Preventing Loops with Spanning Tree

STP (IEEE 802.1D) has two immediate drawbacks. The first issue is with convergence. It just takes too long for Layer 2 networks to converge. Depending on the type of failure, it could take anywhere from 30 to 50 seconds to converge the network. The other issue with STP is potential loops. Loops can be catastrophic, as discussed on numerous occasions throughout this book. Cisco introduced features such as portfast, Root Guard, and Loop Guard to help mitigate loop problems, as discussed in the next sections.


The portfast feature skips through various stages of spanning-tree states, and it immediately brings a port from blocking to forwarding state. There are two reasons behind portfast. First, the ...

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