ACI Multi-Site MP-BGP EVPN Control Plane MTU Defaults

300-620 By islever Editorial Last updated

Describe Multi-Site

Short answer

A By default, spine nodes generate 9000-bytes packets to exchange endpoints routing information, so the Inter-Site network must support carrying 9000-bytes packets.

Question

Regarding the MTU value of MP-BGP EVPN control plane packets in Cisco ACI, which statement about communication between spine nodes in different sites is true?
  1. By default, spine nodes generate 9000-bytes packets to exchange endpoints routing information. As a result, the Inter-Site network should be able to carry 9000-bytes packets. Correct Answer
  2. By default, spine nodes generate 1500-bytes packets to exchange endpoints routing information. As a result, the Inter-Site network should be able to carry 1800-bytes packets.
  3. By default, spine nodes generate 1500-bytes packets to exchange endpoints routing information. As a result, the Inter-Site network should be able to carry 1500-bytes packets.
  4. By default, spine nodes generate 9000-bytes packets to exchange endpoints routing information. As a result, the Inter-Site network should be able to carry 9100-bytes packets.

Key Takeaways

The trap is confusing the control-plane MTU (default 9000 bytes) with data-plane MTU requirements or adding unnecessary overhead calculations; the key is knowing the default generation size matches the minimum ISN requirement for control traffic.

This question tests knowledge of the default MTU size for MP-BGP EVPN control-plane packets in Cisco ACI Multi-Site, which is 9000 bytes, requiring the Inter-Site Network to support at least 9000 bytes.

Common mistake: Option D is the most plausible distractor because administrators often add overhead (like VXLAN headers) when calculating total path MTU, leading them to believe 9100 or higher is required even for control plane packets, whereas the default specification focuses on the 9000-byte payload generation.

Expert Analysis

The core concept tested here is the default MTU behavior of MP-BGP EVPN control-plane traffic in Cisco ACI Multi-Site deployments and the resulting requirements for the Inter-Site Network (ISN).

Default Control Plane MTU: In Cisco ACI, the Multiprotocol Border Gateway Protocol (MP-BGP) Ethernet Virtual Private Network (EVPN) control plane is used to synchronize endpoint learning information between spine nodes in different sites. By default, the spine nodes generate these control-plane packets with a size of 9000 bytes. This is distinct from data plane traffic, which might be smaller depending on endpoint configuration.

ISN Requirements: The question asks about the communication between spine nodes. Since the spines are generating 9000-byte packets, the underlying transport network (the ISN) must be able to carry these frames without fragmentation or dropping. Therefore, the ISN must support an MTU of at least 9000 bytes. While some documentation suggests adding overhead (leading to 9100 or 9200 byte recommendations for end-to-end paths including VXLAN encapsulation), the specific constraint for the control plane packets themselves as they traverse the ISN links is that the links must support the packet size being sent. Option A correctly states that spines generate 9000-byte packets and the ISN should be able to carry 9000-byte packets. Option D suggests 9100 bytes, which is a common point of confusion due to VXLAN overhead calculations for data planes or end-to-end path MTU discovery, but for the specific control plane packet exchange described, 9000 bytes is the baseline requirement cited in official Cisco whitepapers for the ISN's ability to handle the generated control plane traffic.

Why other options are wrong:

  • Options B and C suggest 1500-byte default generation, which is incorrect for the default ACI Multi-Site control plane configuration.
  • Option D suggests 9100-byte ISN requirement. While often recommended for robustness or to account for encapsulation overhead in mixed scenarios, the strict statement regarding the generated packets (9000 bytes) and the ISN's ability to carry them aligns most directly with Option A in the context of standard exam questions referencing the Cisco ACI Multi-Site White Paper. The white paper explicitly states: "By default, the spine nodes generate 9000-byte packets... If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes." (Note: Some interpretations argue for 9100/9200 to include headers, but Option A is the direct reflection of the 'generated packet size' matching the 'minimum ISN capability' for those specific packets as per the primary reference text often used for this exam question).

Official Reference

https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When answering questions about ACI Multi-Site networking, always distinguish between data plane and control plane MTU requirements. For control plane (MP-BGP EVPN), memorize the default packet size (9000 bytes) and ensure the ISN can handle it. Do not automatically add VXLAN overhead unless the question specifically asks about end-to-end path MTU including encapsulation for data traffic.

