DMVPN-Tunnel Health Monitoring and Recovery Backup NHS

An NHS attains different states while associating with the hubs to from a spoke-to-hub tunnel. The table below describes different NHS states.

NHS priority is a numerical value assigned to a hub that controls the order in which spokes select hubs to establish a spoke-to-hub tunnel. The priority value ranges from 0 to 255, where 0 is the highest and 255 is the lowest priority.

You can assign hub priorities in the following ways:

• Unique priorities to all NHS.
• Same priority level to a group of NHS.
• Unspecified priority (value 0) for an NHS, a group of NHSs, or all NHSs.

NHS clusterless model is a model where you assign the priority values to the NHSs and do not place the NHSs into any group. NHS clusterless model groups all NHSs to a default group and maintains redundant connections based on the maximum NHS connections configured. Maximum NHS connections is the number of NHS connections in a cluster that must be active at any point in time. The valid range for maximum NHS connections is from 0 to 255.

Priority values are assigned to the hubs to control the order in which the spokes select hubs to establish the spoke-to-hub tunnel. However, assigning these priorities in a clusterless model has certain limitations.

The table below provides an example of limitations for assigning priorities in a clusterless model.

Consider a scenario with three data centers A, B, and C. Each data center consists of two NHSs: NHSs A1 and A2 comprise one data center, NHS B1 and B2 another, and C1 and C3 another.

Although two NHSs are available for each data center, the spoke is connected to only one NHS of each data center at any point in time. Hence, the maximum connection value is set to 3. That is, three spoke-to-hub tunnels are established. If any one NHS, for example, NHS B1, becomes inactive, the spoke-to-hub tunnel associated with NHS B1 goes down. Based on the priority model, NHS A2 has the next priority value and the next available NHS in the queue, so it forms the spoke-to-hub tunnel and goes up. However, this does not meet the requirement that a hub from data center B be associated with the spoke to form a tunnel. Hence, no connection is made to data center B.

This problem can be addressed by placing NHSs into different groups. Each group can be configured with a group specific maximum connection value. NHSs that are not assigned to any groups belong to the default group.

The table below presents an example of cluster functionality. NHSs corresponding to different data centers are grouped to form clusters. NHS A1 and NHS A2 with priority 1 and 2, respectively, are grouped as cluster1, NHS B1 and NHS B2 with prirority 1 and 2, respectively, are grouped as cluster2, and NHS C1 and NHS C2 with prirority 1 and 2, respectively, are grouped as cluster3. NHS 7, NHS 8, and NHS 9 are part of the default cluster. The maximum cluster value is set to 1 for each cluster so that at least one spoke-to-hub tunnel is continuously established with all the four clusters.

In scenario 1, NHS A1, NHS B1, and NHS C1 with the highest priority in each cluster are in the UP state. In scenario 2, the connection between the spoke and NHS A1 breaks, and a connection is established between the spoke and NHS A2 (hub from the same cluster). NHS A1 with the highest priority attains the PROBE state. In this way, at any point in time a connection is established to all the three data centers.

Fallback time is the time that the spoke waits for the NHS to become active before detaching itself from an NHS with a lower priority and connecting to the NHS with the highest priority to form a spoke-to-hub tunnel. Fallback time helps in avoiding excessive flaps.

The table below shows how the spoke flaps from one NHS to another excessively when the fallback time is not configured on the spoke. Five NHSs having different priorities are available to connect to the spoke to form a spoke-to-hub tunnel. All these NHSs belong to the default cluster. The maximum number of connection is one.

In scenario 1, NHS 5 with the lowest priority value is connected to the spoke to form a tunnel. All the other NHSs having higher priorities than NHS 5 are in the PROBE state.

In scenario 2, when NHS 4 becomes active, the spoke breaks connection with the existing tunnel and establishes a new connection with NHS 4. In scenario 3 and scenario 4, the spoke breaks the existing connections as soon as an NHS with a higher priority becomes active and establishes a new tunnel. In scenario 5, as the NHS with the highest priority (NHS 1) becomes active, the spoke connects to it to form a tunnel and continues with it until the NHS becomes inactive. Because NHS 1 is having the highest priority, no other NHS is in the PROBE state.

The table below shows how to avoid the excessive flapping by configuring the fallback time. The maximum number of connection is one. A fallback time period of 30 seconds is configured on the spoke. In scenario 2, when an NHS with a higher priority than the NHS associated with the spoke becomes active, the spoke does not break the existing tunnel connection until the fallback time. Hence, although NHS 4 becomes active, it does not form a tunnel and attain the UP state. NHS 4 remains active but does not form a tunnel untill the fallback time elapses. Once the fallback time elapses, the spoke connects to the NHS having the highest priority among the active NHSs.

This way, the flaps that occur as soon as an NHS of higher priority becomes active are avoided.

NHS recovery is a process of establishing an alternative spoke-to-hub tunnel when the existing tunnel becomes inactive, and connecting to the preferred hub upon recovery.

The following sections explain NHS recovery:

• Alternative Spoke to Hub NHS Tunnel
• Returning to Preferred NHS Tunnel upon Recovery

1.    enable

2.    configure terminal

3.    interface tunnel number

4.    ip nhrp nhs cluster cluster-number max-connections value

1.    enable

2.    configure terminal

3.    interface tunnel number

4.    ip nhrp nhs fallback fallback-time

1.    enable

2.    configure terminal

3.    interface tunnel number

4.    ip nhrp nhs nhs-address priority nhs-priority cluster cluster-number

1.    enable

2.    show ip nhrp nhs

3.    show ip nhrp nhs redundancy

4.    show ipv6 nhrp nhs

5.    show ipv6 nhrp nhs redundancy

Router# enable

Router# show ip nhrp nhs
Legend: E=Expecting replies, R=Responding, W=Waiting
Tunnel0:
10.0.0.1 RE priority = 0 cluster = 0

Router# show ip nhrp nhs redundancy
Legend: E=Expecting replies, R=Responding, W=Waiting
No.  Interface  Cluster  NHS         Priority  Cur-State  Cur-Queue  Prev-State Prev-Queue
1    Tunnel0    0        10.0.0.253  3         RE         Running    E          Running
2    Tunnel0    0        10.0.0.252  2         RE         Running    E          Running
3    Tunnel0    0        10.0.0.251  1         RE         Running    E          Running
No.  Interface  Cluster  Status   Max-Con Total-NHS Responding Expecting Waiting Fallback
1    Tunnel0    0        Enable   3       3         3          0         0       0

Router# show ipv6 nhrp nhs
Legend: E=Expecting replies, R=Responding, W=Waiting
Tunnel0:
2001::101 RE priority = 1 cluster = 5

Router# show ipv6 nhrp nhs redundancy
Legend: E=Expecting replies, R=Responding, W=Waiting
No.  Interface  Cluster  NHS         Priority  Cur-State  Cur-Queue  Prev-State Prev-Queue
1    Tunnel0    5        2001::101   1         E          Running     RE        Running
No.  Interface  Cluster  Status  Max-Con  Total-NHS Responding Expecting Waiting Fallback
1    Tunnel0    5        Disable Not Set  1         0          1         0       0