This 300-410 practice question tests your understanding of span, rspan, and erspan. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer runs the following command to troubleshoot an ERSPAN issue:
R1# show ip interface brief | include Gi0/0
GigabitEthernet0/0 10.1.1.1 YES NVRAM up up
R1# show monitor session 7 detail
Session 7
---------
Type : ERSPAN Source Session
Source Ports :
Both : Gi0/1
Destination IP : 10.1.1.2
ERSPAN ID : 300
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ERSPAN session is correctly configured and the destination IP is reachable.
The output shows that interface Gi0/0 has IP address 10.1.1.1 and is up/up, and ERSPAN session 7 has destination IP 10.1.1.2. This indicates that the ERSPAN source session is configured to send traffic to 10.1.1.2, which is reachable via Gi0/0.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The ERSPAN session is correctly configured and the destination IP is reachable.
Why this is correct
The interface is up and the destination IP is likely reachable.
The ERSPAN session is misconfigured because the source port must have an IP address.
Why it's wrong here
The source port does not need an IP address for ERSPAN.
✗
The ERSPAN session is misconfigured because the destination IP must be on the same subnet as the source.
Why it's wrong here
The destination IP can be on a different subnet.
✗
The ERSPAN session is misconfigured because the ERSPAN ID must be unique across all sessions.
Why it's wrong here
ERSPAN IDs must be unique per session, but this output does not indicate a conflict.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Trap categories for this question
Command / output trap
ERSPAN IDs must be unique per session, but this output does not indicate a conflict.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The ERSPAN session is correctly configured and the destination IP is reachable. — The output shows that interface Gi0/0 has IP address 10.1.1.1 and is up/up, and ERSPAN session 7 has destination IP 10.1.1.2. This indicates that the ERSPAN source session is configured to send traffic to 10.1.1.2, which is reachable via Gi0/0.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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