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The Hello Protocol


It's really better not to use HELLO unless you have a specific need for it. We plan to drop it some time around GateD 4.0.

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, table 4.2 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 Hello Metrics

HELLO and network masks

HELLO derives the network mask of received networks and hosts from the network mask of the interface the packet via which the packet was received. If a received network or host is on the same natural network as the interface over which it was received and that network is subnetted (the specified mask is more specific than the natural netmask), the subnet mask is applied to the destination. If bits outside the mask are set it is assumed to be a host, otherwise it is assumed to be a subnet.

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.

The Hello Statement

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.

The default preference is 90. The default metric is 30000.

Tracing options

The policy option logs info whenever a new route is announced, 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):

All HELLO packets

next up previous contents index
Next: OSPF Up: Configuration Guide Previous: RIP

Laurent Joncheray
Wed Jun 12 15:35:22 EDT 1996