[chassis] Default route learnt from peer linecard is not installed

[arlakshm]

Description

In a SONiC chassis in T2 topology, on the linecard with no upstream(T3) neighbors the default route is learnt over the internal iBGP session from the remote linecard which has connected to upstream routers.

Example

admin@str2-sonic-lc7-1:~$ show ip route 0.0.0.0/0
Routing entry for 0.0.0.0/0
  Known via "bgp", distance 200, metric 0
  Last update 02:05:49 ago
    10.0.0.1 (recursive), weight 1
      10.0.0.1, via Ethernet-IB0 onlink, weight 1
    10.0.0.9 (recursive), weight 1
      10.0.0.9, via Ethernet-IB0 onlink, weight 1
    10.0.0.13 (recursive), weight 1
      10.0.0.13, via Ethernet-IB0 onlink, weight 1
    10.0.0.17 (recursive), weight 1
      10.0.0.17, via Ethernet-IB0 onlink, weight 1
    10.0.0.21 (recursive), weight 1
      10.0.0.21, via Ethernet-IB0 onlink, weight 1
    10.0.0.25 (recursive), weight 1
      10.0.0.25, via Ethernet-IB0 onlink, weight 1
    10.0.0.29 (recursive), weight 1
      10.0.0.29, via Ethernet-IB0 onlink, weight 1
    10.0.0.33 (recursive), weight 1
      10.0.0.33, via Ethernet-IB0 onlink, weight 1
    10.0.0.37 (recursive), weight 1
      10.0.0.37, via Ethernet-IB0 onlink, weight 1
    10.0.0.39 (recursive), weight 1
      10.0.0.39, via Ethernet-IB0 onlink, weight 1
    10.0.0.41 (recursive), weight 1
      10.0.0.41, via Ethernet-IB0 onlink, weight 1
    10.0.0.43 (recursive), weight 1
      10.0.0.43, via Ethernet-IB0 onlink, weight 1
    10.0.0.45 (recursive), weight 1
      10.0.0.45, via Ethernet-IB0 onlink, weight 1
    10.0.0.47 (recursive), weight 1
      10.0.0.47, via Ethernet-IB0 onlink, weight 1
    10.0.0.49 (recursive), weight 1
      10.0.0.49, via Ethernet-IB0 onlink, weight 1
    10.0.0.51 (recursive), weight 1
      10.0.0.51, via Ethernet-IB0 onlink, weight 1
    10.0.0.53 (recursive), weight 1
      10.0.0.53, via Ethernet-IB0 onlink, weight 1
    10.0.0.55 (recursive), weight 1
      10.0.0.55, via Ethernet-IB0 onlink, weight 1
    10.0.0.57 (recursive), weight 1
      10.0.0.57, via Ethernet-IB0 onlink, weight 1
    10.0.0.59 (recursive), weight 1
      10.0.0.59, via Ethernet-IB0 onlink, weight 1
    10.0.0.61 (recursive), weight 1
      10.0.0.61, via Ethernet-IB0 onlink, weight 1

Routing entry for 0.0.0.0/0
  Known via "static", distance 200, metric 0, best
  Last update 1d01h47m ago
  * 10.3.146.1, via eth0, weight 1

However, the default route learnt from BGP is not installed in the kernel because it is not considered as best route.
The static route programmed for the management gateway address has the same distance, so it is considered as the best route
This results in no default route present in the APP_DB and ASIC_DB

Describe the results you received:

Default route install in kernel

admin@str2-sonic-lc7-1:~$ sudo ip route show 0.0.0.0/0
default via 10.3.146.1 dev eth0 proto 196 metric 20

Default route missing in APP_DB, as routes with nexthop as eth0 is not added to APP_DB

admin@str2-sonic-lc7-1:~$ redis-cli keys *0.0.0.0/0*
(empty array)
admin@str2-sonic-lc7-1:~$

Describe the results you expected:

The default route learnt from the remote linecard should be installed

Output of show version:

SONiC Software Version: SONiC.202106.41991-7aa9fde3e
Distribution: Debian 10.10
Kernel: 4.19.0-12-2-amd64
Build commit: 7aa9fde3e
Build date: Thu Oct  7 13:39:47 UTC 2021
Built by: AzDevOps@sonic-build-workers-000RRM

Output of show techsupport:

(paste your output here or download and attach the file here )

Additional information you deem important (e.g. issue happens only occasionally):