ccna2-v6-0

ccna2-v6-0.192.168.1.0 E0 0 192.168.2.0 E1 0 192.168.3.0 E1 1 192.168.4.0 E1 2Routing Table Network Interface Metric for Router B192.168.2.0 E0 0 192.168.3.0 E1 0 192.168.1.0 E0 1 192.168.4.0 E0 2Figure 14.7 Example of a simple routing loopDistance vector protocols use a variety of mechanisms to cope with routing loops, however these solutions reduce the convergence rate. Link-state protocols and some hybrid protocols cope with routing loops in a better way, and they use methods that do not reduce the convergence rate. The next few subsections describe the possible solutions to the routing loop problem that distance vector protocols can implement.Avoiding infinite counting: maximum hop countOne of the problems posed by routing loops is infinite counting. When routing loops occur, packets may enter such loops and they are continuously transmitted in the loop, wasting both the bandwidth of the network segment and the CPU cycles of the router.To avoid constant transmission of packets in the loop, distance vectoring protocols usually specify a maximum number of hops, which determines how far the packet can travel at most. During packet transmission, the router keeps track of the number of hops in the TTL field of the IP datagram header: for each hop the packet passes, the value of its TTL field is subtracted by 1. After subtracting 1 from the TTL, if it is found to become 0, the router discards the packet (the function of the TTL field is described in Chapter 5).However, setting a maximum hop limit on packets does not solve the routing loop problem - the loop still exists. This solution can only prevent packets from getting caught in a loop that they cannot get out of. Another problem that comes with setting a hop limit on packets is that sometimes packets have to travel to destinations that are far away and exceed the maximum hop count. The router checks the TTL field and drops the packet as soon as it reaches the maximum hop limit, regardless of whether this is due to the destination being far away or the packet being caught in a loop.RIP sets the hop limit to 14o When a packet enters an interface, the router decrements the value of its TTL field by 1. If the hop count is reduced to 0, the router drops the packet immediately. If the destination is far away and exceeds this limit, you can modify the routing protocol's maximum hop count setting, but you need to do so on every router in the network.