Juniper JUNOSE 11.0.X MULTICAST ROUTING, Руководство по настройке

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the process. The router makes up to eight attempts to connect to a destination for a
domain—one attempt for each preference level.
If all destinations at a preference level are marked as unreachable, the router chooses
the destination that failed first and tries to make a connection. The key is to
understand that the router chooses a single destination at each level of preference,
even if all destinations have recently failed. Thus the 5-minute timer normally used
to reinstate failed destinations is ignored under certain conditions.
For example, suppose you have three destinations for a domain: A, B, and C. You
assign the following preferences:
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A, B, and C at preference 0
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A, B, and C at preference 1
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A, B, and C at preference 2
A, B, and C are all considered reachable.
If a PPP user tries to connect to the domain, suppose the router randomly selects
destination A from preference 0. If this connection attempt fails, the router excludes
destination A for 5 minutes and goes to the next level (preference 1). From here, it
randomly selects destination B, one of the two remaining choices. If the second
connection attempt also fails, the router excludes destination B, as well as destination
A, and attempts to connect to destination C, the only destination available with
preference 2. The router has had an opportunity to connect to every destination
available for the domain.
Support for multiple destinations affects the procedure for mapping a user domain
name to an L2TP tunnel. To learn how to complete this mapping, see “Mapping a
User Domain Name to an L2TP Tunnel Overview” on page 352.
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To enable tunnel selection failover between preference levels:
This tunnel selection method is the default method. If you do not set any tunnel
selection parameters, the router uses this method.
Configuring the Failover Within a Preference Level Method
You use the l2tp fail-over-within-preference command to enable tunnel selection
failover within a preference level.In this selection method, if the router tries to connect
to a destination and is unsuccessful, it selects a new destination at the same
preference level. If all destinations at a preference level are marked as unreachable,
the router does not attempt to connect to a destination at that level. It drops to the
next lower preference level to select a destination.
If all destinations at all preference levels are marked as unreachable, the router
chooses the destination that failed first and tries to make a connection. If the
connection fails, the router rejects the PPP user session without attempting to contact
the remote router.
For example, suppose there are four tunnels for a domain: A, B, C, and D. All tunnels
are considered reachable, and the preference levels are assigned as follows:
Configuring the Failover Within a Preference Level Method
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Chapter 12: Configuring an L2TP LAC