VRRP Virtual MAC Address Calculation for VRID 3
Short answer
D The VRRP virtual MAC address is derived from the VRID using the standard format 00-00-5E-00-01-VRID, resulting in 00-00-5E-00-01-03 for VRID 3.
Question
Key Takeaways
The main trap is confusing the VRID value with other numbers like the IP address octets or using the wrong MAC prefix (01-00 instead of 00-00).
This page explains how to calculate the VRRP virtual MAC address given a specific VRID. The correct answer is D, based on the standardized MAC prefix for VRRP groups.
Common mistake: Choosing Option C because it ends in '64', which might be mistakenly associated with the decimal value 100 or confused with hex 'A'. However, the VRID is 3, so the last byte must be 03.
Expert Analysis
VRRP Virtual MAC Address Structure
The virtual MAC address for a VRRP router is strictly defined by RFC 3768 (and its successors like RFC 5798). The format is 00-00-5E-00-01-{VRID}. This structure ensures that the MAC address remains within the IANA-assigned range for IPv4 virtual routers and uniquely identifies the VRRP group.
Calculating the Specific Address
Given the question parameters:
- VRID: 3
- Virtual IP: 100.1.1.10 (irrelevant to MAC calculation)
We substitute the VRID into the hex portion of the MAC address: 00-00-5E-00-01- + 03 (hexadecimal representation of decimal 3).
Result: 00-00-5E-00-01-03.
Evaluating Options
- Option D (00-00-5Е-00-01-03) matches the calculated standard format exactly.
- Option C (00-00-5Е-00-01-64) incorrectly uses
64(decimal 100) or potentially confuses it with the last octet of the IP address, which is not part of the MAC generation logic. - Options A and B start with
01-00-5E..., which is incorrect; the prefix must be00-00-5E.
Operational Consequence
Using an incorrect MAC address (e.g., based on IP octets) would cause ARP resolution failures for hosts relying on the VRRP gateway, leading to network outages. Understanding this fixed mapping is critical for troubleshooting Layer 2/Layer 3 boundary issues in high-availability setups.
Official Reference
https://www.rfc-editor.org/rfc/rfc3768
This link opens the official vendor documentation in a new tab.
Exam Strategy
Memorize the VRRP MAC address formula: 00-00-5E-00-01-VRID. When you see a VRRP question asking for the MAC, immediately convert the decimal VRID to hexadecimal and append it to the fixed prefix. Do not look at the Virtual IP address for this calculation unless asked for the priority or state.
Frequently Asked Questions
Why does the VRRP MAC address start with 00-00-5E?
Does the Virtual IP address affect the MAC address?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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Adi_1 👍 1 Answered DThe VRRP virtual MAC address follows a fixed, well-defined format defined in RFC 3768:
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