The HELLO protocol is an interior protocol that uses a routing metric based on the length of time it takes a packet to make the trip between the source and the destination. HELLO packets carry timestamp information which allows receivers to compute the shortest delay paths to destinations. The "best" route is the route with the shortest time delay. The unit of time used in HELLO is milliseconds. If a HELLO update packet takes less than 100 milliseconds to travel between two routers, a minimum value of 100 is used for that hop. Thus on networks built of high-speed interfaces HELLO essentially defaults to using hop counts. As in any routing algorithm, HELLO cannot change routes too rapidly or it would be unstable. To avoid instabilities, implementations of HELLO build in hysteresis and "hesitate" to change routes until they have confidence that the change will be lasting.
By default HELLO, like RIP, uses the kernel interface metric set by the
ifconfig command to influence metric added to routes as
they are installed in the routing table (metricin).
Since the kernel interface metric is in hops, it must be translated
into HELLOs millisecond metric. In order to do that, the following
table is used:
Hops HELLO metric 0 0 1 100 2 148 3 219 4 325 5 481 6 713 7 1057 8 1567 9 2322 10 3440 11 5097 12 7552 13 11190 14 16579 15 24564 16 30000
On point-to-point interfaces, the netmask is applied to the remote address. The netmask on these interfaces is ignored if it matches the natural network of the remote address or is all ones.
Unlike in previous releases, the zero subnet mask (a network that matches the natural network of the interface, but has a more specific, or longer, network mask) is ignored. If this is not desirable, a route filter may be used to reject it.
hello yes | no | on | off [ {
broadcast ;
nobroadcast ;
preference preference ;
defaultmetric metric ;
interface interface_list
[nohelloin] | [helloin]
[nohelloout] | [helloout]
[metricin metric]
[metricout metric] ;
trustedgateways gateway_list ;
sourcegateways gateway_list ;
traceoptions trace_options ;
} ] ;
the hello statement enables or disables HELLO. If the
hello statement is not specified the default is
hello off. If enabled, HELLO will assume
nobroadcast when there is only one interface and
broadcast when there is more than one interface.
broadcast when only one network
interface is present can cause data packets to traverse a single
network twice.
sourcegateways clause is present, routes will still
be unicast directly to that gateway.
Note that if there are multiple interfaces configured on the same subnet, HELLO updates will only be sent from first one one which HELLO output is configured. This limitation is required because of the way the Unix kernel operates. It will hopefully be removed in a future release.
The possible parameters are:
nohelloin is used on a wildcard interface
descriptor.
broadcast mode. The sending of HELLO on
point-to-point interfaces must be manually configured.
nohelloin is used on a
wildcard interface descriptor.
trustedgateways clause is specified only updates
from the gateways in the list are accepted.
noripout on the interface.
The default preference is 90. The default metric is 30000.
policy option logs info whenever a new route is
announce, the metric being announced changes or a route goes or leaves
holddown.
Packet tracing options (which may be modified with
detail, send and/or recv):