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Cisco Real World Problems

ISR 4000 OSPF neighbor stuck 2WAY broadcast DR election: 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

I run a Cisco shop in Bengaluru and spend most weeks elbow-deep in ISR 4000 chassis either on Putty 0.78 over a console cable or on SecureCRT 9.4 in tabbed mode against a stack of devices in a Mumbai DC. Ospf neighbor stuck in 2way on a broadcast segment during dr election is one of those issues that does not show up in CCNA labs but absolutely shows up in production at 11 pm on a Friday. I have shipped this fix on customer sites between Chennai, Hyderabad, Bengaluru, and Pune over the last three years on hardware bought through Redington, Ingram Micro, and Cisco GeM tenders. The fix below is what I trust under a maintenance window.

The short version: ISR 4000 routers on a broadcast segment never progress beyond 2WAY because the DR election logic is mis-tuned and no router considers itself the DR. Root cause: On a broadcast network, only the DR and BDR build FULL adjacencies with all other routers; DROTHER-to-DROTHER pairs are intentionally left at 2WAY. If every router has ip ospf priority 0, no router is eligible for DR, so the entire segment collapses to 2WAY.. The fix involves a small number of CLI changes plus a verification pass. Total time on a clean run: 10 to 30 minutes including the soft-reset wait. If you are reading this at 7 pm because the change has to land before midnight, skip to the step-by-step section and come back for the context later.

This article gives you the working network-engineer view: what the CLI looks like on a ISR 4000, what to verify after the change, which gotchas have bitten me on real customer sites, and what the rollback looks like if it goes sideways. SmartNet 8x5xNBD on a top-of-rack ISR 4000 pair costs roughly 65,000 to 1,80,000 INR per year per pair depending on the SKU; the procedures below are all covered under that contract. If you are running the chassis without SmartNet, your TAC escalation will be paid per-incident at roughly 15,000 to 30,000 INR per case.

What you need before you start

Bench the change before you touch production. The list below is what lives on my engineering desk for any Cisco IOS XE config change:

Why this matters on a ISR 4000

On a broadcast network, only the DR and BDR build FULL adjacencies with all other routers; DROTHER-to-DROTHER pairs are intentionally left at 2WAY. If every router has ip ospf priority 0, no router is eligible for DR, so the entire segment collapses to 2WAY.

The way this typically presents in production is that ISR 4000 routers on a broadcast segment never progress beyond 2WAY because the DR election logic is mis-tuned and no router considers itself the DR. The fingerprint is consistent enough that once you have seen it twice, you can rule it in or out inside five minutes of opening a console session. The reason it confuses junior engineers is that the symptom looks like a generic network problem - flapping adjacency, lost prefix, high CPU - and the actual root cause is buried in a specific config line or capability negotiation that does not show up in the headline status output.

The log signature you should expect during the failure is:

%OSPF-5-ADJCHG: Process 1, Nbr 10.0.0.2 on GigabitEthernet0/0/1 from EXSTART to DOWN, Neighbor Down: Too many retransmissions

For an Indian customer running on Redington-supplied hardware with a SmartNet 8x5xNBD contract, this kind of failure typically gets to TAC within an hour of detection if the NOC is on-the-ball. TAC will ask for the show tech, the crashinfo (where applicable), and the timestamps of the relevant log entries. Have all three ready before opening the case and the resolution time drops from hours to minutes.

Step-by-step on a ISR 4000

The procedure below is what I follow during a maintenance window. Each step should complete cleanly before moving to the next; if any step does not match the expected output, stop and re-check rather than continuing forward.

  1. Confirm the symptom. show ip ospf neighbor on every router on the segment - if every neighbour is at 2WAY/DROTHER, you have the DR-election failure.
  2. Identify the priority on each interface: show ip ospf interface GigabitEthernet0/0/1 | include Priority. If every interface shows Priority 0, no router can become DR.
  3. Pick two routers to be DR and BDR candidates. Usually the two most stable, central routers on the segment. Loopback-anchored routers are good candidates.
  4. Set ip ospf priority 100 on the intended DR and ip ospf priority 50 on the intended BDR. Leave the rest at default 1 (DROTHER but eligible).
  5. Force a fresh election: clear ip ospf process on the new DR candidate. The election fires within the dead interval (default 40 seconds on broadcast).
  6. Verify: show ip ospf neighbor should now show the DR and BDR clearly, with DROTHERs adjacent to both at FULL.
  7. Save: write memory on every router touched, plus on the new DR / BDR.

Total wall-clock time for a clean run is typically 10 to 30 minutes including the soft-reset waits. If the change blows past that, you have either picked the wrong fix or the underlying cause is something deeper than what this article addresses.

