Route Withdrawal

Watch a prefix leave the system. Establish forwarding for a test prefix, then withdraw it at the source and observe the removal cascade — control plane (FRR), application state (APPL_DB), and hardware state (ASIC_DB) — while testing forwarding before and after. The point is to see teardown propagate in the same order as install, and to time it.

  peer withdraws 10.2.2.0/24 ; observe removal on sonic-a

Prerequisites: Lab 2.2 completed (you can trace the prefix). · Lesson 2.3 lifecycle.

Tasks

1. 1

   Baseline: confirm the route forwards

   SONiC CLI Copy

   show ip route 10.2.2.0/24

   Linux Copy

   ping -c 2 10.2.2.1

   Expected:

   • Route present at all layers; ping succeeds (VS limits noted).

   What happened internally: Establishes the 'before' state so the withdrawal delta is unambiguous.

2. 2

   Withdraw the prefix at the source

   On the peer (AS 65002), stop advertising the test prefix.

   FRR / vtysh Disruptiveon the peer — removes the advertisement Copy

   vtysh -c 'conf t' -c 'router bgp 65002' -c ' address-family ipv4 unicast' -c '  no network 10.2.2.0/24' -c 'end'

   Expected:

   • The peer stops advertising; a BGP withdraw is sent toward sonic-a.

   What happened internally: Withdrawing the network origin triggers a BGP UPDATE with the prefix withdrawn, starting the teardown cascade.

3. 3

   Observe removal at the control plane

   FRR / vtysh Copy

   vtysh -c 'show ip bgp 10.2.2.0/24'

   FRR / vtysh Copy

   vtysh -c 'show ip route 10.2.2.0/24'

   Expected:

   • Prefix gone from bgpd and no longer selected in zebra.

   What happened internally: bgpd removes the path and informs zebra, which withdraws the route and signals FPM to delete it downstream.

4. 4

   Observe removal at application and hardware state

   SONiC DBexpect empty Copy

   sonic-db-cli APPL_DB HGETALL 'ROUTE_TABLE:10.2.2.0/24'

   SONiC DBIllustrative outputthe dest should no longer be present Copy

   sonic-db-cli ASIC_DB EVAL "return redis.call('SCAN',0,'MATCH','ASIC_STATE:SAI_OBJECT_TYPE_ROUTE_ENTRY:*','COUNT',200)" 0

   Expected:

   • APPL_DB entry deleted; no ASIC_DB route object for the dest.

   What happened internally: fpmsyncd deletes the APPL_DB entry; orchagent issues SAI remove; syncd tears down the hardware entry. Same chain as install, in reverse.

5. 5

   Confirm forwarding stopped

   Linux Copy

   ping -c 3 10.2.2.1

   Expected:

   • Ping now fails / no route — forwarding removed, no stale black-hole-avoiding entry lingering.

   What happened internally: Verifies teardown reached the data plane. Lingering forwarding after full withdrawal would indicate stale state (Lesson 2.4).

6. 6

   Restore the advertisement

   FRR / vtyshon the peer — re-advertise Copy

   vtysh -c 'conf t' -c 'router bgp 65002' -c ' address-family ipv4 unicast' -c '  network 10.2.2.0/24' -c 'end'

   Expected:

   • Prefix relearned and reprogrammed; forwarding returns.

   What happened internally: Confirms the install cascade re-runs cleanly, and returns the lab to the Lab 2.1/2.2 baseline for later labs.