352-001 · Question #395
Which mechanism can you use to improve network performance over PPPoE, IPsec, and GRE networks?
The correct answer is A. Path MTU Discovery. Path MTU Discovery (PMTUD) dynamically determines the largest packet size that can traverse a path without fragmentation, which is critical for tunneling protocols like PPPoE, IPsec, and GRE that add header overhead and reduce the effective MTU.
Question
Which mechanism can you use to improve network performance over PPPoE, IPsec, and GRE networks?
Options
- APath MTU Discovery
- BSimple Service Discovery Protocol
- CICMP Router Discovery Protocol
- DBidirectional Forwarding Detection
How the community answered
(42 responses)- A90% (38)
- B2% (1)
- C5% (2)
- D2% (1)
Why each option
Path MTU Discovery (PMTUD) dynamically determines the largest packet size that can traverse a path without fragmentation, which is critical for tunneling protocols like PPPoE, IPsec, and GRE that add header overhead and reduce the effective MTU.
PPPoE, IPsec, and GRE all add encapsulation headers that shrink the usable MTU, causing fragmentation and retransmissions that degrade performance. Path MTU Discovery uses ICMP 'Fragmentation Needed' (Type 3, Code 4) messages to signal the sender to reduce packet size to fit within the path MTU. By dynamically adjusting to the correct MTU, PMTUD eliminates fragmentation overhead and improves throughput across these tunnel types.
Simple Service Discovery Protocol (SSDP) is a UPnP-based mechanism for discovering network services and has no function related to MTU optimization or tunnel performance.
ICMP Router Discovery Protocol (IRDP) allows hosts to discover default gateway routers and has no mechanism for resolving MTU mismatches or improving tunnel throughput.
Bidirectional Forwarding Detection (BFD) is a rapid link-failure detection protocol used to accelerate routing protocol convergence, not to address MTU or packet fragmentation issues.
Concept tested: Path MTU Discovery for encapsulation tunnels
Source: https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html
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