CCNA -A Short Notes - 2 - Switching Technologies

Part II – Switching Technologies

Switch Functions:

Address Learning: when a host transmits a frame, it’s hardware address is recorded in the MAC Address Table, along with the port the frame has been received on.

Forward/Filter Decisions: If the address is unknown, the frame is forwarded to all ports except the one on which the frame was received. In other cases, the frame is only sent to the appropriate interface.

Loop Avoidance: Loops occur when there are multiple links between switches. A broadcast storm occurs when two switches constantly rebroadcast the same frame. Devices may receive the same frames several times, and from different origins. The same problem can cause MAC Address Table confusion (called trashing) if the device is a switch trying to determine the entry port of a MAC address. These problems can be avoided by the Spanning Tree Protocol (STP).

Bridges are software based and can only have one Spanning Tree instance, switches are hardware based (ASIC – Application Specific Integrated Circuit) and have lower latency.

Spanning Tree Protocol (STP): Standard IEEE 802.1d that uses the STA (Spanning Tree Algorithm) to prevent network loops.

Bridge Protocol Data Units (BPDUs): packets of information exchanged between switches to support the STP. They are sent every 2 seconds by default. MaxAge is a timer indicating how long before the bridge should wait before concluding the topology has changed.

Bridge ID: composed of a priority from 1 to 32768 (default) and the MAC address of the bridge, this is communicated using BPDUs.

Root Bridge: elected by the lowest bridge ID. The ports on the Root Bridge are Designated Ports (forwarding) and if the route bridge is not connected to the redundant link, the one determined by the lowest-cost link (or bridge ID in the event of a tie) will be a designated port. All other switches will have non-designated ports onto the redundant link (blocking).

Root Ports: ports linking to the Root Bridge in non-root bridges. They are determined by the lowest-cost path to the Root Bridge.

Blocked Ports: Ports other than the root port that will not forward frames, but will still receive BPDUs.

Port States:

Blocking: does not forward frames, but listen to BPDUs. All ports are in blocking state by default when a switch is powered up.

Listening: Listens to BPDUs to ensure no loops occur on the network before passing data frames.

Learning: Learns MAC addresses and builds a filter table but does not forward frames.

Forwarding: Sends and receives all data on the bridged port.

Disabled: No frame forwarding or BPDUs are sent or received.

Convergence: transition time from blocking to forwarding state to allow the device enough time to learn the latest network topology (default is 50 seconds). When a switch determines a blocked port has to be activated due to a down-link, the port will first go into listening mode to ensure no loops will be created. 

Latency: time elapsed between the receiving of a frame and its forwarding. 

LAN Switch Types:

Store-and-forward: The complete frame is received, checked, and then forwarded. Unchangeable default on Catalyst 5000 switches.

Cut-through: Only the destination hardware address is looked up and the frame is then forwarded.

FragmentFree or modified cut-through: Default for Catalyst 1900. Checks for the first 64 bytes in the data field of a frame before forwarding it.

Both Cut-through and FragmentFree have fixed latency, and Store-and-Forward has variable latency.