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    Configuring LAC Tunnel Selection Parameters

    This section presents the capabilities of the LAC’s tunnel selection process. L2TP allows you to specify:

    • Up to 31 destinations for a domain.
    • Up to eight levels of preference. Preference indicates the order in which the router attempts to connect to the destinations specified for a domain. Zero (0) is the highest level of preference.
    • Up to 31 destinations for a single preference level.

    For information about setting up destinations and preference levels for a domain, see Mapping a User Domain Name to an L2TP Tunnel Overview.

    When the E Series LAC determines that a PPP session should be tunneled, it selects a tunnel from a set of tunnels associated with either the PPP user or the PPP user’s domain. The router provides the following methods for selecting tunnels:

    • Tunnel selection failover between preference levels (the default behavior)
    • Tunnel selection failover within a preference level
    • Maximum sessions per tunnel
    • Weighted load balancing
    1. Configuring the Failover Between Preference Levels Method
    2. Configuring the Failover Within a Preference Level Method
    3. Configuring the Maximum Sessions per Tunnel
    4. Configuring the Weighted Load Balancing Method

    Configuring the Failover Between Preference Levels Method

    When a user tries to log into a domain, in the default method, the router attempts to connect to a destination in that domain with the highest preference level. If more than one destination in the preference level is considered reachable, the router randomly selects a destination and attempts to contact it. If the router is unsuccessful, it marks the destination as unreachable and does not try to connect to that destination for five minutes. The router then moves to the next lower preference level and repeats 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:

    • A, B, and C at preference 0
    • A, B, and C at preference 1
    • 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.

    • 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:

    • A and B at preference 0
    • C and D at preference 1

    When the router attempts to connect to the domain, suppose it randomly selects tunnel B from preference 0. If it fails to connect to tunnel B, the router excludes tunnel B for five minutes and attempts to connect to tunnel A. If this attempt also fails, the router drops to preference 1. Then suppose the router selects tunnel C. If it also fails to connect to tunnel C, the router excludes tunnel C for five minutes and attempts to connect to tunnel D.

    • To enable tunnel selection failover within a preference level:
      host1(config)#l2tp fail-over-within-preference

    Configuring the Maximum Sessions per Tunnel

    You can configure the maximum number of sessions per tunnel, either through a RADIUS server or the command-line interface. If you set the maximum sessions per tunnel parameter, the router takes the setting into consideration when it selects a tunnel. If a randomly selected tunnel has a current session count equal to its maximum session count, the router does not attempt to contact that tunnel. Instead, it makes an alternate tunnel selection from the set of reachable tunnels at the same preference level. If no additional reachable tunnels exist at the current preference level, the router drops to the next lower preference level to make the next selection. This process is consistent, regardless of which fail-over scheme is currently running on the router. A tunnel without a configured maximum sessions value has no upper limit on the number of sessions it can support.

    The router uses a default value of 0 (zero), which allows unlimited sessions in the tunnel.

    • To configure the maximum sessions per tunnel.
      host1(config)#aaa domain-map lacOne host1(config-domain-map)#tunnel 1 host1(config-domain-map-tunnel)#max-sessions 1500

    Configuring the Weighted Load Balancing Method

    With the weighted load-balancing method, the router uses the maximum sessions per tunnel to choose among multiple tunnels that share the same preference level.

    The weight of a tunnel is proportional to its maximum session limit and the maximum session limits of the other tunnels at the same preference level. The tunnel with the largest maximum session value has the largest weight; the tunnel with the next largest maximum session value has the next largest weight, down to the tunnel with the smallest maximum session value that has the smallest weight. The router uses a round-robin tunnel selection method by default.

    • To configure the router to base tunnel selection within a preference level on the maximum sessions per tunnel.
      host1(config)#l2tp weighted-load-balancing

    Published: 2014-08-20