Frequently Asked Questions

Does the ISN MTU need to be higher than 9000 bytes for ACI Multi-Site?

For control plane traffic alone, the ISN must support at least 9000 bytes. However, for end-to-end connectivity including data plane VXLAN encapsulation, a higher MTU (e.g., 9100 or more) is typically recommended to avoid fragmentation.

Can I change the MP-BGP control plane MTU in ACI?

Yes, the System > Control Plane MTU setting allows you to adjust the MTU for MP-BGP control plane traffic, but changing it requires consistent configuration across all sites and the ISN.

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Community Votes

A
70%
D
30%

70% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 27 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 27

  1. therufus 👍 15
    Correct answer is A https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html Excerpt: MTU of MP-BGP control-plane communication between spine nodes in different sites: By default, the spine nodes generate 9000-byte packets for exchanging endpoint routing information. If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes
  2. anasham 👍 6
    @Jey10; what you have written is about Data Plane MTU. The question is about Control plane MTU. So the answer is A as therufus said
  3. pepper_ 👍 4
    Also there is this doc: https://www.cisco.com/c/en/us/support/docs/cloud-systems-management/application-policy-infrastructure-controller-apic/214270-configure-aci-multi-site-deployment.html By default, the spine nodes generate 9000-byte packets to exchange endpoint routing information. If that default value is not modified, the Inter Site Network (ISN) must support an MTU size of at least 9100 bytes. I think correct answer is D, because IPN devices can use some encapsulation too.
  4. JJCR86 👍 3
    Correct answer is A. The question is specific for control plane traffic as per the CISCO documentation the MTU on both spines and IPN devices must support 9000 bytes at least. The 9100 MTU applies to data plane traffic (VXLAN and 8021q header included).
  5. RTL_dude 👍 3
    I agree, the question is about exchanging endpoint routing information (i.e. control plane traffic) between spines in different sites, not local endpoint to spine traffic for which the 9100 byte MTU applies to.
Show 22 more comments
  1. korthab 👍 2
    This question has two answers on the internet. But if you want to support endpoint traffic of a MTU of 9000, it is required to increase the MTU to at least 9100 on the ISN. So what is this question about? Something that is minimal required for the MP-BGP protocol to work? Or an recommended setup looking further than only the communication between Spines. I would like to pick MTU 9100 for the ISN, but not sure if it is a trick question. LINK 1 = MTU 9100 https://www.cisco.com/c/en/us/support/docs/cloud-systems-management/application-policy-infrastructure-controller-apic/214270-configure-aci-multi-site-deployment.html LINK 2 = MTU 9000 https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html
  2. bizzar777 👍 2 Answered D
    Maximum transmission unit (MTU) of Multiprotocol Border Gateway Protocol (MP-BGP) Ethernet Virtual Private Network (EVPN) control plane communication between spine nodes in different sites - By default, the spine nodes generate 9000-byte packets to exchange endpoint routing information. If that default value is not modified, the Inter Site Network (ISN) must support an MTU size of at least 9100 bytes. In order to tune the default value, modify the corresponding system settings in each APIC domain.
  3. designated 👍 2 Answered A
    A is correct.. There is not overhead from VXLAN for control plane traffic. Go to System > Control Plane MTU and you will see that the value that could be set is between 576 and 9000. And we can also set this option to 1500 for remote-leaf architecture. https://www.cisco.com/c/en/us/td/docs/dcn/aci/apic/6x/system-management-configuration/cisco-apic-system-management-configuration-guide-60x/basic-operations-60x.html
  4. Jey10 👍 2
    It is D from the link you give => "Maximum MTU of the frames generated by the endpoints connected to the fabric: If the endpoints are configured to support jumbo frames (9000 bytes), then the ISN should be configured with at least a 9100-byte MTU value. If the endpoints are instead configured with the default 1500-byte value, then the ISN MTU size can be reduced to 1600 bytes"
  5. eddyedwards257 👍 1
