Switching
A switch's MAC address table: learning, forwarding, flooding, filtering, ageing; collision and broadcast domains; the loop and the spanning tree.
- 23 cards
- 6 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
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- On the diagram
- Scenarios
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A few cards from the deck
MAC address table: what is it?
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The list a switch keeps that pairs each source address it has seen with the port it was seen on, and that it forgets after a period without traffic.
The switch learns from the source address of every frame received and looks up the destination address in the table. Ageing lets it follow a host that changes socket.
A four-port switch has just started, its table is empty. A, on port 1, sends a frame to C. What does it do?
- It learns A on port 1 and floods ports 2, 3 and 4
- It learns A on port 1 and sends the frame out of C's port only
- It drops the frame, having no entry for C in its table
- It floods ports 1, 2, 3 and 4 without learning anything
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It learns A on port 1 and floods ports 2, 3 and 4
It records the source (A, port 1) then, not knowing C, floods every port except the arrival one. It cannot guess C's port and never sends a frame back out of its arrival port.
The switch learns from the … address of every frame received, and picks the output port from the … address.
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The switch learns from the source address of every frame received, and picks the output port from the destination address.
Learning and forwarding are two separate acts on two different addresses: the source says where the sender is, the destination says where to send.
Flooding: what is it?
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With no entry in its table, the switch sends the frame out of every port except the one it arrived on.
Broadcast frames are always flooded. For a frame with an unknown destination, flooding stops as soon as the machine answers: its source is then learned.