IGRP Behavior

Distance vector protocols are protocols that solely depend on neighbor routing advertisements to determine the best path to a destination. The advantage of the distance vector protocols is their simplicity to implement. However, because of the long convergence time, IGRP is not suitable for large networks. IGRP and RIP are both classical distance vector protocols. Although IGRP and RIP differ in metric calculation update timers, they exhibit the same behavior when it comes to sending and receiving updates.

IGRP sends its entire routing update over the broadcast address of 255.255.255.255, with the IP Protocol field set to 9. IGRP handles discontiguous network and variable-length subnet masking (VLSM) in exactly the same way that RIP does. IGRP does not support discontiguous networks; in these networks, IGRP autosummarizes over a major network boundary. Therefore, the subnet information is not advertised to the remote site, causing routing problems. Because IGRP does not send subnet mask information as part of the routing update, IGRP does not support VLSM.