Cisco Cisco IOS Software Release 12.2(15)T
OSPF Support for Fast Hello Packets
Prerequisites for OSPF Support for Fast Hello Packets
2
Cisco IOS Release: Multiple releases (see the Feature History table)
Prerequisites for OSPF Support for Fast Hello Packets
OSPF must be configured in the network already or configured at the same time as the OSPF Support
for Fast Hello Packets feature.
for Fast Hello Packets feature.
Information About OSPF Support for Fast Hello Packets
The following sections describe concepts related to OSPF support for fast hello packets:
•
•
•
OSPF Hello Interval and Dead Interval
OSPF hello packets are packets that an OSPF process sends to its OSPF neighbors to maintain
connectivity with those neighbors. The hello packets are sent at a configurable interval (in seconds). The
defaults are 10 seconds for an Ethernet link and 30 seconds for a non broadcast link. Hello packets
include a list of all neighbors for which a hello packet has been received within the dead interval. The
dead interval is also a configurable interval (in seconds), and defaults to four times the value of the hello
interval. The value of all hello intervals must be the same within a network. Likewise, the value of all
dead intervals must be the same within a network.
connectivity with those neighbors. The hello packets are sent at a configurable interval (in seconds). The
defaults are 10 seconds for an Ethernet link and 30 seconds for a non broadcast link. Hello packets
include a list of all neighbors for which a hello packet has been received within the dead interval. The
dead interval is also a configurable interval (in seconds), and defaults to four times the value of the hello
interval. The value of all hello intervals must be the same within a network. Likewise, the value of all
dead intervals must be the same within a network.
These two intervals work together to maintain connectivity by indicating that the link is operational. If
a router does not receive a hello packet from a neighbor within the dead interval, it will declare that
neighbor to be down.
a router does not receive a hello packet from a neighbor within the dead interval, it will declare that
neighbor to be down.
OSPF Fast Hello Packets
OSPF fast hello packets refer to hello packets being sent at intervals of less than 1 second. To understand
fast hello packets, you should already understand the relationship between OSPF hello packets and the
dead interval. See the section
fast hello packets, you should already understand the relationship between OSPF hello packets and the
dead interval. See the section
OSPF fast hello packets are achieved by using the ip ospf dead-interval command. The dead interval is
set to 1 second, and the hello-multiplier value is set to the number of hello packets you want sent during
that 1 second, thus providing subsecond or “fast” hello packets.
set to 1 second, and the hello-multiplier value is set to the number of hello packets you want sent during
that 1 second, thus providing subsecond or “fast” hello packets.
When fast hello packets are configured on the interface, the hello interval advertised in the hello packets
that are sent out this interface is set to 0. The hello interval in the hello packets received over this
interface is ignored.
that are sent out this interface is set to 0. The hello interval in the hello packets received over this
interface is ignored.
The dead interval must be consistent on a segment, whether it is set to 1 second (for fast hello packets)
or set to any other value. The hello multiplier need not be the same for the entire segment as long as at
least one hello packet is sent within the dead interval.
or set to any other value. The hello multiplier need not be the same for the entire segment as long as at
least one hello packet is sent within the dead interval.