ACSP VLANs and Link Aggregation 4 — Questions and Answers
Question 1: Two Aruba CX switches are connected with a LAG. The administrator notices only half the expected bandwidth is being utilized. What should be investigated first?
- The load-balancing hash may be producing skewed distribution (Correct answer)
- One of the LAG member ports may be operating at half-duplex
- The LAG MTU is set too low
- LACP timers are mismatched between the switches
Correct answer: The load-balancing hash may be producing skewed distribution
Uneven LAG utilization is most commonly caused by hash polarization, where most flows map to the same member port.
Question 2: What IEEE standard defines the 802.1Q VLAN tagging mechanism used on Aruba switches?
- 802.1Q (Correct answer)
- 802.3ad
- 802.1D
- 802.1X
Correct answer: 802.1Q
IEEE 802.1Q defines the standard for VLAN tagging, including the 4-byte tag inserted into Ethernet frames to identify the VLAN.
Question 3: On an Aruba CX switch, a port configured as 'trunk' with native VLAN 1 receives an 802.1Q-tagged frame with VLAN ID 1. How is this frame handled?
- The tag is stripped and the frame is treated as an untagged native VLAN frame (Correct answer)
- The frame is forwarded with the tag intact to VLAN 1 members
- The frame is dropped due to VLAN ID conflict
- The frame is forwarded only if VLAN 1 is in the allowed VLAN list
Correct answer: The tag is stripped and the frame is treated as an untagged native VLAN frame
On most implementations, a tagged frame with the native VLAN ID is accepted but the behavior depends on the native VLAN configuration — by default the tag is stripped.
Question 4: Which protocol does Aruba use for dynamic LAG negotiation that is also an IEEE standard?
- LACP (802.3ad) (Correct answer)
- PAgP (Cisco proprietary)
- VRRP (RFC 5798)
- LLDP (802.1AB)
Correct answer: LACP (802.3ad)
LACP (Link Aggregation Control Protocol), defined in IEEE 802.3ad (now part of 802.3), is the industry-standard protocol for dynamic LAG negotiation.
Question 5: An administrator wants to prevent a specific VLAN from being pruned from a trunk on an Aruba CX switch. Which action achieves this?
- Explicitly add the VLAN to the trunk's allowed VLAN list (Correct answer)
- Set the VLAN as the native VLAN on the trunk
- Configure the VLAN as a voice VLAN
- Enable GVRP on the trunk port
Correct answer: Explicitly add the VLAN to the trunk's allowed VLAN list
Explicitly listing a VLAN in the trunk's allowed VLAN list ensures it is always carried and not subject to automatic pruning.
Question 6: What is the function of MLAG (Multi-chassis Link Aggregation) on Aruba CX switches?
- It allows a LAG to span two physical switches, providing redundancy at the switch level (Correct answer)
- It aggregates links across multiple VLANs
- It enables a single switch to connect to multiple upstream routers
- It combines LACP and static LAG on the same port
Correct answer: It allows a LAG to span two physical switches, providing redundancy at the switch level
MLAG enables a single downstream device to form a LAG with two separate upstream switches, eliminating single-switch failure as a point of network downtime.
Question 7: In an Aruba CX MLAG deployment, what is the purpose of the ISL (Inter-Switch Link)?
- To synchronize MLAG state and forward traffic between the two MLAG peers (Correct answer)
- To carry management traffic between the switches
- To provide a backup path for spanning tree
- To replicate VLAN databases between the switches
Correct answer: To synchronize MLAG state and forward traffic between the two MLAG peers
The ISL carries both MLAG control plane synchronization and dataplane traffic that needs to traverse from one MLAG peer to the other.
ACSP VLANs and Link Aggregation 4
Two Aruba CX switches are connected with a LAG.
The administrator notices only half the expected bandwidth is being utilized.
What should be investigated first?