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NATCCNA 200-301

PAT UDP Timeout Too Short — Gaming/Streaming Sessions Dropping

Presenting Symptom

Users report that online gaming sessions and video streaming services drop after a few minutes of inactivity, requiring manual reconnection.

Network Context

A small branch office with 50 users connects to the internet via a Cisco 4321 ISR running IOS XE 16.9. PAT (NAT overload) is configured on the outside interface. The issue occurs specifically with UDP-based applications like online gaming and streaming, while TCP-based services (web browsing, email) work fine.

Diagnostic Steps

1

Check NAT translations for UDP flows

show ip nat translations | include udp
Pro Inside global      Inside local       Outside local
udp 203.0.113.10:12345 192.168.1.10:12345  198.51.100.1:12345
--- 203.0.113.10:12346 192.168.1.11:12346  203.0.113.1:12346

Look for UDP translations. If you see many translations with short lifetimes or missing entries for active sessions, it indicates premature timeout.

2

Check NAT timeout values

show ip nat translations timeout
NAT Translation Timeout values:
  udp-timeout             300 seconds
  dns-timeout             60 seconds
  tcp-timeout             86400 seconds
  finrst-timeout          60 seconds
  icmp-timeout            60 seconds
  syn-timeout             60 seconds
  port-block-timeout      60 seconds

The default UDP timeout is 300 seconds (5 minutes). If this is too short for the application, sessions may drop. For gaming/streaming, a longer timeout (e.g., 600 seconds) is often needed.

3

Monitor NAT timeouts in real time

debug ip nat detailed
NAT: [0] created translation 203.0.113.10:12345 -> 192.168.1.10:12345
NAT: [0] deleting translation 203.0.113.10:12345 -> 192.168.1.10:12345 (timeout)

If you see translations being deleted due to timeout while the application is still active (e.g., during a game lobby or streaming buffer), the timeout is too short.

4

Check application-specific behavior

show ip nat statistics
Total active translations: 45 (0 static, 45 dynamic; 30 extended)
Outside interfaces: GigabitEthernet0/0/0
Inside interfaces: GigabitEthernet0/0/1
Hits: 12345  Misses: 5
Expired translations: 100

High number of expired translations relative to hits may indicate timeouts causing re-translations. Also check if the outside interface is overloaded.

Root Cause

The default NAT UDP timeout (ip nat translation udp-timeout) is set to 300 seconds (5 minutes), which is too short for UDP-based gaming and streaming applications that may have idle periods longer than 5 minutes (e.g., game lobbies, streaming buffering). When the timeout expires, the NAT translation is removed, causing the application to lose its connection and drop the session.

Resolution

Increase the UDP timeout to a higher value, such as 600 seconds (10 minutes) or 1200 seconds (20 minutes), depending on application requirements. Commands: ``` configure terminal ip nat translation udp-timeout 600 end write memory ``` This sets the UDP idle timeout to 600 seconds (10 minutes). Adjust as needed.

Verification

After applying the fix, verify the new timeout value: ``` show ip nat translations timeout ``` Expected output: ``` NAT Translation Timeout values: udp-timeout 600 seconds dns-timeout 60 seconds tcp-timeout 86400 seconds finrst-timeout 60 seconds icmp-timeout 60 seconds syn-timeout 60 seconds port-block-timeout 60 seconds ``` Also monitor active translations during a gaming session to ensure they are not being deleted prematurely.

Prevention

1. Set appropriate NAT timeouts based on application requirements; for UDP applications, consider values between 600 and 1200 seconds. 2. Use application-specific inspection (e.g., fixup protocol) if available to dynamically manage timeouts. 3. Monitor NAT translation statistics regularly to detect abnormal expiration rates.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why UDP-based applications are dropping. The exam tests knowledge of NAT timeout values and their impact on different protocols. A candidate must know the default UDP timeout (300 seconds) and how to modify it using the 'ip nat translation udp-timeout' command.

Exam Tips

1.

Remember the default NAT timeout values: UDP 300s, TCP 86400s, DNS 60s, ICMP 60s.

2.

The exam may present a scenario where users complain about dropped gaming/streaming; the fix is often increasing the UDP timeout.

3.

Know the command 'ip nat translation udp-timeout <seconds>' and that it is configured globally.

Commands Used in This Scenario

debug ip nat detailed

Use this command to enable detailed debugging of NAT translations, showing packet-by-packet translation details including inside/outside addresses and port numbers, typically for troubleshooting NAT issues.

show ip nat statistics

Displays statistics about NAT translations, including active translations, hit counts, and configuration parameters, used to verify NAT operation and troubleshoot translation issues.

show ip nat translations

Displays the current active Network Address Translation (NAT) translations on the router, used to verify NAT operations and troubleshoot connectivity issues.

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