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The following will be discussed in this chapter: A redundant switched topology includes multihomed switches and EtherChannel; a redundant switched topology causes looping issues such as broadcast storms; the 802.1D STP establishes a loop-free network; the original STP has been enhanced by PVST+ and RSTP. | Medium-Sized Switched Network Construction Improving Performance with Spanning Tree Interconnection Technologies Technology Use Fast Ethernet Connects end-user devices to the access layer switch Gigabit Ethernet Connects access switch to distribution switch and high use servers to switches 10-Gigabit Ethernet Provides high-speed switch to switch links, backbones EtherChannel Provides high-speed switch to switch links, backbones with redundancy 142,142,149 Determining Equipment and Cabling Needs Each link provides adequate bandwidth for the total aggregate traffic over that link. Advantages of EtherChannel Logical aggregation of similar links between switches Load-shares across links Viewed as one logical port to STP Redundancy Redundant Topology Redundant topology eliminates single points of failure. Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems. Emphasize: Layer 2 has no mechanism (like a TTL) to stop loops. Station D sends a broadcast frame. Broadcast frames are flooded to all ports except the originating port. Broadcast Frames Broadcast Storms Host X sends a broadcast. Switches continue to propagate broadcast traffic over and over. Layer 3 of 3 Emphasize: Layer 2 has no TTL mechanism to stop looping frames. Multiple Frame Copies Host X sends a unicast frame to router Y. The MAC address of router Y has not been learned by either switch. Router Y will receive two copies of the same frame. Layer 2 of 2 Host X sends a unicast frame to router Y. The MAC address of router Y has not been learned by either switch. Switches A and B learn the MAC address of host X on port 1. The frame to router Y is flooded. Switches A and B incorrectly learn the MAC address of host X on port 2. MAC Database Instability Layer 2 of 2 Provides a loop-free redundant network topology by placing certain ports in the blocking state Published in the IEEE 802.1D specification Enhanced with the Cisco PVST+ implementation . | Medium-Sized Switched Network Construction Improving Performance with Spanning Tree Interconnection Technologies Technology Use Fast Ethernet Connects end-user devices to the access layer switch Gigabit Ethernet Connects access switch to distribution switch and high use servers to switches 10-Gigabit Ethernet Provides high-speed switch to switch links, backbones EtherChannel Provides high-speed switch to switch links, backbones with redundancy 142,142,149 Determining Equipment and Cabling Needs Each link provides adequate bandwidth for the total aggregate traffic over that link. Advantages of EtherChannel Logical aggregation of similar links between switches Load-shares across links Viewed as one logical port to STP Redundancy Redundant Topology Redundant topology eliminates single points of failure. Redundant topology causes broadcast storms, multiple frame copies, and MAC address table instability problems. Emphasize: Layer 2 has no mechanism (like a TTL) to stop loops. .
