Most campus and data center network designs have been following the standard 3 tier design model established by Cisco. New network designs are pushing layer 3 routing out to the access switches creating a routed edge. The maintaining of the routing tables is by means of provisioning or by an automated routing protocol.With the advent of new technologies as well as the ability to have multi-layer switches at the access layer, Cisco is starting to change the game. The next hop MAC address of the neighbor is determined via ARP. The routing table defines for each reachable destination the appropriate next hop system (neighbor IP address) and the port to reach this neighbor. In the example above the routing is based upon IP address information which is maintained in the IP routing tables. The assigned IP address to this interface can be used as a universal id to reach the system. In some systems an extra loopback interface is created. The 元 switch system itself is reachable via any of the 3 IP addresses, unless special security measures are taken to limit this access. The VLANs themselves are interconnected by routing based upon IP addressing. It presents 6 distributed host systems interconnected by means of VLANs and MAC addresses. The above figure shows an example network with 3 VLANs (also, we have depicted the IP stack profile of the 元 switch). It can support routing protocols like RIP, OSFP, EIGRP. Router: it connects subnets or VLANs using its IP routing intelligence which makes it act as a router.Switch: it performs the Layer 2 functionality of connecting devices that are on the same subnet or virtual LAN at lightning speeds.A Layer 3 switch is also referred to as Multilayer switch, since it plays a dual role: Layer 3 switches are high performance routers optimized for campus LANs or intranets. Layer 3 switches were introduced into the market to efficiently perform inter-VLAN routing augmenting all layer 2 functionalities. Traditional routers were slower in forwarding packets when compared to switches They use dynamic routing protocols to identify best path to a destination. Routers, operating at layer 3, offer packet forwarding based on logical addresses (IP Addressing). This brings in the need for a Layer 3 device. Layer 2 switches are capable of intra-VLAN communication, isolating data transfer between devices within each VLAN, but they lack the capability of inter-VLAN communication and they cannot route packets between VLANs. Hence a Layer 2 network has limited scalability and flexibility. A network built using Layer 2 switches appears as a Single Flat Address space. Layer 2 Switches offer frame forwarding based on the physical addresses (MAC).
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