and learning state.
- default value is 15 seconds can be 15 to 30 seconds with the command spanning-tree vlan vlan-id forward-time forward-time.
Election
- Hello BPDUs are exchanged between all switches at the beginning of the root bridge election process
- each switch considers itself as the root bridge, in fact it sends its BPDUs with its bridge ID as the root bridge ID
- when a switch receives a BPDU with a lower bridge ID, it “adjusts” its BPDUs by sending them with the received lower bridge ID, otherwise it would just discard that BPDU with a higher bridge ID.
- That process continues until all switches agree on the root bridge which has the lowest bridge ID.
- In case all switches have same bridge priority then the tiebreaker here would be the mac address, the switch with the lowest mac address would be the root bridge.
- The bridge ID is combined by the bridge priority, System ID Extension (based on IEEE 802.1t standard) and the mac address of the switch.
Finding Topology Change Causes
- determining why TCNs are occurring involves checking a port to see whether it is connected to a host or to another switch.
- If it is connected to another switch, you need to connect to that switch and repeat the process of examining the STP details.
- You might need to examine CDP tables or your network documentation.
- You can execute the show spanningtree [vlan vlan-id] detail command again to find the last switch in the topology to identify the problematic port.
RSTP (802.1W) Port States
- Discarding: The switch port is enabled, but the port is not forwarding any traffic to ensure that a loop is not created. This state combines the traditional STP states disabled, blocking, and listening.
- Learning: The switch port modifies the MAC address table with any network traffic it receives. The switch still does not forward any other network traffic besides BPDUs.
- Forwarding: The switch port forwards all network traffic and updates the MAC address table as expected. This is the final state for a switch port to forward network traffic.
STP Tuning
- Ideally the root bridge is placed on a core switch, and a secondary root bridge is designated to minimize changes to the overall spanning tree
Root and Secondary Placement
- spanning-tree vlan vlan-id priority priority: The priority is a value between 0 and 61,440, in increments of 4,096.
- spanning-tree vlan vlan-id root {primary | secondary} [diameter diameter]: This command executes a script that modifies certain values. The primary keyword sets the priority
- Generally, root switches are at Layer 2/Layer 3 boundaries.
- minimize the number of hops to the furthest switch in the topology.
- BEST PRACTICE: set the priority to 0 for the primary root switch and to 4096 for the secondary root switch.
Oh Snap, There’s a Loop!
- Catalyst switches detect a MAC address that is flapping between interfaces and notify via syslog.
- MAC address of the host, VLAN, and ports between which the MAC address is flapping.
- These messages should be investigated to ensure that a forwarding loop does not exist
Root Guard
- Enabled on a port-by-port basis; it prevents a configured port from becoming a root port.
- Stops downstream switches from becoming the root bridge – Errdisables ports that receive the BPDU
- Command spanningtree guard root – Root guard is placed on designated ports toward other switches that should never become root bridges.
STP Portfast
- Placed on host ports, disables TCNs
- Bypass listen and learning state go straight to forwarding
- Saves time, used on Access ports
- Beneficial in environments where computers use Dynamic Host Configuration Protocol (DHCP) or Preboot Execution Environment (PXE).
- Command spanning-tree portfast trunk. ONLY USED ON SINGLE HOST EX: ESXI SERVER
- command spanning-tree portfast or globally on all access ports with the command spanning-tree portfast default
BPDU Guard
- BPDU guard is a safety mechanism that shuts down portfast ports upon receipt of a BPDU.
- Ensures that a loop cannot accidentally be created if an unauthorized switch is added to a topology.
- spanning-tree portfast bpduguard default. BPDU guard can be enabled or disabled on a specific interface with the command spanning-tree bpduguard {enable | disable}.
- BPDU guard is typically configured with all host-facing ports that are enabled with portfast.
- Use Error Recovery to recover ports that were shut down from BPDU guard errdisable recovery cause bpduguard
- The period that the Error Recovery checks for ports is configured with the command errdisable recovery interval time-seconds
BPDU Filter
- Blocks BPDUs from being transmitted out a port
- Most network designs do not require BPDU filter, which adds an unnecessary level of complexity and also introduces risk.
UDLD Fiber Trunking
- Unidirectional Link Detection (UDLD) allows for the bidirectional monitoring of fiber-optic cables.
