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Oct 14

Troubleshooting Spanning Trunk Protocol (STP)

Troubleshooting Spanning Trunk Protocol (STP)

There area unit specific commands that apply to the Catalyst 6500/6000 only; but you’ll be able to apply most of the principles to any Cisco Catalyst switch that runs Cisco IOS package.

Most STP troubleshooting revolves around 3 issues:

forwarding loops

excessive flooding because of a high rate of STP Topology Changes (TC)

issues associated with convergence time

Because bridging doesn’t have any mechanism to trace whether or not an exact packet is being forwarded multiple times (for example, associate informatics Time to measure [TTL] is employed to discard traffic that’s current too long within the network), only 1 path will exist between 2 devices within the same Layer two (L2) domain.

The purpose of STP is to dam redundant ports supported associate STP formula, to resolve redundant topology into a tree-like topology. A forwarding loop (such as associate STP loop) happens once no port in a very redundant topology is blocked, and traffic is forwarded in a circle indefinitely.

Once the forwarding loop starts, it’ll probably choke off the lowest-bandwidth links on its path—if all the links area unit of a similar information measure, all links can probably be full. This congestion can cause packet loss and can cause a network down within the affected L2 domain.

With excessive flooding, the symptoms won’t be as clear. Some slow links may become full by flooded traffic, and devices or users behind these full links may ability slowness or total loss of property.

Requirements

Cisco recommends that you simply have information of those topics:

Various Spanning Tree sorts and the way to tack together them. check with Configuring STP and IEEE 802.1s Mountain Standard Time for added data.

Various Spanning Tree options and the way to tack together them. check with Configuring STP options for added data.

Components Used

The information during this document is predicated on these package and hardware versions:

Catalyst 6500 with Supervisor two engine

Why STP Fails?

STP makes sure assumptions concerning its operational surroundings. These area unit the assumptions most relevant to the current document:

Each bridge that’s running STP is ready to often receive, process, and send STP Bridge Protocol knowledge Units (BPDUs), additionally referred to as STP packets.

While these assumptions seem logical and obvious, there area unit things once they aren’t met. Most of those things involve hardware issue; but package defects may additionally cause STP failures.

many hardware failures, misconfigurations, or mis cabling cause the bulk of STP failures, while package failures account for the minority. STP failures can even occur because of unessential extra connections that exist between the switches. VLANs enter a down state due to these extra connections. To resolve this drawback, take away all the unwanted connections between the switches.

When one in every of these assumptions isn’t met, one or added bridges may now not receive or method the BPDUs. this implies that the bridge (or bridges) won’t be able to discover the constellation. while not information of the right topology, the switch cannot block the loops. Therefore, the flooded traffic can flow into over the whorled topology, consume all information measure, and convey down the network.

Before you begin to troubleshoot, you need to get this information:

An actual topology diagram that details all the switches and bridges

Their corresponding (interconnecting) port numbers

STP configuration details, like that switch is that the root and backup root, that links have a non-default value or priority, and the location of obstruction ports

Generally, troubleshooting involves these steps (depending on true, some steps might not be necessary):

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