Verification you actually trust

The change is not finished until you can prove the behaviour on the ISR 4000. Run the following commands and capture the output in your change ticket:

Then take a 30-second Wireshark 4.2 capture on the relevant interface and confirm the protocol behaviour matches the CLI counters. I keep saved Wireshark profiles for BGP, OSPF, EIGRP, and PIM - each one pre-filtered to the right protocol and DSCP class. Costs nothing to build, saves about five minutes per incident later.

If the device is registered to Cisco DNA Center / Catalyst Center 2.3.5, log into the Assurance view and verify that no new alarms have fired in the five minutes since the change. The Assurance event list usually picks up routing-protocol or hardware changes within a minute or two; missing alarms can mean the controller is out of sync, which is worth flagging in the ticket regardless.

Common gotchas on a ISR 4000

Field anecdote - what this actually looked like

Lab in Bengaluru, customer was prepping a campus core migration and pre-staged five ISR 4451 routers on a shared L2 segment. show ip ospf neighbor showed every neighbour at 2WAY/DROTHER. We stared at the config for ten minutes - every router had ip ospf priority 0 because the staging template had been written for a hub-and-spoke design where the central router was meant to be DR. The lab segment had no central router. Set priority 1 on the two intended DR candidates, watched DR election fire, and FULL adjacencies came up across the whole segment in 45 seconds.

The takeaway from that incident, and from the dozen-or-so similar ones I have run since, is that diagnosing this scenario is more about pattern recognition than about running every possible show command. Once you have seen the failure mode once, you can identify it inside a five-minute triage and move straight to the fix.

Rollback if the change misbehaves

Every routing-protocol or hardware change on a ISR 4000 should have a documented rollback path before you start. For the scenario covered in this article, the rollback is usually a simple negation of the new lines you just added, plus a soft-reset of the affected protocol session. The pre-change snapshot lives in tac-tracker; diff against it to confirm exactly what needs to be reverted.

If the rollback itself does not restore the previous behaviour, escalate to TAC. SmartNet 8x5xNBD on a ISR 4000 pair gives you a 4-to-8 hour response SLA on severity-2 issues, which is usually enough to get a TAC engineer engaged before the next business day. For severity-1 (production-down), expect a callback inside 30 to 60 minutes from Cisco TAC India.

The pre-change snapshot from tac-tracker is your insurance policy. If you do not have one, take one now even if you are mid-incident - having it at the end of the case will make the post-incident review easier.

Costs and licensing in India

An Indian buyer looking at a top-of-rack ISR 4000 pair plus SmartNet 8x5xNBD should budget roughly 85,000 to 2,00,000 INR per year per pair depending on the SKU. Redington and Ingram Micro carry standard pricing; GeM tenders (for government-registered orgs) typically pull 15 to 25 percent off list. SmartNet renewal is usually 12 to 18 percent of the original list price annually.

License-wise, most of the features in this article are covered by Network Advantage or Network Essentials on IOS XE platforms. Smart Licensing is required - the device must reach Cisco CSSM or an on-prem SSM satellite (about USD 2,500-4,000 to deploy on-prem SSM). Without a working call-home, licences eventually drop to Out of compliance, but the protocols and features themselves keep working - enforcement is reporting-only for routing and switching.

If you are buying second-hand through a Burrabazar broker (USD 600-1,800 per used chassis is common), be aware that SmartNet contracts cannot be transferred without Cisco's letter of relinquishment. Either buy a fresh contract at full list (USD 800-2,400 per chassis per year) or run the box self-supported and budget for downtime when it fails. ESS Bengaluru, Redington, and Anuh Pharma's IT arm in Mumbai are common channels for new and refurb gear with the paperwork done properly.

Frequently asked questions

Does this change require a maintenance window?

Yes. Even when the protocol-level change is technically non-disruptive, soft-resets and adjacency renegotiation can leak user-visible behaviour. Always run inside a planned window with a backup-on-call assigned and a rollback path documented.

How long should the recovery take?

For most cases covered in this article, allow 15 to 45 minutes the first time you walk through it. Repeat incidents are usually under 10 minutes once you recognise the pattern.

Will this exact procedure work on every ISR 4000 model?

The procedure reflects current ISR 4000 behaviour on IOS XE 17.6 through 17.12. CLI syntax has been stable across these trains; if you are on an older release, verify against the platform release notes before pasting.

Does SmartNet cover this kind of operational fix on a ISR 4000?

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

Is the procedure safe to run in production?

Inside a maintenance window with a clean pre-change snapshot in tac-tracker, yes. Outside a window, no - even a clean change has a non-zero chance of triggering a follow-on adjacency or hardware behaviour you did not predict.

Does this affect my ISR 4000 warranty?

Standard CLI configuration and official IOS XE upgrades do not affect warranty. Opening sealed hardware components, using third-party transceivers without Cisco approval, or running unsupported software can void warranty - check before going further.

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

References