pimd, tests: Add pim test showing that registers work with no path -> rp

[donaldsharp]

Add a test that creates a test router r1 that does not actually have a path to the rp at first, start a *,G and a S,G stream on r1 and then introduce the ability to actually reach the RP.

[greptile-apps]

Greptile Summary

Added a new topotest validating PIM register functionality when the Rendezvous Point (RP) is initially unreachable, then becomes reachable during active multicast traffic.

The test creates a topology with three routers: rp (RP), r1 (first-hop router), and r2 (multicast source). Key test phases include:

• Verifying r1 initially has no route to the RP loopback (10.255.0.3/32)
• Establishing PIM neighbor adjacencies between rp and r1
• Starting multicast traffic from r2 (10.0.2.2) to group 229.1.1.1 while RP is unreachable
• Dynamically adding routes to make the RP reachable
• Validating proper PIM upstream state transitions (RegPrune on r1, RegNoInfo on rp)

This test validates the important edge case where multicast streams start before RP reachability is established, ensuring PIM registers work correctly when paths are later introduced.

Confidence Score: 5/5

• This PR is safe to merge with no issues identified
• This is a pure test addition with no production code changes. The test follows established topotest patterns, includes proper topology setup/teardown, comprehensive state validation, and memory leak testing. All configuration files are syntactically correct and align with the test objectives.
• No files require special attention

Important Files Changed

Filename	Overview
tests/topotests/pim_no_path_to_rp/test_pim_no_path_to_rp.py	Added comprehensive test validating PIM register behavior when RP is initially unreachable, then becomes reachable
tests/topotests/pim_no_path_to_rp/r1/frr.conf	Configuration for r1 router with PIM and IGMP enabled, RP set to 10.255.0.3
tests/topotests/pim_no_path_to_rp/r2/frr.conf	Minimal configuration for r2 as multicast source, no PIM configuration needed
tests/topotests/pim_no_path_to_rp/rp/frr.conf	Configuration for RP router with loopback address 10.255.0.3/32 serving as the rendezvous point

Sequence Diagram

sequenceDiagram
    participant Test as Test Framework
    participant r1 as Router r1
    participant rp as Router rp (RP)
    participant r2 as Router r2 (Source)
    
    Note over Test,r2: Setup Phase
    Test->>r1: Configure PIM, IGMP, set RP to 10.255.0.3
    Test->>rp: Configure PIM, set loopback 10.255.0.3/32
    Test->>r2: Configure basic IP only
    
    Note over Test,r2: Phase 1: No RP Reachability
    Test->>r1: Verify no route to 10.255.0.3/32
    r1-->>Test: "Network not in table"
    
    Test->>r1: Check PIM neighbor with rp
    Test->>rp: Check PIM neighbor with r1
    r1-->>Test: PIM neighbor on r1-eth0
    rp-->>Test: PIM neighbor on rp-eth0
    
    Note over Test,r2: Phase 2: Start Multicast Traffic
    Test->>r1: Start mcast-tester for 229.1.1.1
    Test->>r2: Start mcast-tx from 10.0.2.2 to 229.1.1.1
    
    r2->>r1: Multicast traffic (S,G)
    Test->>r1: Verify mroute exists
    r1-->>Test: Mroute (10.0.2.2, 229.1.1.1) present
    Test->>r1: Verify IGMP group
    r1-->>Test: IGMP group 229.1.1.1 on r1-eth1
    
    Note over Test,r2: Phase 3: Add RP Reachability
    Test->>rp: Add route: 10.0.2.0/24 via 10.0.1.1
    Test->>r1: Add route: 10.255.0.3/32 via 10.0.1.3
    
    Test->>r1: Verify route to RP loopback
    r1-->>Test: Route via r1-eth0
    Test->>rp: Verify route to source
    rp-->>Test: Route via rp-eth0
    
    Note over Test,r2: Phase 4: Verify PIM State
    Test->>r1: Check PIM RP info
    r1-->>Test: RP reachable via r1-eth0
    
    Test->>r1: Check upstream state
    r1-->>Test: State=J,RegP, regState=RegPrune
    
    Test->>rp: Check upstream state
    rp-->>Test: State=NotJ, regState=RegNoInfo
    
    Test->>r1: Terminate multicast processes
    Test->>r2: Terminate multicast processes

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[greptile-apps]

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[Jafaral]

@Mergifyio backport stable/10.5 stable/10.4

[mergify]

backport stable/10.5 stable/10.4

✅ Backports have been created

Details

• #20594 pimd, tests: Add pim test showing that registers work with no path -> rp (backport #20577) has been created for branch stable/10.5
• #20595 pimd, tests: Add pim test showing that registers work with no path -> rp (backport #20577) has been created for branch stable/10.4