Switching modes: store-and-forward vs. cut-through

When a switch starts sending a frame, and what that means for latency and errors.

Intermediate · 5 min read

Switching modes are the methods a switch uses to decide when to start forwarding a frame it is receiving. Store-and-forward receives the whole frame and checks its FCS first, cut-through starts forwarding as soon as it has read the destination MAC address, and fragment-free waits for the first 64 bytes, which is enough to filter out most collision fragments.

In simple terms: A switch can wait for the whole frame and check it for errors before sending it on, or start sending as soon as it knows where the frame goes. Waiting is safer; starting early is slightly faster.

The question

A frame arrives bit by bit. The switch needs the destination MAC to decide where it goes, and that's in the first 6 bytes. Should it start sending straight away, or wait for the whole frame?

Where each mode starts forwarding

Dest MAC

Src MAC

Type

Payload …

FCS

↑ Cut-through starts

↑ Fragment-free starts (64 bytes)

↑ Store-and-forward starts (after FCS)

Not to scale: the payload can be up to 1500 bytes.

Mode	Starts forwarding after	Latency	Bad frames
Store-and-forward	The whole frame, after checking the FCS	Highest; grows with frame size	Dropped: corrupt frames never leave the switch
Cut-through	The destination MAC	Lowest, constant	Forwarded: the error is found too late (counted, not stopped)
Fragment-free	The first 64 bytes	Low	Collision fragments dropped; other corruption forwarded

Which is used where?

Campus switches (for example most Cisco Catalyst models) use store-and-forward. The extra microseconds don't matter for users, and dropping corrupt frames keeps errors local.
Data-centre switches (for example many Cisco Nexus models) often use cut-through where low, predictable latency matters, such as storage and trading networks.
A switch must store-and-forward when the ports run at different speeds (e.g. 1 Gbps in, 10 Gbps out), or when it's congested and has to buffer anyway.
Fragment-free mattered in the era of collisions; it's rare today.

Check yourself

Predict · scenario 1

Which mode drops a frame whose FCS is wrong before forwarding it?

Predict · scenario 2

Why must a switch store-and-forward from a 1 Gbps port to a 10 Gbps port?