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Exams352-001Questions#82
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352-001 · Question #82

352-001 Question #82: Real Exam Question with Answer & Explanation

The correct answer is B: Ensure that the spanning tree diameter for one or more VLANs is not too large.. Bridging two data center STP domains over EoMPLS extends the spanning tree diameter, and exceeding the IEEE 802.1D maximum of 7 hops causes BPDU delivery delays that can produce topology instability or lost connectivity.

Question

Network designers plan to interconnect two geographically separated data centers using an Ethernet-over-MPLS pseudowire. Within that design, the link between the sites is stable, there are no apparent loops in the topology, and the root bridges for the respective VLANs are stable and unchanging. What additional aspect of the design should be adjusted to mitigate the chance of connectivity issues to the peer data center when the connection takes place?

Options

  • AEnable 802.1d on one data center, and 802.1w on the other.
  • BEnsure that the spanning tree diameter for one or more VLANs is not too large.
  • CEnable UDLD on the link between the data centers.
  • DEnable root guard on the link between the data centers.

Explanation

Bridging two data center STP domains over EoMPLS extends the spanning tree diameter, and exceeding the IEEE 802.1D maximum of 7 hops causes BPDU delivery delays that can produce topology instability or lost connectivity.

Common mistakes.

  • A. Running 802.1d on one site and 802.1w on the other forces RSTP to fall back to slow 802.1d convergence timers network-wide, which is poor design practice and does not address the STP diameter risk.
  • C. UDLD detects physical unidirectional link conditions at the data-link layer and does not address spanning tree diameter or timer-related issues that arise when merging two large Layer 2 broadcast domains.
  • D. Root guard prevents a port from accepting superior BPDUs to protect the root bridge placement, but it does not mitigate connectivity issues caused by a spanning tree diameter that exceeds timer-supported limits.

Concept tested. Spanning tree diameter limits in EoMPLS data center design

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html

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