When changing nameservers for a domain, most ISPs follow standard DNS propagation timelines (typically 24-48 hours). However, some providers like Vodafone Ireland in this case exhibit different behavior by:

• Persisting with old nameserver queries despite NS record updates
• Only updating when old nameservers return NXDOMAIN (non-existent domain)
• Maintaining unusually long TTLs (Time To Live) in their cache

According to RFC 2181 Section 5.4, resolvers should respect the TTL values provided in DNS records. However, some ISPs implement aggressive caching strategies that exceed standard practices.

// Example DNS query showing TTL values
$ dig +nocmd example.com NS +noall +answer +ttlid
example.com.    3600    IN    NS    ns1.newnameserver.com.
example.com.    3600    IN    NS    ns2.newnameserver.com.

When facing nameserver propagation issues:

// Python script to monitor DNS propagation
import dns.resolver

def check_ns_propagation(domain, expected_ns):
    try:
        answers = dns.resolver.resolve(domain, 'NS')
        current_ns = [str(rdata) for rdata in answers]
        return set(current_ns) == set(expected_ns)
    except dns.resolver.NoAnswer:
        return False

# Usage:
target_domain = "example.com"
correct_nameservers = ["ns1.newnameserver.com", "ns2.newnameserver.com"]
print(check_ns_propagation(target_domain, correct_nameservers))

For persistent ISP-specific issues:

• Implement DNS monitoring from multiple locations using services like Pingdom or DNSChecker
• Contact the ISP's technical support with traceroute and DNS query evidence
• Consider temporary solutions like adding A records pointing to your IP at the registrar level

# Bash script for pre-migration testing
#!/bin/bash
OLD_NS="old.nameserver.com"
NEW_NS="new.nameserver.com"
DOMAIN="example.com"

echo "Testing old nameserver ($OLD_NS):"
dig @$OLD_NS $DOMAIN ANY
echo "\nTesting new nameserver ($NEW_NS):"
dig @$NEW_NS $DOMAIN ANY

Always remember to:

1. Lower TTL values before migration (to 300-600 seconds)
2. Maintain both old and new nameservers during transition
3. Monitor using global DNS checking tools

When transitioning between DNS nameservers, most network operators follow the standard practice of checking for NS record updates at the TTL-specified interval. However, some ISPs (particularly Vodafone in this case) implement a more conservative approach that waits for explicit NXDOMAIN responses before switching.

What this ISP appears to be referencing is a strict interpretation of RFC 1912 (Common DNS Operational Errors), which states:

; Sample DNS zone file snippet showing TTL settings
$TTL 3600       ; Default TTL 1 hour
@ IN SOA ns1.example.com. hostmaster.example.com. (
    2023081501  ; serial
    3600        ; refresh
    900         ; retry
    604800      ; expire
    86400 )     ; minimum

; NS records demonstrating migration scenario
@    IN NS ns1.oldprovider.com.
@    IN NS ns2.oldprovider.com.
@    IN NS ns1.newprovider.com.
@    IN NS ns2.newprovider.com.

To verify if an ISP is properly following your nameserver changes, you can use DNS inspection tools:

# Bash script to check DNS propagation
#!/bin/bash
DOMAIN="example.com"
ISPS=("8.8.8.8" "1.1.1.1" "VODAFONE_NS_IP")

for resolver in "${ISPS[@]}"; do
    echo "Checking $DOMAIN via $resolver"
    dig @$resolver $DOMAIN NS +short
    echo "-------------------"
done

When planning a nameserver migration, consider this phased approach:

1. Dual-host NS records for at least 72 hours before cutover
2. Gradually reduce TTL values in advance (to 300 seconds)
3. Maintain old nameservers for 7-14 days post-migration

For networks that don't properly respect DNS changes, you might need to:

// Python script to force DNS cache clearing at some ISPs
import dns.resolver

def clear_isp_cache(domain, isp_ns):
    resolver = dns.resolver.Resolver()
    resolver.nameservers = [isp_ns]
    try:
        # Query with recursion desired flag set to 0
        answer = resolver.resolve(domain, 'NS', raise_on_no_answer=False)
        return answer.rrset
    except dns.resolver.NXDOMAIN:
        return "Domain not found - cache cleared"

In this specific case, Vodafone's eventual resolution involved internal escalation to their networking team, suggesting they had an overly aggressive caching implementation that needed manual intervention.