Frame Forwarding Concepts PDF

Switching Concepts ITEP 309  Frame Forwarding  Frame Forwarding Two terms are associated with frames entering or leaving an interface: Ingress – entering the interface Egress – exiting the interface  Frame Forwarding A switch forwards based on the ingress interface...

Switching Concepts ITEP 309  Frame Forwarding  Frame Forwarding Two terms are associated with frames entering or leaving an interface: Ingress – entering the interface Egress – exiting the interface  Frame Forwarding A switch forwards based on the ingress interface and the destination MAC address. A switch will never allow traffic to be forwarded out the interface it received the traffic A switch uses its MAC address table to make forwarding decisions. Frame Forwarding  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.  Frame Forwarding The Switch MAC Address Table 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.  Frame Forwarding The Switch Learn and Forward Method The switch uses a two step process: Step 1. Learn – Examines Source Address. Step 2. Forward – Examines Destination Address.  Frame Forwarding The Switch Learn and Forward Method 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  Frame Forwarding The Switch Learn and Forward Method Step 2. Forward – Examines Destination Address. If the destination MAC is in the MAC address table it is forwarded out the specified port.  Frame Forwarding The Switch Learn and Forward Method Step 2. Forward – Examines Destination Address. If a destination MAC is not in the table, it is flooded out all interfaces except the one it was received. Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding Frame Forwarding  Frame Forwarding Switch Forwarding Methods Switches use software on application-specific-integrated circuits (ASICs) to make very quick decisions.  Frame Forwarding Switch Forwarding Methods A switch will use one of two methods to make forwarding decisions after it receives a frame: Store-and-forward switching Cut-through switching  Frame Forwarding Switch Forwarding Methods Store-and-forward switching Receives the entire frame and ensures the frame is valid. Store-and-forward switching is Cisco’s preferred switching method.  Frame Forwarding Switch Forwarding Methods Cut-through switching Forwards the frame immediately after determining the destination MAC address of an incoming frame and the egress port.  Frame Forwarding Store-and-Forward Switching Two primary characteristics: 1. Error Checking The switch will check the Frame Check Sequence (FCS) for CRC errors. Bad frames will be discarded.  Frame Forwarding Store-and-Forward Switching Two primary characteristics: 2. Buffering The ingress interface will buffer the frame while it checks the FCS.  Frame Forwarding Store-and-Forward Switching Two primary characteristics: 2. Buffering This also allows the switch to adjust to a potential difference in speeds between the ingress and egress ports.  Frame Forwarding Store-and-Forward Switching  Frame Forwarding Cut-Through Switching Cut-through forwards the frame immediately after determining the destination MAC.  Frame Forwarding Cut-Through Switching Fragment (Frag) Free method will check the destination and ensure that the frame is at least 64 Bytes. This will eliminate runts.  Frame Forwarding 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  Frame Forwarding Concepts of Cut-Through switching: May lead to bandwidth issues if the switch propagates too many errors Cannot support ports with differing speeds going from ingress to egress  Frame Forwarding Cut-Through Switching Switching Domains  Switching Domains Collision Domains  Switching Domains Collision Domains Switches eliminate collision domains and reduce congestion. When there is full duplex on the link the collision domains are eliminated.  Switching Domains Collision Domains Switches eliminate collision domains and reduce congestion. When there is one or more devices in half-duplex there will now be a collision domain.  Switching Domains Full Duplex vs Half Duplex  Switching Domains Auto -negotiation Fragment (Frag) Free method will check the destination and ensure that the frame is at least 64 Bytes. This will eliminate runts.  Switching Domains Broadcast Domains  Switching Domains Broadcast Domains A broadcast domain extends across all Layer 1 or Layer 2 devices on a LAN. When the layer 2 switch receives the broadcast it will flood it out all interfaces except for the ingress interface.  Switching Domains Broadcast Domains Too many broadcasts may cause congestion and poor network performance. Increasing devices at Layer 1 or layer 2 will cause the broadcast domain to expand  Switching Domains Broadcast Domain vs Collision Domain  Switching Domains Alleviated Network Congestion Switches use the MAC address table and full-duplex to eliminate collisions and avoid congestion. END OF PRESENTATION