[snmpagent][rfc1213] Optimize ipRouteNextHop default-route lookup

[vpandian-nokia]

What I did

Optimized ipRouteNextHop handling in rfc1213.py to avoid scanning all route keys in APPL_DB, while preserving RFC1213 default-route behavior.

• Replaced ROUTE_TABLE:* scan + filter with direct lookup of ROUTE_TABLE:0.0.0.0/0.
• Kept behavior of selecting the first valid IPv4 nexthop.
• Added robust parsing for nexthop tokens (strip() + invalid-token handling).
• Skip non-IPv4 nexthops (IPv6 tokens are ignored with warning), aligned with RFC1213 IPv4 semantics.
• Ensured route_list is updated only after successful IPv4 nexthop parse.
• Updated unit tests in tests/test_rfc1213.py for direct lookup path, invalid-token fallback, and IPv6/IPv4 nexthop cases.

How I did it

Modified NextHopUpdater.update_data() to:

1. Query only ROUTE_TABLE:0.0.0.0/0 from APPL_DB.
2. Read nexthop field from the returned route entry.
3. Parse nexthops left-to-right and pick the first valid IPv4 token.
4. Log warning for invalid/non-IPv4 tokens and continue scanning remaining tokens.
5. Populate SNMP route cache only when a valid IPv4 nexthop is found.

How to verify it

• Unit tests: tests/test_rfc1213.py
• DUT functional validation on a Broadcom-based SONiC switch with large BGP route scale (~250k routes):
  • Verified optimized code path (ROUTE_TABLE:0.0.0.0/0 direct lookup + IPv4-only nexthop filter)
  • No default route: snmpwalk 1.3.6.1.2.1.4.21.1.7 returns No Such Instance in <200ms (stable across repeated polls)
  • With temporary static default route (0.0.0.0/0 via valid IPv4 nexthop): snmpwalk/get returns expected IPv4 nexthop in <200ms
  • After route removal: behavior restored to No Such Instance

- Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202605

Tracking issue/work item for backport/cherry-pick request (GitHub issue or Microsoft ADO):
Failure type: N/A (optimization; no backport requested)

- Tested branch

• master
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511
• 202605
• N/A

- Test result

• master:
  • Unit tests in tests/test_rfc1213.py passed
  • DUT SNMP validation passed on a large-scale route table (~250k BGP routes), covering:
    • no default route (No Such Instance, fast response)
    • temporary default route (expected IPv4 nexthop returned, fast response)
  • Repeated polling showed stable behavior with no timeout/hang

Description for the changelog

Optimize SNMP ipRouteNextHop default-route lookup by replacing full route-table scan with direct ROUTE_TABLE:0.0.0.0/0 query and improving IPv4 nexthop parsing robustness.

[mssonicbld]

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[mssonicbld]

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[vpandian-nokia]

@qiluo-msft This PR is a follow-up optimization of ipRouteNextHop (default-route-only) in PR #26.

Changes in this PR:

• replace ROUTE_TABLE:* scan with direct lookup of ROUTE_TABLE:0.0.0.0/0
• preserve existing default-route behavior
• add robustness for invalid nexthop tokens

Would appreciate your review when you have a chance.

[mssonicbld]

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[vpandian-nokia]

@deepak-singhal0408: The relevant sonic-mgmt SNMP tests passed for this change. I would appreciate a reviewer taking a look and, if everything looks good, helping approve the PR.

[vpandian-nokia]

@qiluo-msft Gentle follow-up on this PR when you have a chance.

The relevant sonic-mgmt SNMP tests have passed, and the change is ready for review. Please let me know if any additional information is needed.

[mssonicbld]

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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[mssonicbld]

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[vpandian-nokia]

Hi @deepak-singhal0408 @qiluo-msft —

Latest update:

• Rebased on current master
• Added c986fc1 (IPv4-only nexthop handling + unit tests)
• DUT validation completed on ~250k-route scale (no-default and temporary default-route cases)
• PR description updated

CI is green on c986fc1. If Semgrep approval is still needed for this fork PR, please help approve/run it.

Ready for review/merge.
Thanks!