cisco ccna v7

cisco ccna v7.The routers determine the cost of packet forwarding based on the number of routers separated from the destination network and choose the best path to the destination network accordingly. This is by no means the best solution for choosing a packet forwarding path. For example, if there are two paths to the destination network, the first path consists of three routers, all with a link bandwidth of 1 Gbit/s, and the second path consists of two routers, all with a link bandwidth of 1 Mbit/s. Then it is obvious that one path should be chosen to forward packets to that destination network, because the bandwidth of the first path is too much higher than the second one.The router is not fully aware of the actual network topology (it can only "hear" from neighboring routers), so the best path calculated to the destination network may not actually be the best.The router has to transmit the full destination network information and the corresponding forwarding cost for each routing update, which consumes a significant amount of link bandwidth. Routing loops cannot be eliminated. Thanks to the principles of the DV algorithm, in some cases packets can form forwarding loops in the network, which is known as the counttoinfinity problem. To solve this problem, DV-based routing protocols implement split-horizon and reverse-poison techniques. However, these techniques in turn increase the time consumed by the routing protocol to converge to a stable state.2. Advanced distance vectoring or hybrid algorithmsIn order to avoid several shortcomings of traditional DV routing protocols (e.g., RIPv2), a new routing protocol has been created-which, while based on DV, makes several structural improvements to the DV protocol's operating mechanism. Such routing protocols are called advanced distance vector or hybrid routing protocols, of which Cisco's EIGRP protocol is better known.Figure 1.60 shows an example of an EIGRP message being exchanged between a pair of neighboring routers. Initially, those two routers send out Neighbor Discovery Hello packets to discover each other. Once the neighbor relationship is established, the two routers exchange complete routing information with each other, in a manner similar to a router running the traditional DV routing protocol. When performing routing updates (for example, when a network topology is sent for change), only specified, not complete, routing information is exchanged.68 Chapter 1 Network Protocols and Network Device BasicsNeighbor Discovery Message I.V-Neighbor Discovery MessageComplete Routing Update Message I AV-Full Routing Update Message