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You can influence root port ( forwarding port ) selection based on costs and port priorities but essentially the placement of the root switch is how the forwarding path is built. The lack of a TTL field will cause Layer 2 packets to infinitely loop causing a network meltdown.Ĥ) Incompetent path selection: The shortest path for a Layer 2 network depends on the placement of the Root switch. In comparison to Layer 3 headers, the Layer 2 packet header does not have a TTL field. Also, in the Layer 2 network, all the MAC addresses are populated in ALL switches leading to large requirements in the Layer 2 table sizes.ģ) Instability of Layer 2 networks: Layer 3 networks have an eight-bit Time to Live ( TTL ) field that prevents datagrams from persisting ( e.g going in circles ) on the internet. Traditionally, Layer 3 networks can overcome this by introducing ABRs in OSPF or summarization/filtering in EIGRP. For example, you cannot have one MAC address represent a stub of networks. There have been enhancements to the protocol with the introduction of PVST per VLAN load balancing but this feature can only load balance on VLANs.Ģ) MAC address scalability: Layer 2 end hosts are discovered by their MAC address and this type of host addressing cannot be hierarchical and summarized. 1) No Multipathing support at Layer 2: Spanning Tree Protocol ( STP ) lacks any good Layer 2 multipathing features that are needed for large data centers.