Examkin

Question

Difficulty: MediumInside Source NAT and PAT Configuration and Verification

A network administrator is configuring Port Address Translation (PAT) on a Cisco IOS router to allow multiple internal hosts on the 192.168.1.0/24 subnet to share the single public IPv4 address assigned to the GigabitEthernet0/1 interface. The administrator enters the following CLI commands:

Router(config)# access-list 1 permit 192.168.1.0 0.0.0.255
Router(config)# ip nat inside source list 1 interface GigabitEthernet0/1

During testing, only the first internal host that generates outbound traffic successfully connects to the Internet. Subsequent traffic from all other internal hosts is dropped. Which configuration change resolves this issue and enables all internal hosts to share the interface IP address simultaneously?

  1. Append the overload keyword to the ip nat inside source list 1 interface GigabitEthernet0/1 command.Answer
  2. B
    Modify access-list 1 to permit 172.32.0.0 0.0.255.255 to match the Class B RFC 1918 private address space.
  3. C
    Configure dynamic 1-to-1 NAT pool translation by omitting the interface parameter and overload keyword.
  4. D
    Add an explicit deny any statement at the end of access-list 1 to clear exhausted translation table entries.

Answer

Appending the overload keyword to the NAT configuration command resolves the issue by enabling Port Address Translation (PAT).
In Cisco IOS, specifying a NAT inside source list with an interface but without the overload keyword configures dynamic 1-to-1 NAT using only the IP address of that interface. Because only one IP address is available, only the first host receives a translation entry in the NAT table. Appending the overload keyword enables Port Address Translation (PAT), allowing the router to distinguish traffic from multiple internal hosts using unique source port numbers.

Step-by-Step Solution

1
Analyze the existing NAT command
The command 'ip nat inside source list 1 interface GigabitEthernet0/1' configures dynamic 1-to-1 NAT without port multiplexing.
Without the 'overload' keyword, Cisco IOS binds the single IP address of GigabitEthernet0/1 to the first inside local IP address requesting translation.
2
Identify the cause of traffic drop for subsequent hosts
Once the single outside IP address is allocated to the first host, no additional public IP addresses are available in the pool.
Standard dynamic NAT requires one unique public IPv4 address per active internal host.
3
Apply the correction for Port Address Translation (PAT)
Executing 'ip nat inside source list 1 interface GigabitEthernet0/1 overload' enables PAT.
The 'overload' keyword instructs the router to track source port numbers along with IP addresses, allowing up to 65,000+ simultaneous internal connections on a single public IP address.

Key Concept

Port Address Translation (PAT) Overload Keyword Requirement
Rate this question

Topics

Inside Source NAT and PAT Configuration and Verification

More questions from this topic