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Get the Big Picture on Layer 3 Switching

If you can't see the content, please view the online version here. Aug. 2019

Why and when do you need a Layer 3 switch?
An extensive area is recommended to be divided into separate LANs. Because each LAN is a broadcast domain, all devices can reach each other through the Layer 2 (physical layer) environment. Layer 2 switches transmit traffic based on the MAC address of other devices on the network. A broadcast storm is a state in which a message that has been broadcast across a network results in even more responses, and each response results in still more responses in a snowball effect. In order to prevent this from occurring in a large network, it is essential to segregate the network into isolated LANs.

The main function of a Layer 3 switch is to connect two or more LANs. Large areas are preferably subdivided into separated LANs to reduce the number of broadcasts that are received and propagated. A Layer 3 switch connects these separated LANs and uses a routing protocol to determine the best path between them.

Layer 3 switches use IP addresses to transmit traffic and separate broadcast domains. A Layer 2 switch forwards traffic between network hosts such as a server and a client PC within the same subnet. The traffic-forwarding decision is based on its MAC address table entries. However, if the server and the client PC are located on different subnets, and data traffic needs to be sent between them, then a Layer 3 switch is needed. It makes traffic-forwarding decisions based on a map of the IP network maintained in its routing table.

In what scenario do you need to utilize Layer 3 switches for subnet planning?
The network topology that contains two or more subnets needs a Layer 3 switch for interconnection between these subnets. In the following figure, Control area (, Area A ( and B ( are isolated areas. In this figure, there is a Layer 3 switch on each area.

How do you leverage dynamic routing in Layer 3 routing function
In this figure, each Layer 3 switch is connected to its own subnets and has two links to the other layer 3 switches. There are options for Layer 3 switches to perform routing functions for these subnets. Possibilities include static routing, Routing Information Protocol (RIP), and Open Shortest Path First (OSPF) routing. RIP and OSPF are dynamic routing protocols; routers exchange routing information and allow other routers to learn about remotely connected networks dynamically. This routing information is then added to their routing tables for forwarding packets.

How to choose a Layer 3 switch?




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Routing for up to 64 VLANs  
Max. number of routes 64 entries
IPv4 routing Static route, RIP v2, OSFP v2
IPv4 Multicast routing N/A
IPv6 routing N/A
IPv6 Multicast routing N/A
IPv4 Multicast routing N/A
Virtual router redundancy VRRP



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Routing for more than 65 VLANs
L3 unicast (IPv4): 8192 (8K)
L3 unicast (IPv6): 2048 (2K)
L3 multicast (IPv4): 8192 (8K)
Static route, RIP v2, OSPF v2
RIPng, OSPF v3
PIM-DM v6 / PIM-SM v6


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