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Cisco 300-540 Exam - Topic 4 Question 2 Discussion
Topic #: 4
Refer to the exhibit.
Refer to the exhibit. An engineer must configure Cisco IOS SLB for DNS on router R1 to meet these requirements:
The first DNS request to www.ccnp.test
must be redirected to the DNS server at 10.1.1.1;
The second DNS request to www.ccnp.test
must be redirected to the DNS server at 10.2.1.1;
The third DNS request to www.ccnp.test
must be redirected to the DNS server at 10.3.1.1.
In each case, the other two addresses must also be attempted if the first one fails. The indicated configuration was applied to R1; however, the load balancing failed. Which command must be run on R1 to resolve the issue?
On R1 the configuration (simplified) is:
ip domain lookup
ip domain name ccnp.test
ip host www.ccnp.test 10.1.1.1 10.2.1.1 10.3.1.1
The ip host command statically maps the hostname www.ccnp.test
to three IP addresses. By default, Cisco IOS will always return these IP addresses to DNS queries in the same order they are configured (10.1.1.1, then 10.2.1.1, then 10.3.1.1). This means that clients will always attempt 10.1.1.1 first and will not achieve per-query load balancing across all three servers.
To enable DNS-based load balancing so that each successive query rotates the order of the addresses, Cisco IOS provides the command:
ip domain round-robin
This command enables round-robin rotation of multiple A records associated with a single hostname defined by ip host. With this feature enabled:
1st query: response order 10.1.1.1, 10.2.1.1, 10.3.1.1
2nd query: response order 10.2.1.1, 10.3.1.1, 10.1.1.1
3rd query: response order 10.3.1.1, 10.1.1.1, 10.2.1.1
Clients will typically try the first IP address in the list and use the others if the first one fails, exactly matching the requirement.
Why other options are incorrect:
A . ip domain retry 3 controls how many times the router retries DNS queries to a server; it does not control the order of multiple A records.
C . ip dns server turns the router into a DNS server but does not itself provide round-robin behavior for statically defined hosts.
D . maximum-paths 3 is a routing (IP forwarding) parameter for equal-cost multipath, unrelated to DNS resolution.
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