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472 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
Retransmit Interval When a router sends a link state advertisement to its neighbor, it keeps a
copy of the LSA until the neighbor acknowledges receipt of the LSA with
a link state acknowledgment packet. If the sending router does not
receive a link state acknowledgment from its neighbor, it then retransmits
the LSA. The retransmit interval (in seconds) determines how long the
sending router waits for an acknowledgement before retransmitting the
LSA to its neighbors.
To prevent needless retransmissions, the value that you specify must be
greater than the roundtrip delay between any two routers on the
attached network.
Dead Interval The dead interval determines how long neighbor routers wait for a Hello
packet before they determine that a neighbor is inactive. Each time that a
router receives a Hello packet from a neighbor, the router resets the dead
interval timer for that neighbor. The dead interval must be the same for
all routers on the network. The default value for the dead interval is 4
times the default value for the Hello interval — 40 seconds.
Password OSPF supports simple password authentication. You can set security
passwords for OSPF interfaces so that only routers that know the
password participate in OSPF exchanges. Therefore, configure routers in
the same area that want to participate in the routing domain with the
same password.
When you configure a password on a router interface, the interface
inserts the specified password in the OSPF header of every packet that it
transmits and receives only those OSPF packets that contain the same
password.
Simple password authentication prevents routers from inadvertently
joining the area and helps ensure that only trusted routers participate in
the routing domain.
By default, OSPF interfaces on your system do not have associated
passwords. When no password is assigned to an interface, OSPF
exchanges are not authenticated so that, although a password may exist
in an OSPF packet header, it is not examined when it is received.