ccna 7 chapter 3 exam

ccna 7 chapter 3 exam.The next 4 fields are used to identify the overhead of the root bridge and the path to the root bridge.The last 4 fields are all timer fields that are used to determine how often to send BPDU messages and how long thehow long to retain the message. In the example, the BPDU frame contains more fields than previously described. When a BPDU message is transmitted over the network, it is encapsulated in a158 Chapter 6 Protecting Local Area Networksthe Ethernet frame. The 802.3 header indicates the source and destination addresses of the BPDU frame. The frame's current destination MAC address is 01:80:C2:00:00:00, which is a spanning-tree multicast address. When the frame is sent with this MAC address, every switch configured for spanning tree accepts the frame and reads the information inside; other devices on the network ignore the frame.In this example, the root ID and BID are the same in the captured BPDU frame. This indicates that the frame was captured from the root bridge. The timer pair is set to the default value.6.2.4.6 BPDU Propagation and ProcessEach switch in the broadcast domain assumes at the outset that it is itself the root bridge in a spanning tree instance, and therefore sends BPDU frames containing the BID of the native switch, which it uses as the root ID. by default, BPDU frames are sent every 2 seconds after the switch reboots; that is, the default value of the Hello timer specified in the BPDU frame is 2 seconds. Each switch maintains a local information related to its BID, root ID, and path overhead to the root bridge.When a neighboring switch receives a BPDU frame, it compares the root ID from the BPDU frame with the local root ID. If the root ID in the BPDU frame is lower than the local root ID, the switch updates the local root ID and ID in its BPDU message. this information indicates that there is a new root bridge in the network. The distance to the root bridge is also indicated by the updated path overhead. For example, if a BPDU is received on a Fast Ethernet switch port, the path overhead may increase by 19. If the local root ID is less than the root ID received from the BPDU frame, the BPDU frame is discarded.After a Root 1D has been updated to identify a new Root Bridge, then all subsequent BPDU frames sent by that adjacent switch will contain the new Root ID and the updated path overhead. In this way, the determined minimum root ID is visible to all other adjacent switches at all times, and as BPDU frames are passed between other adjacent switches, the path overhead is constantly updated to refer to the total path overhead to the root bridge. Each switch in the spanning tree uses its own path overhead to determine the best possible path to the root bridge.