Add W-ECMP test plan (sonic-net#12933)

 Add W-ECMP test plan re https://github.com/sonic-net/SONiC/blob/master/doc/wcmp/wcmp-design.md

Author: weiguo-nvidia

docs/testplan/Img/W-ECMP_disabled.png

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+# W-ECMP Test Plan
+
+## Related documents
+
+| **Document Name** | **Link** |
+|-------------------|----------|
+| SONiC Weighted ECMP | [https://github.com/sonic-net/SONiC/blob/master/doc/wcmp/wcmp-design.md]|
+
+## Overview
+In ECMP (Equal-Cost Multi-Path) scenario, traffic is distributed equally across all available paths and each path has same cost.
+
+In W-ECMP (Weighted-Cost Multi-Path) scenario, it can distribute traffic base on the weight value assigned to each path. This is useful when the paths have different bandwidth capacities. In a W-ECMP setup, each path is assigned a weight based on various criteria, such as bandwidth, delay, or administrative preferences. BGP then uses these weights to distribute outbound traffic among the multiple available paths
+
+
+## BGP link bandwidth extended community
+The BGP link bandwidth extended community contains information about the bandwidth of a link. This information is advertised along with BGP route updates to inform other routers about the capacity of the link.
+RFC: [https://datatracker.ietf.org/doc/html/draft-ietf-idr-link-bandwidth]
+
+```
+router bgp 65200
+ bgp router-id 10.1.0.2
+ no bgp ebgp-requires-policy
+ neighbor SPINE1 peer-group
+ neighbor SPINE1 remote-as 65100
+ neighbor 10.10.1.1 peer-group SPINE1
+ neighbor 10.10.2.1 peer-group SPINE1
+ !
+ address-family ipv4 unicast
+  network 20.20.20.0/24
+  neighbor SPINE1 route-map TO_BGP_PEER_V4 out      <<<<<<
+ exit-address-family
+exit
+!
+route-map TO_BGP_PEER_V4 permit 100
+ set extcommunity bandwidth num-multipaths          <<<<<<
+exit
+!
+route-map TO_BGP_PEER_V6 permit 100
+ set extcommunity bandwidth num-multipaths
+exit
+```
+
+### Disable W-ECMP
+If W-ECMP is `disabled`, the BGP update message contains three default path attributes `ORIGIN`/`AS_PATH`/`NEXT_HOP` when advertising network prefix 10.0.0.0/24
+![Use case scenario](Img/W-ECMP_disabled.png)
+
+
+### Enable W-ECMP
+If W-ECMP is `enabled`, the BGP `EXTENDED_COMMUNITIES` attribute is added in the update message (yellow part)
+![Use case scenario](Img/W-ECMP_enabled.png)
+
+
+## Related DUT CLI commands
+
+### Config
+The following command can be used to enable/disable W-ECMP:
+```
+config bgp device-global wcmp enabled
+config bgp device-global wcmp disabled
+```
+
+### Show
+The following command can be used to show W-ECMP status:
+```
+show bgp device-global
+show bgp device-global --json
+```
+Example:
+```
+admin@router:~$ show bgp device-global
+TSA       WCMP
+--------  -------
+disabled  enabled
+```
+```
+admin@router:~$ show bgp device-global --json
+{
+    "tsa": "disabled",
+    "wcmp": "enabled"
+}
+```
+
+
+## Related DUT configuration files
+```
+"BGP_DEVICE_GLOBAL": {
+    "STATE": {
+        "tsa_enabled": "false",
+        "wcmp_enabled": "true"
+    }
+}
+```
+
+
+## Testbed
+![Testbed topology](Img/W-ECMP_topology.png)
+We can use BGP VRF to split the DUT host into DUT1 and DUT2
+
+1. BGP neighbor relationship
+- DUT1 establish BGP neighbor with SPINE1 and SPINE2, each neighbor has two BGP sessions
+- DUT2 establish BGP neighbor with SPINE1 and SPINE2, each neighbor has two BGP sessions
+
+2. BGP route advertise
+- DUT2 advertises ip prefix 100.0.0.1/32 to SPINE1, then SPINE1 advertises it to DUT1
+- DUT2 advertises ip prefix 100.0.0.1/32 to SPINE2, then SPINE2 advertises it to DUT1
+
+Finally, DUT1 has four route to the destination 100.0.0.1
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 1
+        nexthop via 10.10.27.1 dev Ethernet104 weight 1
