3Com corebuilder 3500 Implementation Manual page 461

AppleTalk Routes 461
When a router receives an RTMP packet that contains a routing entry
currently not in it's table, the router adds the entry to its routing table,
and increments the route's distance (hop count) by 1.
When a network is removed from the RTMP table (whether manually,
or though the aging process), the router also scans the Zone
Information Table (ZIT), and removes ZIT entries that contain the
deleted network number.
If the zone Information Table contains an entry whose network
number range is not in the RTMP table, it then concludes that the
network is no longer on the Internet, and deletes the network's
ZIT entry.
An overburdened network with many routers can prevent some
routers from sending RTMP updates every 10 seconds. Because
routers begin to age out routes after the loss of 2 successive RTMP
updates, the failure for RTMP packets to arrive may result in
unnecessary route changes, known as route flapping. For this reason,
network segments should be kept to a reasonable size.