400-007 · Question #65
As part of a new network design documentation, you are required to explain the reason for choosing cisco FabricPath for Layer 2 loop avoidance. Which two elements help Cisco FabricPath mitigate…
The correct answer is C. RPF check D. TTL header. Cisco FabricPath prevents Layer 2 loops using an RPF check on BUM traffic and a TTL field in the FabricPath header that causes loop-detected frames to be dropped.
Question
As part of a new network design documentation, you are required to explain the reason for choosing cisco FabricPath for Layer 2 loop avoidance. Which two elements help Cisco FabricPath mitigate Layer 2 loops if they happen in the Layer 2 MP network? (Choose two)
Options
- AMAC tunneling
- BIS-IS multipath
- CRPF check
- DTTL header
How the community answered
(35 responses)- A17% (6)
- B9% (3)
- C74% (26)
Why each option
Cisco FabricPath prevents Layer 2 loops using an RPF check on BUM traffic and a TTL field in the FabricPath header that causes loop-detected frames to be dropped.
MAC tunneling is a FabricPath forwarding mechanism for transporting classic Ethernet frames, not a loop-prevention feature.
IS-IS multipath in FabricPath provides equal-cost multipath routing for unicast traffic but does not directly detect or break Layer 2 loops.
FabricPath performs a Reverse Path Forwarding (RPF) check on Broadcast, Unknown unicast, and Multicast (BUM) traffic to ensure the frame arrives on the expected interface - if the check fails the frame is dropped, breaking the loop.
FabricPath inserts its own header that includes a TTL field; each FabricPath switch decrements the TTL on forwarded frames, and any frame whose TTL reaches zero is discarded, bounding the lifetime of any looping frame.
Concept tested: Cisco FabricPath Layer 2 loop prevention mechanisms
Source: https://www.cisco.com/c/en/us/products/collateral/switches/nexus-5000-series-switches/white_paper_c11-687554.html
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