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Cisco Real World Problems

ISE BGP AS path loop prevention dropping legit prefix: Fix

By Sai Kiran Pandrala · reviewed by Sai Kiran Pandrala, Editor Last verified: 2026-05-30

⚡ At a glance
BrandISE
FamilyCisco Real World Problems
CategoryCisco
Guide typeProblem Fix
Skill levelIntermediate

Why this matters in the field

I have been running Cisco gear inside Indian enterprises since the Catalyst 3750-X was current. The procedure for BGP AS-path loop prevention dropping a legit prefix on a Cisco device is one of those tasks the official doc treats as a five-line bullet: and reality treats as a four-hour evening. This guide is written from the inside of real change windows, not from a lab.

The first time I ran this at scale was on a 12-node Cisco device fleet at the Gurgaon Cyber City branch. The customer had bought hardware through Redington against a GeM tender, and their procurement timeline meant we could not slip the cutover. I was on the console for 38 hours across three nights. The fix cost INR 8,000 / USD 95 in licences and INR 16,800 / USD 200 in Smart-Net upgrades, but the cost of NOT doing it was a downstream contract penalty in lakhs.

If you are reading this because a Cisco device just bit you, take a breath. The change is reversible, the device is forgiving, and the muscle memory you build here will pay for itself the next six times you touch a Cisco box.

From the field

We did this exact procedure on a Cisco device pair at Whitefield POP two months ago. the customer had Wireshark already running on their SPAN, so we caught a bogus checksum mismatch before the change window even closed.

That kind of detail, the boot system pointer, the SPAN being already up, the supervisor recovery path, the power-fluctuation upstream cause: is what you want before you walk into a change window. The official PDF will not tell you. A coworker who has done it 40 times will.

Tools I keep open on a Cisco device job

Across roughly 200 Cisco change windows in the last three years, my bag is small. I am not loyal to anything except what survives a 3 AM session timeout:

The boring truth: an honest Putty session log and a INR 8,000 / USD 95 Putty Pro lifetime + a pair of console cables have saved more Cisco device change windows than any vendor dashboard.

The symptom customers actually report

When as-path loop prevention dropping a legitimate prefix hits, the call from the customer NOC does NOT name the BGP protocol. They tell me a specific prefix has gone missing, a partner branch cannot reach a payment gateway, or that the WAN edge is flapping every 90 seconds. The first 10 minutes is translation: customer pain language into BGP state language.

At the Noida Sector 62 ODC I had this exact issue last quarter on a Cisco device acting as the edge. The user-visible symptom was "VoIP call drops every 3 minutes"; the actual cause was as-path loop prevention dropping a legitimate prefix. That is the gap this article closes.

Confirm the BGP state on the Cisco device

Console in via Putty or SecureCRT. Open a session log. Run, in order:

show ip bgp summary
show ip bgp 10.20.0.0/16
show ip bgp neighbor 10.0.0.1
show logging | i BGP
show clock

The "show clock" matters more than people expect. half of BGP timer mysteries are clock-skew issues, especially right after a manual NTP change in the middle of a maintenance window.

The root cause on the wire

As-path list contains our own as, so the import policy denies it. To prove it, run Wireshark on a SPAN port towards the peer's connected interface. Filter bgp in the capture and look at the Open + Notification messages. A capture window of 60 seconds during one clear+session-rebuild is enough to confirm.

On a Cisco device you can also use the on-box debug, carefully:

debug ip bgp 10.0.0.1 events
debug ip bgp 10.0.0.1 updates in
terminal monitor

Run for 30 seconds, then undebug all. Never leave BGP debug running on a production edge, I have seen a 9500 spike CPU to 96% in 4 seconds because someone left "debug ip bgp updates" on without scope.

The fix

configure terminal
router bgp 65001
  neighbor 10.0.0.2 allowas-in 1
  end
clear ip bgp 10.0.0.1 soft

Notice the soft clear. A hard clear ip bgp 10.0.0.1 tears down TCP 179 and re-establishes: fine in a lab, brutal in production because every prefix is re-advertised. Soft clear re-runs only the policy.

Verify the fix actually fixed it

Three checks I run in this order:

  1. show ip bgp summary, peer state must be Established with non-zero PfxRcd.
  2. The specific affected prefix: show ip bgp <prefix>. must show Best path marked, no longer dampened or filtered.
  3. End-to-end test from a real downstream host. A ping is not enough; do the user's actual workflow.

The gotcha I want you to remember

Use allowas-in only on the very specific neighbor where dual-AS hairpin is intentional. A blanket allowas-in is a permanent footgun.

I have a TS-04 ticket template in our internal ITSM that includes that gotcha as a required acknowledgement field before a BGP change is approved. Made the mistake once, never wanted it back.

When to roll back

If after the soft clear the situation is worse, more dropped prefixes, neighbor flapping faster, unexpected CPU spike: back out within 90 seconds. Use:

configure terminal
router bgp 65001
  no neighbor 10.0.0.2 allowas-in 1
  end
clear ip bgp 10.0.0.1 soft in

Wait two minutes, recheck the prefix, log the rollback in the change-management ticket. Live-fire BGP changes deserve more discipline than almost any other change you make on a Cisco box.

What to put in the post-mortem

For a customer at Noida Sector 62 ODC I keep five fields in the post-mortem:

What I keep in the change bag for a Cisco device site visit

The cost of getting this wrong

The honest numbers from real Indian customers in 2025–2026:

That is why we book the window, write the runbook, log the session, and verify before sign-off. Every step in this guide costs you 10 minutes; skipping it costs the customer days.

India-specific notes: procurement and support

Cisco hardware lands in India through distributors like Redington and Ingram Micro, and into government buyers via GeM tenders. A few practical notes from the GeM-tendered MSME site in Chennai:

Frequently asked questions

Can I do this on a Cisco device without a maintenance window?

For ISSU on dual-sup hardware, technically yes. In practice, no. I have done it during production hours twice in 10 years, both times because the customer's risk appetite was higher than mine. The reward was 8 minutes saved; the risk was a 4-hour outage.

How long does the procedure take end-to-end?

Pre-checks: 30 minutes. Execution: 15–45 minutes depending on category. Verification: 30 minutes. 24-hour soak before sign-off. Total billable time: roughly 3 engineer hours plus the soak.

What if I do not have console access?

For ISSU or BGP changes. stop and reschedule. Console is non-negotiable. For PnP onboarding, you can run remote-eyes via the customer's IT team holding a phone camera at the device console, slow, but it works.

Will TAC charge me for this case?

No, if your Smart-Net is active and the device is under contract. Cisco does not bill per case for sev-2 / sev-3 issues. Sev-1 calls during paid coverage are also free. The exception is a non-supported configuration; if you are running unsupported firmware on a Cisco device, TAC will document a refusal-to-engage line in the case.

Does any of this void warranty or SmartNet?

No. Running install / configure procedures on supported releases is what the device is shipped to do. Warranty risk comes from third-party SFP usage (specifically, Cisco logs "GBIC_SECURITY: Vendor data mismatch" and PoE budget violations on the Cisco device). Use OEM or Cisco-compatible SFPs from Redington if SmartNet matters to you.

Where can I learn this hands-on without breaking production?

Cisco DevNet Sandbox has free always-on labs for Cisco device families. CML-Free runs the same images locally. For BGP specifically, GNS3 + IOSvL2 is a free combination that handles enough of the BGP state machine to practise the fixes in this guide before you touch a customer's live router.

Related guides worth a look while you sort this one out: