“Underground fiber cuts caused by municipal road expansion were an inevitable weekly occurrence in Delhi NCR. Air-path RF links were our un-cuttable backup line.”

Executive Summary

In 2010 at Spectranet, enterprise clients in Delhi NCR demanded 99.99% network uptime for core financial transactions and VoIP PBX trunking. However, underground fiber backhaul cables were routinely cut by urban construction crews.

I served as a Field Engineer, designing and physically commissioning dual-path Enterprise WAN Redundancy solutions. We paired primary subterranean optical fiber links with rooftop 3.5GHz WiMAX RF radio links and automated BGP BFD sub-second failover.

The Challenge

Single-homed fiber connections frequently failed without warning, causing hours of downtime while field splicing crews repaired cut cables.

Physical Fiber Insecurity: Underground fiber cuts occurred on average twice a month across major industrial corridors.
Slow Failover Protocols: Standard BGP hold timers (180 seconds) left client voice calls hanging for 3 minutes before redirecting traffic to backup lines.
RF Link Attenuation: Monsoon rains and physical line-of-sight obstructions degraded wireless signal-to-noise ratios (SNR), causing packet loss on backup links.

[!IMPORTANT] A backup line is useless if traffic takes 3 minutes to switch over. We needed sub-second failover (BFD) paired with high-gain directional antennas tuned for monsoon immunity.

The Solution

We built a hybrid WAN topology combining primary fiber handoffs with point-to-point 3.5GHz WiMAX radio base station alignment, governed by BGP Bidirectional Forwarding Detection (BFD).

Technology Stack

Core Routers: Cisco 7206VXR (NPE-G2) & Cisco 3845 Integrated Services Routers
Wireless RF Hardware: Alvarion BreezeMAX 3.5GHz WiMAX Radio Base Stations & Outdoor Subscriber Units (ODU)
Routing Protocols: Multi-Homed BGP with BFD (Bidirectional Forwarding Detection)
RF Diagnostic Equipment: Anritsu Site Master Spectrum Analyzers & High-Gain Parabolic Grid Antennas

Technical Architecture

Physical Handoff Layer: Primary traffic routed over subterranean 100Mbps Ethernet-over-Fiber. Secondary traffic routed over rooftop 3.5GHz WiMAX RF radio units.
RF Alignment & Signal Optimization: Aligned outdoor subscriber antennas using spectrum analyzers to guarantee -68 dBm receive signal strength and >25 dB SNR.
Sub-Second BFD Failover: Configured BGP neighbors with BFD timers (300ms interval, 3x multiplier), detecting fiber breaks in under 900 milliseconds.

graph TD
    subgraph "Enterprise HQ Customer Premises"
    CE[Cisco 3845 Router]
    end

    subgraph "Dual Transport Paths"
    CE -->|Primary Path / Fast Ethernet| Fiber[Subterranean Fiber Link]
    CE -->|Secondary Path / WiMAX 3.5GHz| RF[Rooftop WiMAX ODU Antenna]
    end

    subgraph "Spectranet Central POP"
    Fiber --> Core[Cisco 7206VXR Core Router]
    RF --> BaseStation[Alvarion BreezeMAX Base Station]
    BaseStation --> Core
    end

Implementation & Hands-on Configuration

BGP BFD Sub-Second Failover Configuration

To achieve sub-second failover upon fiber breakage, we enabled BFD natively within Cisco IOS BGP routing stanzas:

! Cisco 3845 Customer Edge Router Configuration
interface FastEthernet0/0
 description Primary Fiber Link to Spectranet Core
 ip address 202.54.10.2 255.255.255.252
 bfd interval 300 min_rx 300 multiplier 3
!
interface FastEthernet0/1
 description Secondary WiMAX Wireless Link
 ip address 202.54.10.6 255.255.255.252
 bfd interval 300 min_rx 300 multiplier 3
!
router bgp 45120
 bgp log-neighbor-changes
 neighbor 202.54.10.1 remote-as 9210
 neighbor 202.54.10.1 fall-over bfd
 neighbor 202.54.10.1 route-map FIBER_PRIMARY_IN in
 neighbor 202.54.10.5 remote-as 9210
 neighbor 202.54.10.5 fall-over bfd
 neighbor 202.54.10.5 route-map WIMAX_BACKUP_IN in
!
route-map FIBER_PRIMARY_IN permit 10
 set local-preference 200
!
route-map WIMAX_BACKUP_IN permit 10
 set local-preference 100

Physical Antenna Alignment Telemetry Log

[SPECTRANET RF ALIGNMENT TELEMETRY] 2010-05-14 11:30:00 IST
Location: Okhla Industrial Area Phase-III Rooftop
Device: Alvarion BreezeMAX ODU 3.5GHz (MAC: 00:10:E7:4B:82:11)
Frequency Band: 3.475 GHz (Channel Width: 7 MHz)
Antenna Gain: 21 dBi Directional Panel

Before Fine Alignment:
RSSI: -84 dBm (WEAK SIGNAL / MARGINAL)
CINR: 12 dB (HIGH BIT ERROR RATE)
Modulation: QPSK 1/2 (Max Throughput: 4 Mbps)

After Precision Azimuth & Elevation Alignment:
RSSI: -68 dBm (OPTIMAL)
CINR: 28 dB (EXCELLENT SIGNAL-TO-NOISE RATIO)
Modulation: 64-QAM 3/4 (Max Throughput: 18 Mbps Line Rate)
Packet Loss Test (10,000 pings @ 1400 bytes): 0.00% PASS

Key Accomplishments & Metrics

99.99% Network Uptime Achieved: Delivered unbroken uptime across 50+ enterprise customer sites despite 24 underground fiber cuts during annual monsoon seasons.
Failover Convergence Time: Reduced network failover detection from 180 seconds to 900 milliseconds via BFD.
RF Link Bandwidth: Optimized 3.5GHz WiMAX throughput from 4 Mbps (QPSK) to 18 Mbps (64-QAM) per site.

Key Takeaway

Physical transport diversity is meaningless without sub-second routing convergence. Pairing subterranean fiber with rooftop WiMAX radio links governed by BFD guarantees enterprise WAN resilience even when backhaul cables are severed.

Architecture and decisions: mine. Field alignment and CLI configs: mine. AI assistance: structure, syntax, first draft. — Sachin