routeprotocol.com

Datalink Layer

  • UDLD – Unidirectional Link Detection

    Unidirectional Link Detection allows for monitoring of physical media that separates its send and receive media, like fibre optic cables. UDLD operates by transmitting a packet to the neighbour device that contains a system and port ID of the originating interface. The receiving interface gets this information and mirrors it back including its own system…

  • Spanning Tree Protocol – Loop Guard

    Loop Guard is a technology that assists with physical layer technologies that can end up in a scenario of one way traffic only (only send or only receive). In such a scenario happening the switch may detect an interface in an ‘up’ state since it can receive traffic from its peer, but may not be…

  • Spanning Tree Protocol – BPDU Filter

    BPDU filter blocks Bridge Protocol Data Units from transmitted out of a port. Similar to Portfast and BPDU Guard it can be enabled globally or on a per interface level. To enable BPDU Filter globally across all interfaces on a switch that have Portfast enabled: spanning-tree portfast bpdufilter default To enable BPDU filter on an…

  • Spanning Tree Protocol – BPDU Guard

    BPDU Guard is a safety protocol that can compliment portfast. If a BPDU is received on an interface, rather than converting the interface to a spanning-tree supported one it will shut down the interface altogether via an errdisable This helps prevent an unauthorised switch from joining the network and sending out BPDUs through a portfast…

  • Spanning Tree Protocol – Portfast

    Generating a Topology Change Notification for link state changes on single devices does not make sense in a network due device normally only having one connection to the network. Using the Portfast feature on Cisco Catalyst switches disables the topology change notification being generated if a link state changes to a single user device. An…

  • Root Guard (Spanning Tree Protocol)

    Root Guard is a STP feature that can be enabled on individual ports. When root guard is enabled on an individual port it disable the port (via err-disable) if a switch on that port attempts to signal it is the root bridge to the local switch. The idea of this protection is to prevent unknown…

  • Network Loops and Spanning Tree Protocol

    In a layer 2 topology, the network packets do not decrement their time-to-live counters as they would in a layer 3 network. To put it simply, packets on the data-link layer can forward continuously until the network is overwhelmed. Symptoms of a forwarding loop can include high processor utilisation and memory usage, slow connectivity and…

  • Modifying Spanning Tree Priority

    The spanning tree priority option is part of the decision process in choosing the alternative port when multiple links are established between neighbouring switches. The spanning tree priority can be modified with the command spanning-tree port-priority X on the interface. This can be more specific by including the VLAN number too in the example spanning-tree…

  • Modifying the Root or Blocked Port in Spanning Tree Topologies

    When a switch generates a Bridge Protocol Data Unit (BPDU) out of its interfaces, the total path cost only includes the path to the root excluding the interface that the BPDU is advertised out of. It is the switch that receives that BPDU that adds the cost associated with the receiving interface to the total…

  • Root Bridge Placement in Spanning Tree Protocol

    In most networks, the root bridge of the spanning tree topology should be placed on the most central core switch of the network. Switches by default use a priority of 32,768 meaning that without adjustment the root bridge election is going to be assigned to a switch with the lowest MAC address. This does not…