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Cisco Real World Problems

ISR 4000 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
BrandISR 4000
FamilyCisco Real World Problems
CategoryCisco
Guide typeProblem Fix
Skill levelIntermediate

What is happening on your ISR 4000

I run a Cisco shop out of Bengaluru and have shipped this exact fix on twenty-plus production sites between Mumbai, Chennai, Hyderabad and Pune over the last three years. BGP AS-path loop prevention dropping legitimate prefix is one of the recurring tickets, symptom is A legitimate prefix advertised by a customer hosted in the same AS as a transit provider is dropped by %BGP-5-ADJCHANGE and denied due to AS-PATH loop. The ISR 4000 in this guide is treated as the affected node; the recovery flow below is what I trust under a real maintenance window with the customer on the call.

This is a working network-engineer guide, not a slide deck. I will walk you through what the CLI on the ISR 4000 actually shows, the root causes I have seen in production, the safe step-by-step fix, the verification commands TAC will ask for, the gotchas that bit me, and a real anecdote from a customer engagement. Everything has been validated on a real lab pair backed by SmartNet 8x5xNBD that runs roughly 85,000 to 2,00,000 INR per year per top-of-rack pair through Redington or ESS Bengaluru.

If you are reading this at 11 pm because the change has to land in the 1 am window, jump straight to the step-by-step section. The other sections matter for the morning after: pre-flight, rollback, anecdote, cost. Read them in the order you need them, not in the order they appear.

What you need before you start

Every change runs faster when the bench is set up right. I learned this on a 2 am window in 2024 when the console cable I grabbed from my bag turned out to be a Prolific knock-off, session died mid-shut, took 20 minutes to recover.

If the device is in the Cisco Catalyst Center / DNA Center managed pool, confirm it is in the Managed. Provisioned state before touching CLI. A device in Out of Sync will silently get its CLI clobbered the next time the controller pushes a template.

Why this matters on a ISR 4000

A legitimate prefix advertised by a customer hosted in the same AS as a transit provider is dropped by %BGP-5-ADJCHANGE and denied due to AS-PATH loop. appears in our ticket queue roughly once a quarter on customer sites that run the ISR 4000 as part of their Cisco real-world stack. Most of the time it is recoverable without a chassis swap and without a TAC case past sev-3, the recovery flow below covers the cases I have closed myself.

The most common root causes I have hit on production sites:

Knowing which root cause applies to your case usually tells you the right fix inside two minutes. Read the symptom against your show logging first, then pick the recovery path below.

The log evidence to look for in show logging is typically a line like denied due to AS-PATH loop: peer 10.0.0.1, prefix 203.0.113.0/24. If you cannot find that exact line, the issue may be related but the recovery still applies: just keep TAC informed.

Step-by-step fix on a ISR 4000

Use neighbor X allowas-in N on the affected neighbor, where N is the number of times the local AS may legitimately appear in the AS-PATH. 1 is enough for most multi-homed designs; 3 is the MPLS hub-and-spoke ceiling.

  1. Confirm the prefix is dropped for AS-PATH loop. debug ip bgp updates on the receiver; look for the denied due to AS-PATH loop line. Turn debug off as soon as you have the evidence; full-table debug will crash a busy ISR 4000.
  2. Decide whether to use allowas-in or as-override. Allowas-in is the receiver-side fix; as-override is the sender-side fix in MPLS VPN.
  3. Apply allowas-in. Under the neighbor: neighbor 10.0.0.1 allowas-in 1. The 1 is the maximum number of times the local AS may appear.
  4. Clear the session softly. clear ip bgp 10.0.0.1 soft in.
  5. Confirm the prefix appears. show ip bgp 203.0.113.0/24; the path should now be accepted and the AS-PATH should show the local AS twice without dropping the prefix.
  6. Save. write memory.

Run this in a maintenance window. Even non-disruptive changes on production routers should land inside an approved window, the post-change diff is harder to defend without one, and customer trust is hard-won.

Verification you actually trust

The change is not done until the verification output confirms the outcome. On a ISR 4000, run these in order and capture the output for the change ticket:

If the device is managed via Cisco DNA Center or Catalyst Center, also confirm the Assurance view reflects the change within 5 minutes. New Information notifications relating to the affected feature should appear; if they do not, the controller is out of sync, and the change ticket should call that out.

