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    CCNP BCMSN Notes – Spanning Tree Configuration

    By Raiy Wong | November 3, 2009

    Root Bridge Configuration

    The root bridge should be positioned centrally in the network to ensure the spanning tree forms in a predictable manner.

    Two bridge ID formats are available:

             802.1D Standard – 16-bit priority + unique MAC address for the VLAN

             802.1t Extended – 4-bit priority multiplier + 12-bit VLAN ID + non-unique MAC address

    The extended ID format is enabled by default, or with spanning-tree extend system-id.

    An extended system ID priority must be a multiple of 4096.

    There are two ways to designate the root bridge.

    Setting a static priority:

           clip_image002

    Allowing the switch to decide its own priority relative to other switches on the network:

           clip_image004

    Port Cost and ID Tuning

    To manually configure the cost for a port (and override the default cost associated with its speed):

           clip_image006

    The port ID consists of an 4-bit port priority (default value is 128) and a 12-bit port number.

    To manually configure the priority of a port:

           clip_image008

    Tuning Spanning-Tree Convergence

    Manually configuring STP timers:

         clip_image010

    Timers can also be automatically adjusted when designating an automatically determined bridge priority(via the diameter parameter):

         clip_image012

    Redundant Link Convergence

           PortFast – Applied to access ports to allow fast establishment of connectivity

           UplinkFast – Enables fast failover to an alternate uplink toward root

           BackboneFast – Enables fast convergence in the core after a topology change

    PortFast

    Can be enabled globally:

         clip_image014

    Or, per-interface:

         clip_image016

    UplinkFast

    Can only be enabled on switches which do not act as transit to root (and are not root).

    Enabled globally, for all ports and VLANs:

        clip_image018

    The optional max-update-rate (default 150) specifies how fast to flood out spoofed multicast frames

    from sources in the CAM so that upstream switches see them on the new link.

    BackboneFast

    BackboneFast allows a switch to respond to inferior BPDUs (BPDUs from a new switch claiming to be root) immediately, instead of waiting for the Max Age timer to expire.

    Root Link Query (RLQ) protocol requests are sent out to see if upstream switches have a path to root.

    BackboneFast is enabled globally:

        clip_image020

    If enabled, BackboneFast should be enabled on all switches in the domain, due to its reliance on RLQ.

    Troubleshooting Spanning-Tree

          show spanning-tree [detail | summary | …]

             

    Topics: CCNP, CCNP Notes | No Comments »

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