Discuss F5 Networks F5CAB2 Exam Topic 5 Question 15

The BIG-IP Administrator wants to provide quick failover between the F5 LTM devices that are configured as an HA pair with a single-selfip using the MAC Masquerade feature for this quick failover and runs this command: tmsh modify /cm traffic-group traffic-group-1 mac 02:12:34:56:00:00 However, the Network Operations team has identified an issue with the use of the same MAC address being used within different VLANs. As a result, the administrator decides to implement the Per-VLAN Mac Masquerade in order to have a unique MAC address on each VLAN: tmsh modify /sys db tm.macmasqaddr_per_vlan value true. What would be the resulting MAC address on a tagged VLAN of 1501? (Choose one answer)

A02:12:34:56:01:15

B02:12:34:56:dd:05

C02:12:34:56:05:dd

D02:12:34:56:15:01

Suggested Answer: C

According to F5 BIG-IP documentation regarding High Availability and MAC Masquerade behavior, the system allows for more granular control over Layer 2 addresses during failover events.

Standard MAC Masquerade: By default, when a traffic group is assigned a MAC masquerade address (like 02:12:34:56:00:00), the BIG-IP system uses that exact MAC address for all traffic associated with that traffic group across all VLANs. This ensures that upstream switches do not need to relearn ARP entries for the Virtual IP, but it can cause issues in environments where multiple VLANs share the same physical infrastructure or monitoring tools that flag identical MACs across segments.

Per-VLAN MAC Masquerade: When the system database variable tm.macmasqaddr_per_vlan is set to true, the BIG-IP system calculates a unique MAC address for each VLAN. It does this by taking the base MAC masquerade address configured in the traffic group and adding the VLAN ID (tag) to it.

Calculation Logic:

Base MAC: 02:12:34:56:00:00

VLAN ID: 1501

To find the suffix, the VLAN ID is converted from decimal to hexadecimal:

$1501$ in decimal = 05DD in hex.

The system then applies this offset to the last two octets of the base MAC address.

00:00 + 05:DD = 05:DD.

Result: The final MAC address for VLAN 1501 becomes 02:12:34:56:05:dd.

This ensures that every VLAN has a unique Layer 2 identity while still reaping the benefits of 'gratuitous ARP-less' failover provided by MAC masquerading.

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12 comments · newest first

Alonso 1 month ago

I’m leaning towards D. It matches the MAC Masquerade logic well.

Casandra 1 month ago

I feel like B could be right. It seems unique enough for VLAN 1501.

Jesusa 1 month ago

I think the answer is A. The pattern fits the VLAN tagging.

Charlena 2 months ago

I disagree, I think option B is the right one!

Jimmie 2 months ago

I'm leaning towards option A, seems logical for VLAN 1501.

Willard 2 months ago

Wait, does that mean we can have different MACs on the same device?

Wayne 2 months ago

I think the unique MAC per VLAN is a must to avoid conflicts.

Bea 2 months ago

The MAC Masquerade feature is pretty cool for HA setups!

Ezekiel 3 months ago

I remember something about the hexadecimal conversion for VLAN IDs, but I'm not sure how that applies here. Could it be option D?

Janet 3 months ago

This reminds me of a practice question where we had to calculate MAC addresses based on VLANs. I feel like it might be option A, but I'm not confident.

Jacklyn 3 months ago

I think the last two digits of the MAC address are related to the VLAN ID, but I can't recall the exact format we practiced.

Kyoko 3 months ago

I remember we discussed MAC Masquerade in class, but I'm not entirely sure how the unique MAC addresses are generated for each VLAN.

F5 Networks F5CAB2 Exam - Topic 5 Question 15 Discussion