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On some deployments, uplink IPv6 addresses can be programmed in VPP with host prefixes (`/128`) which means there is no connected subnet on the interface. Neighbor discovery can still learn MAC/IP entries, but VPP may create host/adj-fib behavior that effectively treats off-subnet neighbors as attached, causing forwarding to become `UNRESOLVED` for certain peers and intermittently override expected default-route forwarding. The failure is intermittent because it is timing-dependent: whether VPP learns the neighbor (via NDP) before or after the default route is installed, and on neighbor aging/re-learning cycles. Introduced CALICOVPP_DEBUG.uplinkSubnetMask (default: 64) to force all IPv6 uplink interface AddInterfaceAddress calls in vpp-manager to use this mask, regardless of the source netmask from Linux interface. This keeps neighbor/MAC learning behavior while ensuring uplink IPv6 addresses are installed with a connected-prefix mask that avoids host-prefix adjacency edge cases. Signed-off-by: Aritra Basu <aritrbas@cisco.com>
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RCA:
On some deployments, uplink IPv6 addresses can be programmed in VPP with host prefixes (
/128) which means there is no connected subnet on the interface. Neighbor discovery can still learn MAC/IP entries, but VPP may create host/adj-fib behavior that effectively treats off-subnet neighbors as attached, causing forwarding to becomeUNRESOLVEDfor certain peers and intermittently override expected default-route forwarding.The failure is intermittent because it is timing-dependent: whether VPP learns the neighbor (via NDP) before or after the default route is installed, and on neighbor aging/re-learning cycles.
Fix:
When configuring IPv6 addresses on the VPP uplink interface via
AddInterfaceAddress, override the Linux-sourced netmask with a configurable value (default/64). This creates a connected/64prefix on the uplink, so that:The mask override is controlled by a new debug config field
uplinkSubnetMask(default: 64, range: [1, 128]) in theCALICOVPP_DEBUGConfigMap.