1.
6 Frame Forwarding
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Frame Forwarding
Switching in Networking
Two terms are associated with frames entering or
leaving an interface:
• Ingress – entering the interface
• Egress – exiting the interface
A switch forwards based on the ingress interface
and the destination MAC address.
A switch uses its MAC address table to make
forwarding decisions.
Note: A switch will never allow traffic to be
forwarded out the interface it received the traffic.
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Frame Forwarding
The Switch MAC Address Table
A switch will use the destination MAC address to determine the egress interface.
Before a switch can make this decision it must learn what interface the destination is
located.
A switch builds a MAC address table, also known as a Content Addressable Memory
(CAM) table, by recording the source MAC address into the table along with the port
it was received.
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Frame Forwarding
The Switch Learn and Forward Method
The switch uses a two step process:
Step 1. Learn – Examines Source Address
• Adds the source MAC if not in table
• Resets the time out setting back to 5 minutes if source is in the table
Step 2. Forward – Examines Destination Address
• If the destination MAC is in the MAC address table it is forwarded out the specified port.
• If a destination MAC is not in the table, it is flooded out all interfaces except the one it was
received.
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Frame Forwarding
Video – MAC Address Tables on Connected Switches
This video will cover the following:
• How switches build MAC address tables
• How switches forward frames based on the content of their MAC address tables
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Frame Forwarding
Switch Forwarding Methods
Switches use software on application-specific-integrated circuits (ASICs) to make
very quick decisions.
A switch will use one of two methods to make forwarding decisions after it receives a
frame:
• Store-and-forward switching - Receives the entire frame and ensures the frame
is valid. Store-and-forward switching is Cisco’s preferred switching method.
• Cut-through switching – Forwards the frame immediately after determining the
destination MAC address of an incoming frame and the egress port.
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Frame Forwarding
Store-and-Forward Switching
Store-and-forward has two primary characteristics:
• Error Checking – The switch will check the Frame Check Sequence (FCS) for CRC errors.
Bad frames will be discarded.
• Buffering – The ingress interface will buffer the frame while it checks the FCS. This also
allows the switch to adjust to a potential difference in speeds between the ingress and
egress ports.
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Frame Forwarding
Cut-Through Switching • Cut-through forwards the frame immediately
after determining the destination MAC.
• Fragment (Frag) Free method will check the
destination and ensure that the frame is at
least 64 Bytes. This will eliminate runts.
Concepts of Cut-Through switching:
• Is appropriate for switches needing latency
to be under 10 microseconds
• Does not check the FCS, so it can
propagate errors
• May lead to bandwidth issues if the switch
propagates too many errors
• Cannot support ports with differing speeds
going from ingress to egress
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