Mtu¶
22 cards — 🟢 3 easy | 🟡 4 medium | 🔴 3 hard
🟢 Easy (3)¶
1. What is MTU and what is the standard Ethernet MTU?
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MTU (Maximum Transmission Unit) is the largest packet size in bytes that a network interface will transmit without fragmentation. Standard Ethernet MTU is 1500 bytes.Name origin: the 1500-byte standard dates to the 1980 DIX Ethernet spec — a compromise between efficiency and buffer memory costs on early NICs.
Under the hood: 1500 bytes is payload only. Full Ethernet frame = 1518 bytes (14B header + 4B FCS). With 802.1Q VLAN tag: 1522 bytes.
Remember: MTU = Maximum Transmission Unit. Ethernet=1500. Jumbo=9000.
Gotcha: Mismatch = fragmentation/drops. Entire path must match for jumbo.
2. What are jumbo frames and when are they used?
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Jumbo frames use MTU 9000 (up to 9216). They reduce per-packet overhead and CPU usage for bulk transfers. Used within datacenters on dedicated storage/cluster networks where every device in the path supports them.Remember: Jumbo(9000) reduces CPU overhead. Every device in path must support them.
Gotcha: Even one device in the path that doesn\'t support jumbo frames will cause silent packet drops. Test end-to-end with `ping -M do -s 8972`.
Name origin: "Jumbo" because 9000 bytes is 6x the standard 1500-byte Ethernet MTU.
3. How do you check and set the MTU of a Linux interface?
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Check: ip link show eth0 (shows mtu value) or cat /sys/class/net/eth0/mtu. Set: ip link set dev eth0 mtu 9000.Gotcha: ip link set changes are not persistent. Configure MTU permanently in /etc/netplan/*.yaml, /etc/network/interfaces, or a NetworkManager profile.
Remember: MTU issues cause subtle problems. Test: `ping -M do -s 1472 host` (1472+28=1500).
🟡 Medium (4)¶
1. How does Path MTU Discovery (PMTUD) work?
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The sender sets the Don't Fragment (DF) bit on packets. If a router along the path cannot forward a packet because it exceeds the link MTU, it drops it and sends back an ICMP "Fragmentation Needed" message. The sender reduces packet size and retries.Remember: PMTUD uses ICMP. Blocked ICMP → black hole → stalled connections.
Gotcha: Allow ICMP type 3 code 4 to fix PMTUD black holes.
2. What is an MTU blackhole and what are its symptoms?
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An MTU blackhole occurs when ICMP "Fragmentation Needed" messages are blocked by a firewall, breaking PMTUD. Symptoms: small packets work (ping, DNS, SSH login) but large transfers hang (SCP, HTTP downloads). TCP connections establish but data transfer stalls.Remember: MTU = Maximum Transmission Unit. Ethernet=1500. Jumbo=9000.
Gotcha: Mismatch = fragmentation/drops. Entire path must match for jumbo.
3. How do you test path MTU using ping on Linux?
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Use ping -M do -sRemember: PMTUD uses ICMP. Blocked ICMP → black hole → stalled connections.
Gotcha: Allow ICMP type 3 code 4 to fix PMTUD black holes.
4. Why do overlay networks like VXLAN require lower MTU?
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Overlay protocols add encapsulation headers (VXLAN adds 50 bytes, GRE adds 24 bytes). On a 1500 underlay, the effective MTU for VXLAN is 1450. Pods/containers must use this reduced MTU or large packets will be dropped.Remember: MTU issues cause subtle problems. Test: `ping -M do -s 1472 host` (1472+28=1500).
🔴 Hard (3)¶
1. What is TCP MSS clamping and when should you use it?
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MSS clamping forces smaller TCP segments by rewriting the MSS option in SYN packets. Use when you cannot fix MTU everywhere: iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu. This avoids fragmentation without requiring end-host MTU changes.Remember: MTU issues cause subtle problems. Test: `ping -M do -s 1472 host` (1472+28=1500).
2. Why is IP fragmentation problematic and how do you detect it?
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Fragmentation is bad because: one lost fragment requires full retransmission, it increases router CPU, stateful firewalls may drop fragments, and it enables DoS attacks. Detect with: tcpdump -i eth0 'ip[6:2] & 0x3fff != 0' (matches fragmented packets) or netstat -s | grep -i frag.Remember: Fragmentation = performance+security hit. Avoid with correct MTU settings.
3. How does MTU misconfiguration affect Kubernetes pod networking?
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If CNI plugin MTU exceeds the underlay effective MTU (e.g., pod MTU 1500 on VXLAN underlay), large pod-to-pod transfers fail silently. DNS works but HTTP hangs. Fix by setting pod MTU = underlay MTU minus overlay overhead (e.g., 1450 for VXLAN on 1500). Verify with kubectl exec -itRemember: MTU issues cause subtle problems. Test: `ping -M do -s 1472 host` (1472+28=1500).