In an enterprise network, you would need to configure service providers to interconnect the hub and branches. The Path Preference Hierarchy feature allows you to configure three service providers per VRF for traffic classes. The service providers could be primary service provider, fallback service provider, and next-fallback service provider respectively. As the name suggests, the primary service provider is the first preference in the network, followed by fallback and next-fallback, respectively. You cannot have the same service provider for primary and fallback as this results in a “fallback backhole.” In other words, each service provider must be unique.

Use the path-preference command to specify the service provider order. Use the blackhole or routing keywords for a next-fallback service provider to drop the packet if fallback is unavailable or to specify there is no next-fallback service provider, respectively. When a packet reaches “blackhole,” the packet is discarded.

Perform this task to configure Path Preference Hierarchy feature on a hub.

domain default
 vrf green
  master hub
   source-interface Loopback1
   site-prefixes prefix-list HUBPFX
   class HEIRARCHICAL sequence 100
    match dscp ef policy custom
     priority 1 loss threshold 10
    path-preference ISP1 ISP2 fallback ISP3 next-fallback blackhole

The following is a sample output on a device that displays the route change reason and history. In this example, the traffic class jumps from next-fallback service provider to primary service provider, when the fallback is unavailable.

Dst-Site-Prefix: 100.30.0.0/16       DSCP: ef [46] Traffic class id:2
  Clock Time:                 12:57:15 (PST) 03/30/2015
  TC Learned:                 00:22:14 ago
  Present State:              CONTROLLED
  Current Performance Status: in-policy
  Current Service Provider:   ISP2 path-id:2 since 00:03:28
  Previous Service Provider:  ISP3 pfr-label: 0:0 | 0:7 [0x7] for 180 sec
  (A fallback/next-fallback provider. Primary provider will be re-evaluated 00:02:34 later)
  BW Used:                    3 Kbps
  Present WAN interface:      Tunnel20 in Border 100.10.2.1
  Present Channel (primary):  46 ISP2 pfr-label:0:0 | 0:2 [0x2]
  Backup Channel:             42 ISP3 pfr-label:0:0 | 0:7 [0x7]
  Destination Site ID bitmap: 0
  Destination Site ID:        100.30.1.1
  Class-Sequence in use:      10
  Class Name:                 BUSINESS using policy User-defined
    priority 2 packet-loss-rate threshold 10.0 percent
    priority 2 byte-loss-rate threshold 10.0 percent
  BW Updated:                 00:00:14 ago
  Reason for Latest Route Change:    next-fallback to Higher Path Preference
  Route Change History:
             Date and Time                   Previous Exit                         Current Exit                      Reason

    1:  12:53:47 (PST) 03/30/2015   ISP3/100.10.1.1/Tu30 (Ch:42)            ISP2/100.10.2.1/Tu20 (Ch:46)            next-fallback to Higher Path Preference
    2:  12:50:47 (PST) 03/30/2015   None/0.0.0.0/None (Ch:0)                ISP3/100.10.1.1/Tu30 (Ch:42)            Uncontrolled to Controlled Transition
    3:  12:50:15 (PST) 03/30/2015   ISP3/100.10.1.1/Tu30 (Ch:42)            None/0.0.0.0/None (Ch:0)                No Channels Available
    4:  12:48:14 (PST) 03/30/2015   ISP2/100.10.4.1/Tu20 (Ch:43)            ISP3/100.10.1.1/Tu30 (Ch:42)            Exit down
    5:  12:47:57 (PST) 03/30/2015   ISP2/100.10.2.1/Tu20 (Ch:46)            ISP2/100.10.4.1/Tu20 (Ch:43)            Exit down

In the following example, continuation of the above example, the traffic class is now controlled by primary service provider.

 Route Change History:
             Date and Time                   Previous Exit                         Current Exit                      Reason

    1:  12:59:49 (PST) 03/30/2015   ISP2/100.10.2.1/Tu20 (Ch:46)            ISP1/100.10.1.1/Tu10 (Ch:41)            Backup to Primary path preference transition
    2:  12:53:47 (PST) 03/30/2015   ISP3/100.10.1.1/Tu30 (Ch:42)            ISP2/100.10.2.1/Tu20 (Ch:46)            next-fallback to Higher Path Preference
    3:  12:50:47 (PST) 03/30/2015   None/0.0.0.0/None (Ch:0)                ISP3/100.10.1.1/Tu30 (Ch:42)            Uncontrolled to Controlled Transition
    4:  12:50:15 (PST) 03/30/2015   ISP3/100.10.1.1/Tu30 (Ch:42)            None/0.0.0.0/None (Ch:0)                No Channels Available
    5:  12:48:14 (PST) 03/30/2015   ISP2/100.10.4.1/Tu20 (Ch:43)            ISP3/100.10.1.1/Tu30 (Ch:42)            Exit down

In the following example, continuation of the above example, the traffic class is discarded since the packet has reached a blackhole.

Route Change History:
             Date and Time                   Previous Exit                         Current Exit                      Reason

    1:  12:50:15 (PST) 03/30/2015   ISP3/100.10.1.1/Tu30 (Ch:42)            None/0.0.0.0/None (Ch:0)                No Channels Available
    2:  12:48:14 (PST) 03/30/2015   ISP2/100.10.4.1/Tu20 (Ch:43)            ISP3/100.10.1.1/Tu30 (Ch:42)            Exit down
    3:  12:47:57 (PST) 03/30/2015   ISP2/100.10.2.1/Tu20 (Ch:46)            ISP2/100.10.4.1/Tu20 (Ch:43)            Exit down
    4:  12:44:42 (PST) 03/30/2015   ISP1/100.10.1.1/Tu10 (Ch:41)            ISP2/100.10.2.1/Tu20 (Ch:46)            Exit down
    5:  12:44:13 (PST) 03/30/2015   ISP1/100.10.3.1/Tu10 (Ch:44)            ISP1/100.10.1.1/Tu10 (Ch:41)            Exit down