science

Unicast flood

Unicast flood is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Unicast flood rather than just read about it. In short: In computer networking, a unicast flood occurs when a switch receives a unicast frame and the switch does not know that the addressee is on any particular switch port. Since the switch has no information regarding which port, if any, the addressee might be reached through, it forwards the frame through all ports aside from the one through which the frame was received.

Key takeaways

Unicast flood belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Unicast flood to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Unicast flood from memory before moving on to harder problems.

Reference excerpt

In computer networking, a unicast flood occurs when a switch receives a unicast frame and the switch does not know that the addressee is on any particular switch port. Since the switch has no information regarding which port, if any, the addressee might be reached through, it forwards the frame through all ports aside from the one through which the frame was received.

Background Unicast refers to a one-to-one transmission from one node in a network to another. This diagram illustrates the unicast transmission of a frame from one network node to another:

When a switch receives a unicast frame with a destination address not in the switch’s forwarding table, the frame is treated like a broadcast frame and sent to all network segments to which it is attached except the one from which it received the frame:

Causes The learning process of transparent bridging requires that the switch receive a frame from a device before unicast frames can be forwarded to it. Before any such transmission is received, unicast flooding is used to ensure transmissions reach their intended destinations. This is normally a short-lived condition as receipt typically produces a response that completes the learning process. The process occurs when a device is initially connected to a network segment, or after its address and port identifier is purged from the forwarding information base. An entry is purged when the link goes down on the original port or when it expires due to inactivity (five minutes is the default on many switches). A time limit is necessary because a switch does not necessarily see any indication when a network node is moved or disconnected. When a bridge or switch has no room left in its forwarding information base and so cannot add an entry for a new node, it must forward any frame addressed to that node through all ports except the one on which the frame was received. This is a common problem on networks with many hosts. Less common is the artificial flooding of address tables in a MAC flooding attack. Another common cause is a host with an ARP cache timeout longer than the timeout of the forwarding information base (FIB) in a switch—the switch forgets which port connects to the target before the host forgets the MAC address of the target. This can be prevented by configuring the switch with a FIB timeout longer than the ARP cache timeouts of nodes on its network. When a node needs to send a frame to a host after its corresponding ARP cache entry expires it must first send an ARP broadcast frame, which the switch must forward through all ports, to discover the (current) MAC address of the host. Misconfigured features of the networks may lead to unicast flooding as well. If there are two layer-2 paths from Host A to B and Host A uses path 1 to talk to Host B, but Host B uses path 2 to respond to Host A, then intermediate switches on path 1 will never learn the destination MAC address of Host B and intermediate switches on path 2 will never learn the destination MAC address of Host A. A final cause of unicast floods are topology changes. When a link state changes on a network port that participates in Rapid Spanning Tree Protocol, the address cache on that switch will be flushed causing all subsequent frames to be flooded out of all ports until the addresses are relearned by the switch.

Remedies A feature blocking unicast floods is available on Cisco switches but is not enabled by default. After ensuring that timeouts and security features have been configured to maintain table entries on client access ports longer than typical host ARP cache timeouts, this command is used to quiet down the unicast floods on those ports:

Switch(config-if)# switchport block unicast

Other techniques involve isolating hosts at Layer 2. Ports configured as protected ports are forbidden to communicate with other protected ports. Private VLANs implements port isolation such that members of the VLAN are only allowed to communicate via a designated uplink are not allowed to talk to other members of the VLAN.

Effects on Networks When a network is experiencing unicast flooding, network performance may be degraded. Here is a graph of a bridge before and after adjusting the size of the bridge address cache:

80% of the frames were flooded out never to be received by the destination address, while 20% was valid traffic. In high-volume networks, the flooded traffic may cause ports to saturate, leading to packet loss and high latency. Another side effect of exhausted address tables is the compromise of data. The security considerations are discussed in the MAC flooding—one of several causes of unicast floods. If an end user is running a packet sniffer, the flooded frames can be captured and viewed.

See also Broadcast, unknown-unicast and multicast traffic

References

Illustrations

Worked examples

Example 1 — a first encounter with Unicast flood

Start with the simplest possible case. Write down what Unicast flood claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Unicast flood before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Unicast flood ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Unicast flood

In research: Unicast flood appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Unicast flood in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Unicast flood is common in secondary-school and first-year university syllabi. It links to neighbouring topics Internet architecture, so understanding it makes those chapters shorter.
In everyday life: Look for Unicast flood outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Unicast flood in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Unicast flood means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Unicast flood out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Unicast flood in simple terms?

Why does Unicast flood matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Unicast flood?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Unicast flood.