The OSPF: Limit
Simultaneous Adjacency Formations feature allows you to limit to the number of
adjacencies that are in “exchange” or “loading” state at the same time. A
process limit (PL) determines the number of “forming” adjacencies and applies
to all adjacencies for the entire process. The term “forming” refers to
adjacencies that are in “exchange” or “loading” state. Adjacencies form in an
OSPF area during the initial period after the area is created. The Initial
Limit applies when no adjacencies have reached the “full” state in an OSPF
area. If there are any “full” adjacencies in the area, the new adjacencies are
governed by the Process Limit. At a given point of time, process limit and
initial limit are effective in an OSPF area. When there are no adjacencies
“forming” in an area, at least one adjacency is allowed to form regardless of
the maximum limit specified for it. In other words, the maximum number of
adjacencies can be exceeded before adjacencies form in one or more areas. The
maximum limit can be exceeded by the number of areas minus one.
When a limit is
reached, adjacencies in a state less than EXCHANGE are terminated. To terminate
the adjacency, a hello packet is sent to the neighbor which does not have the
neighbor’s device ID. This causes the neighbor to put the adjacency in the INIT
state. This prevents a deadlock with the neighbor, which could otherwise happen
if the neighbor is blocking an adjacency from forming on a different interface.
By causing the neighbor to bring the adjacency to INIT, it allows the neighbor
to form an adjacency on a different interface. Packets from unknown neighbors
are ignored when the limit has been reached or exceeded.
If graceful restart
or Cisco nonstop forwarding is configured, the hello packets must be accepted
from every neighboring device The restarting device must include the neighbors’
device IDs in its hello packets to prevent the adjacency from being dropped by
the neighbor. If graceful restart is in configured, the grace link-state
advertisements (LSAs) must be sent in a normal mode and not in a throttling
mode. When the device is performing graceful restart and if the limit is
reached, new adjacencies are allowed to remain in 2-WAY or EXSTART. However,
they are prevented from proceeding to EXCHANGE until the number of forming
adjacencies is less than the limit.
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