How IPMP Works
IPMP maintains network availability by attempting to preserve the same number of active and standby interfaces that was originally configured when the IPMP group was created.
IPMP failure detection can be link-based, probe-based, or both to determine the availability of a specific underlying IP interface in the group. If IPMP determines that an underlying interface has failed, then that interface is flagged as failed and is no longer usable. The data IP address that was associated with the failed interface is then redistributed to another functioning interface in the group. If available, a standby interface is also deployed to maintain the original number of active interfaces.
Consider a three-interface IPMP group, itops0, with an active-standby configuration, as illustrated in the following figure.
Figure 2-1 IPMP Active-Standby Configuration
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Two data addresses are assigned to the group: 192.168.10.10 and 192.168.10.15.
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Two underlying interfaces are configured as active interfaces and are assigned flexible link names: net0 and net1.
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The group has one standby interface, also with a flexible link name: net2.
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Probe-based failure detection is used, and thus the active and standby interfaces are configured with test addresses, as follows:
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net0: 192.168.10.30
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net1: 192.168.10.32
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net2: 192.168.10.34
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The IPMP group itops0 is configured as follows:
Note - The Active, Offline, Standby, and Failed areas in Figure 2–1, Figure 2–2, Figure 2–3, and Figure 2–4 indicate only the status of underlying interfaces, and not physical locations. No physical movement of interfaces or addresses, or any transfer of IP interfaces, occurs within this IPMP implementation. The areas only serve to show how an underlying interface changes status as a result of either failure or repair.
You can use the ipmpstat command with different options to display specific types of information about existing IPMP groups. For additional examples, see Monitoring IPMP Information.
The following command displays information about the IPMP configuration in Figure 2–1:
# ipmpstat -g GROUP GROUPNAME STATE FDT INTERFACES itops0 itops0 ok 10.00s net1 net0 (net2)
You would display information about the group's underlying interfaces as follows:
# ipmpstat -i INTERFACE ACTIVE GROUP FLAGS LINK PROBE STATE net0 yes itops0 ------- up ok ok net1 yes itops0 --mb--- up ok ok net2 no itops0 is----- up ok ok
IPMP maintains network availability by managing the underlying interfaces to preserve the original number of active interfaces. Thus, if net0 fails, then net2 is deployed to ensure that the IPMP group continues to have two active interfaces. The net2 activation is shown in the following figure.
Figure 2-2 Interface Failure in IPMP
Note - The one-to-one mapping of data addresses to active interfaces in Figure 2–2 serves only to simplify the illustration. The IP kernel module can randomly assign data addresses without necessarily adhering to a one-to-one relationship between data addresses and interfaces.
The ipmpstat command displays the information in the figure as follows:
# ipmpstat -i INTERFACE ACTIVE GROUP FLAGS LINK PROBE STATE net0 no itops0 ------- up failed failed net1 yes itops0 --mb--- up ok ok net2 yes itops0 -s----- up ok ok
After net0 is repaired, it reverts to its status as an active interface. In turn, net2 is returned to its original standby status.
A different failure scenario is shown in Figure 2–3, where the standby interface net2 fails (1). Later, one active interface, net1, is taken offline by the administrator (2). The result is that the IPMP group is left with a single functioning interface, net0.
Figure 2-3 Standby Interface Failure in IPMP
The ipmpstat command displays the information in the figure as follows:
# ipmpstat -i INTERFACE ACTIVE GROUP FLAGS LINK PROBE STATE net0 yes itops0 ------- up ok ok net1 no itops0 --mb-d- up ok offline net2 no itops0 is----- up failed failed
For this particular failure, the recovery process after an interface is repaired is different. The recovery process depends on the IPMP group's original number of active interfaces compared with the configuration after the repair. The following figure represents the recovery process.
Figure 2-4 IPMP Recovery Process
In Figure 2–4, when net2 is repaired, it would normally revert to its original status as a standby interface (1). However, the IPMP group would still not reflect the original number of two active interfaces because net1 continues to remain offline (2). Thus, IPMP instead deploys net2 as an active interface (3).
The ipmpstat command displays the post-repair IPMP scenario as follows:
# ipmpstat -i INTERFACE ACTIVE GROUP FLAGS LINK PROBE STATE net0 yes itops0 ------- up ok ok net1 no itops0 --mb-d- up ok offline net2 yes itops0 -s----- up ok ok
A similar recovery process occurs if the failure involves an active interface that is also configured in FAILBACK=no mode, where a failed active interface does not automatically revert to active status upon repair. Suppose that net0 in Figure 2–2 is configured in FAILBACK=no mode. In that mode, a repaired net0 becomes a standby interface, even though it was originally an active interface. The interface net2 remains active to maintain the IPMP group's original number of two active interfaces.
The ipmpstat command displays the recovery information as follows:
# ipmpstat -i INTERFACE ACTIVE GROUP FLAGS LINK PROBE STATE net0 no itops0 i------ up ok ok net1 yes itops0 --mb--- up ok ok net2 yes itops0 -s----- up ok ok
For more information about this type of configuration, see FAILBACK=no Mode.