    Maximum MTU of the frames generated by the endpoints connected to the fabric: If the endpoints are configured to support jumbo frames (9000 bytes), then the ISN should be configured with at least a 9100-byte MTU value. If the endpoints are instead configured with the default 1500-byte value, then the ISN MTU size can be reduced to 1600 bytes. ● MTU of MP-BGP control-plane communication between spine nodes in different sites: By default, the spine nodes generate 9000-byte packets for exchanging endpoint routing information. If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes, otherwise the exchange of control-plane information across sites would not succeed (despite being able to establish MP-BGP adjacencies). The default value can be tuned by modifying the corresponding system settings in each APIC domain
  6. veld2 👍 1
    MTU of MP-BGP control-plane communication between spine nodes in different sites: By default, the spine nodes generate 9000-byte packets for exchanging endpoint routing information. If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes, otherwise the exchange of control-plane information across sites would not succeed (despite being able to establish MP-BGP adjacencies). https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html
  7. Mr_Certifiable 👍 1
    IPN requirements for a Remote Leaf solution are as follows: MTU: The solution must support an end-to-end MTU that is at least 100 bytes higher than that of the endpoint source traffic. Assuming that 1500 bytes has been configured for data plane MTU, Remote Leaf can be deployed using a minimum MTU of 1600 bytes. An IPN MTU this low, however, necessitates that ACI administrators lower the ACI fabricwide control plane MTU, which is 9000 bytes by default. https://www.ciscopress.com/articles/article.asp?p=3150964&seqNum=3
  8. ostych 👍 1 Answered A
    https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html#CiscoACIMultiSiteoverlaycontrol MTU of MP-BGP control-plane communication between spine nodes in different sites: By default, the spine nodes generate 9000-byte packets for exchanging endpoint routing information. If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes;
  9. Rododendron2 👍 1 Answered D
    The minimum MTU value to configure in the ISN depends on two factors: ● The maximum MTU of the frames generated by the endpoints connected to the fabric: If the endpoints are configured to support jumbo frames (9000 bytes), then the ISN should be configured with at least a 9050-byte MTU value. If the endpoints are instead configured with the default 1500-byte value, then the ISN MTU size can be reduced to 1550 bytes.
  10. kamel86 👍 1 Answered A
    From ACI Multi-site white paper: The MTU of MP-BGP control-plane communication between spine nodes in different sites: By default, the spine nodes generate 9000-byte packets for exchanging endpoint routing information. If that default value is not modified, the ISN must support an MTU size of at least 9000 bytes; otherwise, the MP-BGP exchange of control-plane information across sites would not succeed.
  11. 959836c 👍 1 Answered D
    Answer is D. While it's true the question is about the control plane and a mtu of 9000 is suitable. For the data plane you need to configure 9100 for mtu. In a real world environment you need to account for the data plane and go with the highest value. It could be A, but it's a technically either way.
  12. leetingo 👍 1
    the ISN mtu value needed depends on whether you need jumbo frame support for endpoint or not. In this question it does not say anything about endpoint jumbo frame support, in this case 9000 is enough for ISN control plane packets
  13. Player 👍 1
    A is correct. We are asked exactly about the control plane traffic between sites, not about the size of frames of the EPs connected to the fabric. Link provided by therufus is clearly describing that.
  14. Lalag 👍 1
    A is the right answer as per the link https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-739609.html
  15. JJCR86 👍 1
    I changed my mind to D. However there is some documentation that is confusing, some CISCO documents state it should be 9000 whereas others states it should be 9100.
  16. rltk8029 👍 1 Answered A
    A- is correct, since question is specifically for MP-BGP, so for MP-BGP MTU would be same as default spine MTU. But yes for data-plane 9050 and recommended 9100
  17. Maccc10 👍 1 Answered A
    Difficult question but I am swaying towards A rather than B due to the fact it mentions control plane within the question...
  18. Chanderia 👍 1
    Here the ask is about control plane so answer is A. If it was for data plane then answer would be D.
  19. ashwind123 👍 1 Answered A
    Its A as it explicitly talk about control plane MTU and not data plane MTU
  20. zelya19 👍 1 Answered A
    No mentioning of supporting jumbo frames within the fabric -> A.
  21. mdsuresh 👍 1
    D is correct!
  22. instancepau 👍 1
    I support D

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