4-4
CheetaSwitch Workgroup-3616
Basic Functions of Layer-3 Switching
The two major functions of a Layer-3 switch are:
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Routing Path Management
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Packet Forwarding
Routing Path Management involves the determination and updating of all the
routing information required for packet forwarding, including:
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Handling routing protocols
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Updating the routing table
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Updating the Layer-3 switching database
Packet forwarding, on the other hand, encompasses the tasks required to forward
all packets for both Layer-2 and Layer-3 forwarding. These functions include:
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Layer-2 forwarding (switching) based on the Layer-2 destination MAC address
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Layer-3 forwarding (routing):
• Based on the Layer-3 destination address
• Replacing destination / source MAC addresses for each hop
• Incrementing the hop count
• Decrementing the time-to-live
• Verifying and recalculating the Layer-3 checksum
Routing Protocols
The ES3616 supports both static routing and dynamic routing.
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Static routing requires routing information to be stored in the switch, either
manually or when a connection is set up, by an application outside the switch.
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Dynamic routing uses a routing protocol to exchange routing information,
calculates routing tables, and responds to changes in the status or traffic on the
network.
The ES3616 supports RIP and RIP-2 dynamic routing protocols.
RIP and RIP-2 Dynamic Routing Protocols
The RIP protocol is the most widely used routing protocol. The RIP protocol uses a
distance vector-based approach to routing. Routes are determined on the basis of
minimizing the distance vector, or hop count, which serves as a rough estimate of
transmission cost. Each router broadcasts its advertisement every 30 seconds,
together with any updates to its routing table. This allows all routers on the network
to learn consistent tables of next hop links which lead to relevant subnets.
Just as Layer-2 switches use the Spanning Tree to prevent loops, routers also use
methods for preventing loops that would cause endless retransmission of data
traffic. RIP utilizes the following three methods to prevent loops from occurring:
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Split horizon—never propagate routes back to an interface port from which they
have been acquired.
Summary of Contents for ES3616
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