Topotest test_bgp_instance_del_test test_adjacencies fails constantly on Debian 10

[mwinter-osr]

While in the progress to add Debian 10 (to replace Ubuntu 16.04) on the CI, we found the issue that the test test_bgp_instance_del_test test_adjacencies always fails on Debian 10.

CI Tests: https://ci1.netdef.org/browse/TESTING-TOPO10DEB10

So far only tested with latest FRR master (sha c3ac2e5 as of 4/1/2021) (7.5 Testing in process)

This issue is mostly to track this, as it's holding up the replacement of Ubuntu 16.04 (and with it the python 3 migration of the Topotests)

[mwinter-osr]

Same issue for stable/7.5 branch. Fails same tests

[mwinter-osr]

The problem is with failing to establish BGP across a VRF connection

The issue is in our check if we need to set the sysctl net.ipv4.tcp_l3mdev_accept
We are setting this to 1 for Kernel 4.15-4.18 and for Kernel 5.0+

    if (
        topotest.version_cmp(krel, "4.15") >= 0
        and topotest.version_cmp(krel, "4.18") <= 0
    ):
        l3mdev_accept = 1

    if topotest.version_cmp(krel, "5.0") >= 0:
        l3mdev_accept = 1

Debian 10 uses a 4.19 kernel

If we would do just a check for 4.15 or higher (and include Debian's 4.19 kernel), then the test passes

Why do we exclude Kernels 4.19+ (< 5.0) from setting net.ipv4.tcp_l3mdev_accept to 1 ?
Is there a specific reason to exclude these kernels?

[davischw]

@mwinter-osr Fix in PR #8402