Chapter 7 – Analyzing Ethernet LAN Switching

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LAN switches receive Ethernet frames and then make a switching decision: either forward the frame out some other ports or ignore the frame. To accomplish this primary mission, switches perform three actions: I. 2. 3. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address Preparing to forward frames by learning MAC addresses by examining the source MAC address of each frame received by the switch Preparing to forward only one copy of the frame to the destination by creating a (Layer 2) loop-free environment with other switches by using Spanning Tree Protocol (STP)

Forwarding Unicast Frames

i) Frame Came in FO/I, 2) Destined for 0200.2222.2222 3) Forward out F012 4) Filter (DO Not send) on FOB, FO/4 o Dest 0200.22222222 , Fred Barney 0200222.2222 Figure 7-3 MAC Address Table MAC Address Wilma 0200_3333.3333 Betty 0200.4444.4444 Output 0200.2222.2222@ FOÆ FOB 0200.4444.4444 FO/4 Sample Switch Forwarding and Filtering Decision

MAC address table:  

  • switching table 
  • bridging table 
  • content addressable memory CAM table 
1) Frame Entered FO/I_ 2) Destined for 0200.3333.3333_ 3) MAC table entry lists GO/I 4) Forward out GO/I Fred Barney o Dest 020022222222 FO/I SWI GO,'I GO/2 SW2 020022222222 Figure 7-4 SWI Address Table MAC Address 0200.2222.2222 0200 . 3333 .3333 Output FO/I 0200 @ GO/I . 4444 . 444 4 GO/ I FO/4 SW2 Address Table MAC Address 0200.2222.2222 0200. 3333.3333 0200.4444. 4444 Wilma 0200.3333.3333 Betty 0200.4444_4444 Output GO/2 GOP FO/3 FO/4 Forwarding Decision with Two Switches: First Switch
Fred Dest 0200.3333.3333 SWI Barney 020022222222 SWI Address Table MAC Address 0200.2222.2222 0200 . 3333 .3333 0200 . 4444.4444 1) Frame Entered GO,'2 2) Destined for 0200.3333.3333_ 3) MAC table entry lists FO,'3_ 4) Forward out FOB GO/I GO,'2 SW2 FO/4 Wilma 0200.3333.3333 Betty Output FO/I GO/I GO/I SW2 Address Table MAC Address 0200.2222.2222 0200.4444_4444 Output GO/2 GO/2 0200.3333.3333 FO/3 0200. 4444.4444 Figure 7-5 Forwarding Decision with Two Switches: Second Switch

Learning MAC addresses

Fred FO/I FOÆ Barney 0200.2222.2222 Wilma 0200 3333.3333 FO/4 Betty 0200 4444.4444 Address Table: Before Either Frame Is Sent Address: Output (Empty) (Empty ) Address Table: Atter Frame 1 (Fred to Barney) Address Output Address Table: After Frame 2 (Barney to Fred) Address: Output 0200.2222 .2222 Figure 7-6 Switch Learning: Empty Table and Adding Two Entries

Flooding unknown unicast and broadcast frames: 

Address Table: Address: (Empty) FO/I FOP Barney 0200.2222.2222 Wilma 0200.3333.3333 FO/4 Betty 0200.4444.4444 Before Frame Is Sent Output (Empty) Figure 7-7 Switch Flooding: Unknown Unicast Arrives, Floods out Other Ports

Avoiding loops using Spanning Tree Protocol: 

Key Topic LAN Switching Summary Switches use Layer 2 logic, examining the Ethernet data-link header to choose how to pro- cess frames. In particular, switches make decisions to forward and filter frames, learn MAC addresses, and use STP to avoid loops, as follows: Step I . Switches forward frames based on the destination MAC address: A. If the destination MAC address is a broadcast, multicast, or unknown desti- nation unicast (a unicast not listed in the MAC table), the switch floods the frame. B. If the destination MAC address is a known unicast address (a unicast addrews found in the MAC table): i. If the outgoing interface listed in the MAC address table is different from the interface in which the frame was received, the switch for- wards the frame out the outgoing interface. 56 CCENT/CCNA ICNDI 100-105 Official Cert Guide step 2. Step 3. ii. If the outgoing interface is the same as the interface in which the frame was received, the switch filters the frame, meaning that the switch simply ignores the frame and does not forward it. Switches use the following logic to learn MAC address table entries: A. For each received frame, examine the source MAC address and note the interface from which the frame was received. B. If it is not already in the table, add the MAC address and interface it was learned on. Switches use STP to prevent loops by causing some interfaces to block, mean- ing that they do not send or receive frames.
Table 7-4 Chapter 7 EXEC Command Reference Command show mac address-table show mac address-table dynamic show mac address-table dynamic vlan vlan-id show mac address-table dynamic address MAC-address show mac address-table dynamic interface inter ace-id show mac address-table count show mac address-table aging-time clear mac address-table dynamic show interfaces status Mode/Purpose/Description Shows all MAC table entries of all types Shows all dynamically learned MAC table entries Shows all dynamically learned MAC table entries in that VLAN Shows the dynamically learned MAC table entries with that MAC address Shows all dynamically learned MAC table entries associated with that interface Shows the number of entries in the MAC table, and the total number of remaining empty slots in the MAC table Shows the global and per-VLAN aging timeout for inactive MAC table entries Empties the MAC table of all dynamic entries Lists one line per interface on the switch, with basic status and operating information for each

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