Hiding Network Changes

If the routing table contains a summary of the networks beneath it, any changes in the network at these levels are not seen. This is both a good thing and a bad thing. If the network in the earlier example—140.100.50.128/27, the subnet on the fourth floor of the second building in San Jose, California—were to go down, the router at the core would be oblivious to the LAN problem. This is beneficial because there are no additional updates or recalculation.

The disadvantage is that any traffic destined for that subnet is sent on the assumption that it exists. To be more accurate, the core router sees the inbound IP packet destined for 140.100.50.131 and, instead of applying the /27 mask, uses the mask that it has configured. It employs the /19 mask that sees the subnet 140.100.32.0/19, although in reality the destination subnet is 140.100.50.128/27. If the subnet 140.100.50.128 is no longer available, all traffic is still forwarded to the subnet until it reaches a router that holds the same mask as the IP packet and therefore is aware that it is no longer available. An ICMP message that the network is unreachable is generated to the transmitting host. The host may stop transmitting after hearing that the network is down.

Although unnecessary traffic will traverse the network for a while, it is a minor inconvenience compared to the routing update demands on the network and the CPU utilization on the routers in large networks.

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