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Difficulty: HardInside Source NAT and PAT Configuration and Verification

An enterprise router is configured to perform dynamic NAT for an internal LAN subnet using a NAT pool containing a single public IP address (203.0.113.2203.0.113.2). Network users report that while the first host connecting to an external server successfully establishes a session, all subsequent internal hosts fail to reach external destinations simultaneously.

Review the following excerpt from the router's configuration:

text
interface GigabitEthernet0/0
ip address 10.50.0.1 255.255.255.0
ip nat inside
!
interface GigabitEthernet0/1
ip address 203.0.113.1 255.255.255.248
ip nat outside
!
ip access-list standard LAN_SERVERS
permit 10.50.0.0 0.0.255.255
!
ip nat pool PUBLIC_POOL 203.0.113.2 203.0.113.2 netmask 255.255.255.248
ip nat inside source list LAN_SERVERS pool PUBLIC_POOL

Which configuration change will resolve the issue and permit concurrent internet access for multiple internal hosts?

  1. Append the overload keyword to the ip nat inside source list LAN_SERVERS pool PUBLIC_POOL command.Answer
  2. B
    Modify the ACL statement to permit 10.50.0.0 0.0.0.255 instead of 10.50.0.0 0.0.255.255.
  3. C
    Change the ip nat pool PUBLIC_POOL command to define a range spanning from 203.0.113.2 to 203.0.113.30.
  4. D
    Re-apply the ip helper-address command on interface GigabitEthernet0/1 targeting 203.0.113.2.

Answer

Append the overload keyword to the ip nat inside source list LAN_SERVERS pool PUBLIC_POOL command.
Appending the 'overload' keyword to the dynamic NAT statement transforms the mapping from dynamic 1-to-1 NAT into Port Address Translation (PAT). PAT multiplexes multiple internal IP addresses onto a single public inside global address by varying the layer 4 source port numbers, thereby permitting concurrent sessions for all LAN hosts.

Step-by-Step Solution

1
Analyze the configuration and symptoms.
The statement 'ip nat inside source list LAN_SERVERS pool PUBLIC_POOL' lacks the 'overload' keyword, which instructs Cisco IOS to perform dynamic 1-to-1 NAT rather than 1-to-many PAT.
Dynamic 1-to-1 NAT allocates one public IP address from the pool per host. Because the pool contains only one IP address (203.0.113.2), only one device can be translated at any given time.
2
Identify the required NAT operational mode for single IP multi-host sharing.
Port Address Translation (PAT / NAT Overload) allows thousands of concurrent flows to share one public IP address by assigning unique source port numbers.
Adding the 'overload' keyword at the end of a dynamic NAT mapping command explicitly enables PAT.
3
Formulate the correct Cisco IOS CLI command modification.
Execute 'ip nat inside source list LAN_SERVERS pool PUBLIC_POOL overload' in global configuration mode.
This resolves port translation allocation and permits all internal hosts matching ACL LAN_SERVERS to reach external networks concurrently.

Key Concept

Inside Source NAT and PAT (Overload) Pool Configuration
Estimated Time:2m 0s
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Topics

Inside Source NAT and PAT Configuration and Verification

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