ccna certification number check

ccna certification number check..54.Cisco Networking Lab CCNA Lab Guide (Release 2) Switching is the preferred packet forwarding mechanism used by Cisco routers. CEF will construct the ForwardingInformation Base (FIB) and AdjacencyTable. o The FIB and routing table are synchronized and are generated by the CPU through recursive lookups based on the routing table. the IP routing table is updated when routing or topology changes occur in the network and these changes are It is especially critical that the FIB lookups are performed by hardware and are much faster. The adjacency table is somewhat similar to the ARP table in that it primarily holds the encapsulation information needed for Layer 2 rewrites. The FIB and adjacency table are prepared before data is forwarded so that as soon asThe routers can use them directly for data forwarding and encapsulation without having to query the routing table and send ARP requests, so routing is much more efficient.4.2 Routing Decisions4.2.1 Principles of IP RoutingRouting is the act and action of sending a packet from a source to a destination, and a router is the device that performs this act and action. When a router receives an IP packet from an interface, it determines which interface to use to forward the packet to its destination. Thus the behavior of a router in forwarding a packet includes determining the best path for the packet and forwarding the packet to its destination. The router uses the routing table to determine the best path to forward the packet. When the router receives a packet, it checks its destination IP address and searches the routing table for the best matching network address. Once a matching entry is found, the router encapsulates the IP packet into the corresponding data link frame on the outgoing interface for forwarding. The data link frame can be an Ethernet frame, a PPP frame, or an HDLC frame, etc. The data link encapsulation depends on the type of router interface and the type of media it is connected to.The router routes packets as shown in Figure 4-2. The packet arrives at Server B from Computer A as follows: When Host A wants to send an IP packet to Server B, the IP packet is first encapsulated in the Ethernet frame format and then sent to the default gateway, i.e., Router Rl,R1 receives the Ethernet frame from the G0/0 interface and unencapsulates the packet (removes the Layer 2 frame header and frame tail), R1 searches the routing table using the destination IP address of the packet to find a matching network address. After finding the destination network address in the routing table, the outgoing interface is determined to be S0/0/0. R1 re-encapsulates the packet (Layer 2 rewrite) into a UPPP frame and forwards the packet to R2oR2 then performs a similar process as above, and the packet finally arrives at the server Bo Throughout the packet delivery process, the Layer 2 information is rewritten, but the Layer 3 IP information remains unchanged.Figure 4-2 Router Routing Packet Process4.2.2 Administrative Distance and Metrics1. Administrative Distance (AD)The administrative distance is used to define the trustworthiness of the routing source, and is an integer value in the range of 0-255, with a lower value indicating a higher priority of the routing source, and 0 indicating the highest priority. By default, only directly connected networks have an administrative distance of 0, and this value cannot be changed. The administrative distance for static and dynamic routing protocols can be modified. Table 4.1 lists the default administrative distances for directly connected, static routes, and common dynamic routing protocols.