+        nexthop via 10.10.28.1 dev Ethernet108 weight 1
+        nexthop via 10.10.29.1 dev Ethernet112 weight 1
+```
+
+
+## Test cases
+### Test case #1 - W-ECMP command line test
+1. Enable W-ECMP and verify W-ECMP status on DUT2
+```
+admin@router:~$ sudo config bgp device-global wcmp enabled
+admin@router:~$ show bgp device-global
+TSA       WCMP
+--------  -------
+disabled  enabled
+```
+2. Verify the route weight value on DUT1 is 25, which means W-ECMP is effect
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 25
+        nexthop via 10.10.27.1 dev Ethernet104 weight 25
+        nexthop via 10.10.28.1 dev Ethernet108 weight 25
+        nexthop via 10.10.29.1 dev Ethernet112 weight 25
+```
+3. Disable W-ECMP and verify W-ECMP status on DUT2
+```
+admin@router:~$ sudo config bgp device-global wcmp disabled
+admin@router:~$ show bgp device-global
+TSA       WCMP
+--------  -------
+disabled  disabled
+```
+4. Verify the route weight value on DUT1 is 1, which means W-ECMP is disabled
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 1
+        nexthop via 10.10.27.1 dev Ethernet104 weight 1
+        nexthop via 10.10.28.1 dev Ethernet108 weight 1
+        nexthop via 10.10.29.1 dev Ethernet112 weight 1
+```
+
+
+### Test case #2 - Verify W-ECMP behavior when no link failure
+1. Enable W-ECMP and verify W-ECMP status on DUT2
+```
+admin@router:~$ sudo config bgp device-global wcmp enabled
+```
+2. Verify the route weight value on DUT1 is 25, which means W-ECMP take effective
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 25
+        nexthop via 10.10.27.1 dev Ethernet104 weight 25
+        nexthop via 10.10.28.1 dev Ethernet108 weight 25
+        nexthop via 10.10.29.1 dev Ethernet112 weight 25
+```
+3. DUT1 sends traffic to destination 100.0.0.1
+4. Verify that traffic throughput on each nexthop is the same
+
+
+### Test case #3 - Verify W-ECMP behavior after link failure
+1. Enable W-ECMP and verify W-ECMP status on DUT2
+```
+admin@router:~$ sudo config bgp device-global wcmp enabled
+```
+2. Verify the route weight value on DUT1 is 25, which means W-ECMP take effective
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 25
+        nexthop via 10.10.27.1 dev Ethernet104 weight 25
+        nexthop via 10.10.28.1 dev Ethernet108 weight 25
+        nexthop via 10.10.29.1 dev Ethernet112 weight 25
+```
+3. Shutdown one link between DUT2 and SPINE2
+4. Verify the weight value on DUT1 has updated after link failure
+```
+admin@router:~$ ip route show 100.0.0.1
+100.0.0.1 proto bgp metric 20
+        nexthop via 10.10.26.1 dev Ethernet100 weight 16
+        nexthop via 10.10.27.1 dev Ethernet104 weight 16
+        nexthop via 10.10.28.1 dev Ethernet108 weight 33
+        nexthop via 10.10.29.1 dev Ethernet112 weight 33
+```
+5. DUT1 sends traffic to destination 100.0.0.1
+6. Verify that traffic throughput on each nexthop is based on weight value of the interface
+7. Recover the failed link between DUT and SPINE1
+8. Verify the route weight value on DUT1 is 25
+
+
+### Test case #4 - Verify configuration persists after warm/cold/fast reboot
+1. Enable W-ECMP on DUT, verify the config take effective on DUT2
+2. DUT2 warm/cold/fast reboot
+3. Verify W-ECMP configuration persist after reboot
+4. Traffic test, verify the W-ECMP function still take effect
+
+
+### Test case #5 - Verify interface flap
+1. Enable W-ECMP on DUT2
+2. The interface between DUT2 and SPINE2 flaps 10 times, it will trigger weight recalculation and prefix installation
+3. Verify the W-ECMP function still take effect and no dump generate
+
+
+### Test case #6 - Verify W-ECMP feature status flap
+1. Enable/disable W-ECMP feature 10 times on DUT2
+2. Verify the W-ECMP function still take effect and no dump generate