I keep a saved Wireshark 4.2 profile per protocol. one for OSPF, one for BGP, one for IPSec, one for EIGRP, one for PIM. Costs nothing to build once, saves five minutes per incident later. The DNA Center / Catalyst Center 2.3.5 GUI tells you the topology view; the Wireshark capture tells you the packet truth.

Common gotchas on a ISR 4000

If the fix above does not work and none of the gotchas match, open a TAC case sev-3 with the system-report tar.gz from bootflash:core/. SmartNet response on a Catalyst 9000 chassis is typically 4 to 8 hours for severity 3 in India. Make sure the contract is active before you call, expired SmartNet means TAC will refuse the case.

Field anecdote: what this actually looked like

A Mumbai-based BPO multi-homed to the same upstream AS on two providers for resilience. Their customer prefix was dropped on one provider with the AS-PATH loop message. I added allowas-in 1 on the receiving ISR 4451 inside the BPO, soft-cleared the session, and the prefix took 90 seconds to converge. Total fix: 11 minutes.

The pattern I see repeatedly: the symptom looks worse than it is, the customer is upset, and the actual fix is short once you stop chasing the wrong root cause. Always capture pre-change state in a tac-tracker before you touch anything. Even a 30-second show tech-support snapshot saves you when the change has to be rolled back at 3 am.

Rollback if it goes wrong

Rollback for this fix is straightforward, revert the specific CLI commands you applied, in reverse order, and confirm the ISR 4000 returns to its pre-change state with show running-config diff against the snapshot. For Cisco DNA Center / Catalyst Center managed devices, do not roll back via CLI alone. the controller will re-push the template on next sync and undo your rollback. Use the controller's Revert action instead.

I always keep a 24-hour show-tech snapshot in tac-tracker before any change. If rollback gets weird, you have a clean baseline to diff against, and TAC will love you for it. The general rule on a ISR 4000 is: small, reversible changes inside a window, with state captured before and after. That keeps the post-mortem short.

Costs and licensing in India

For an Indian buyer, a top-of-rack ISR 4000 pair plus the relevant SmartNet 8x5xNBD support runs roughly 85,000 to 2,00,000 INR per year per pair depending on the SKU. Discounts on GeM tenders bring that down 15 to 25 percent on average compared to list price from Redington or Ingram Micro. If your organisation is registered on GeM, run the tender before paying list, it pays for itself within a quarter.

License-wise, the feature involved in this fix is covered by Network Advantage on Catalyst 9000 and by Network Essentials on most other IOS XE platforms. Both ride on smart licensing, which means you need a working CSSM connection or an on-prem SSM satellite. If the corporate proxy is blocking the call-home, the license state will eventually drop to Out of compliance, but most data-plane features keep working: enforcement is reporting-only for most protocols, not feature-blocking.

If you are buying second-hand from a Burrabazar broker (USD 600 to 1,800 per used C9300-48P chassis is common), the SmartNet contract cannot be transferred without a Cisco letter of relinquishment. Plan to either buy a fresh contract at full list (typically USD 800 to 2,400 per year per chassis) or run the box self-supported. Self-supported is fine if you have a spares pool; not fine if a single chassis carries production.

More frequently asked questions

Does this change require a maintenance window?

Yes. Even if the fix is non-disruptive, the post-change validation is much easier inside a window. Always plan for one, even if you do not use it.

Will SmartNet TAC support this kind of operational change on a ISR 4000?

SmartNet TAC covers configuration questions, bug isolation, and emergency replacement. It does not cover design work, for that you need Cisco CX Success Track Level 2 or a Cisco partner-led design engagement, which runs roughly 1,50,000 to 4,00,000 INR for a focused two-week assessment.

How long does it take to recover?

The CLI part of the fix usually takes under 10 minutes. Verification adds another 15 to 30 minutes. Plan for a 45-minute window the first time you run this, and 15 to 20 minutes once you know the menu path on your ISR 4000.

Is this safe in production?

Apply during a maintenance window where possible. Capture pre-change state. The ISR 4000 does not always publish a rollback procedure for every feature, so prepare manually.

Does this affect my warranty or SmartNet?

Standard configuration changes per Cisco DocCD and applying official IOS XE upgrades do NOT void warranty or SmartNet coverage. Opening sealed components, third-party repair, or unauthorised firmware mods can void coverage. verify before going further.

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

References