Lab
SW-01 Configuration
SW-01#sh runn Building configuration… Current configuration : 3810 bytes ! ! Last configuration change at 19:53:21 UTC Sat Mar 13 2021 ! version 15.2 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption service compress-config ! hostname SW-01 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ! ! ! ! ! ! ! ! ip cef no ipv6 cef ! ! errdisable recovery cause bpduguard errdisable recovery interval 30 ! spanning-tree mode rapid-pvst spanning-tree extend system-id ! vlan internal allocation policy ascending ! ! ! ! ! ! ! ! ! ! ! ! ! ! interface GigabitEthernet0/0 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/2 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/3 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/0 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/1 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/2 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/3 description ACCESS-WIRED switchport access vlan 10 switchport mode access media-type rj45 negotiation auto spanning-tree portfast edge spanning-tree bpduguard enable ! ip forward-protocol nd ! no ip http server no ip http secure-server ! ! ! ! ! ! control-plane ! line con 0 line aux 0 line vty 0 4 ! ! end
SW-02 Configuration
SW-02#sh runn Building configuration… Current configuration : 3810 bytes ! ! Last configuration change at 19:53:22 UTC Sat Mar 13 2021 ! version 15.2 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption service compress-config ! hostname SW-02 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ! ! ! ! ! ! ! ! ip cef no ipv6 cef ! ! errdisable recovery cause bpduguard errdisable recovery interval 30 ! spanning-tree mode rapid-pvst spanning-tree extend system-id ! vlan internal allocation policy ascending ! ! ! ! ! ! ! ! ! ! ! ! ! ! interface GigabitEthernet0/0 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/2 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/3 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/0 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/1 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/2 description SHUTDOWN-NO-USERS switchport access vlan 999 switchport mode access shutdown media-type rj45 negotiation auto ! interface GigabitEthernet1/3 description ACCESS-WIRED switchport access vlan 10 switchport mode access media-type rj45 negotiation auto spanning-tree portfast edge spanning-tree bpduguard enable ! ip forward-protocol nd ! no ip http server no ip http secure-server ! ! ! ! ! ! control-plane ! line con 0 line aux 0 line vty 0 4 ! ! end SW-02#
SW-03 Configuration
SW-03#sh runn Building configuration… Current configuration : 3443 bytes ! ! Last configuration change at 19:53:38 UTC Sat Mar 13 2021 ! version 15.2 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption service compress-config ! hostname SW-03 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ! ! ! ! ! ! ! ! ip cef no ipv6 cef ! ! ! spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1-4094 priority 0 ! vlan internal allocation policy ascending ! ! ! ! ! ! ! ! ! ! ! ! ! ! interface GigabitEthernet0/0 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto spanning-tree guard root ! interface GigabitEthernet0/2 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto spanning-tree guard root ! interface GigabitEthernet0/3 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet1/0 media-type rj45 negotiation auto ! interface GigabitEthernet1/1 media-type rj45 negotiation auto ! interface GigabitEthernet1/2 media-type rj45 negotiation auto ! interface GigabitEthernet1/3 media-type rj45 negotiation auto ! ip forward-protocol nd ! no ip http server no ip http secure-server ! ! ! ! ! ! control-plane ! line con 0 line aux 0 line vty 0 4 ! ! end SW-03#
SW-04 Configuration
SW-04#sh runn Building configuration… Current configuration : 3446 bytes ! ! Last configuration change at 19:53:44 UTC Sat Mar 13 2021 ! version 15.2 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption service compress-config ! hostname SW-04 ! boot-start-marker boot-end-marker ! ! ! no aaa new-model ! ! ! ! ! ! ! ! ip cef no ipv6 cef ! ! ! spanning-tree mode rapid-pvst spanning-tree extend system-id spanning-tree vlan 1-4094 priority 4096 ! vlan internal allocation policy ascending ! ! ! ! ! ! ! ! ! ! ! ! ! ! interface GigabitEthernet0/0 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto spanning-tree guard root ! interface GigabitEthernet0/2 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto spanning-tree guard root ! interface GigabitEthernet0/3 switchport trunk encapsulation dot1q switchport trunk native vlan 666 switchport mode trunk media-type rj45 negotiation auto ! interface GigabitEthernet1/0 media-type rj45 negotiation auto ! interface GigabitEthernet1/1 media-type rj45 negotiation auto ! interface GigabitEthernet1/2 media-type rj45 negotiation auto ! interface GigabitEthernet1/3 media-type rj45 negotiation auto ! ip forward-protocol nd ! no ip http server no ip http secure-server ! ! ! ! ! ! control-plane ! line con 0 line aux 0 line vty 0 4 ! ! end
