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PACKET FORWARDING MECHANISMS - Coggle Diagram
PACKET FORWARDING MECHANISMS
FUNCTION
To encapsulate packets in the appropriate data link frame type for the outgoing interface
Cisco Express Forwarding
HOW IT WORKS?
CEF builds a Forwarding Information Base (FIB), and an adjacency table. The table entries are not packet-triggered like fast switching but change-triggered, such as when something changes in the network topology. When a network has converged, the FIB and adjacency tables contain all the information that a router would have to consider when forwarding a packet.
CHARACTERISTICS
Fastest, most recent, and preferred packet-forwarding mechanism
CEF builds a Forwarding Information Base (FIB) and an adjacency table
Table entries are not packet-triggered like fast switching, but change-triggered when something changes in the network topology
When a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet
FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information
Fast Switching
HOW IT WORKS?
Fast switching uses a fast-switching cache to store next-hop information. When a packet arrives on an interface, it is forwarded to the control plane where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is then stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention.
CHARACTERISTICS
Common packet forwarding mechanism which uses a fast-switching cache to store the next-hop information
Packet arrives on an interface, it is forwarded to the control plane where the CPU searches for a match in the fast-switching cache
If no match, it is process-switched and forwarded to the exit interface
Packet flow information stored in the fast-switching cache for quick lookup
Process Switching
HOW IT WORKS?
When a packet arrives on an interface, it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets.
CHARACTERISTICS
Slower and older packet forwarding mechanism
Packet arrives on an interface, it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table in order to determine the exit interface
Slow because it does this for every packet in a stream