Packet Manipulation at Layer 3
How does Layer 3 switching differ from Layer 2 switching? Layer 3 switching requires rewriting the packet. This implies decrementing the TTL field, modifying the MAC addresses, changing the VLAN-ID and recomputing the FCS. Doing all these actions at wire speed is difficult which is why an ASIC is necessary. True Layer 3 switching has all the advantage of routing, therefore it is rich in feature and performance. Layer 2 switching, on the contrary, does not require packet rewriting. Without packet rewriting, no matter how you call it (e.g. virtual routing) it is NOT routing.
What Is Layer 3 Switching?
Layer 3 switching is hardware-based routing. The packet forwarding is handled by specialized hardware, usually ASICs.
A Layer 3 switch can make switching and filtering decisions on both Layer 2 and Layer 3 addresses and can dynamically decide whether to route or switch incoming traffic.
Multilayer switching combines the ease of use of Layer 2 switching with the stability and security of Layer 3 routing.
To make Layer 3 switching decisions, routing table information must be assembled and exchanged between routing entities.
Route calculation is performed by one or more route processors that reside in routers or other devices. These route processors periodically distribute their routing tables to multilayer LAN switches to allow them to make very fast switching decisions.
Layer 3 switching is the favorite for highly scalable, resilient networking.
A Layer 3 Switch Has Two Distinct Components
ASICs:
– High-performance, hardware-based Layer 3 switching and services with consistent low latency
Routing software:
– Routing protocols to provide scalability
– Backbone redundancy
– Dynamic load balancing and fast convergence in the backbone
– Reachability information
– Multiprotocol support for the campus
