๐ฅ When you see the output of <span><span>ip a</span></span>, are you confused by the mysterious parameters like <span><span>LOOPBACK</span></span>, <span><span>fq_codel</span></span>, and <span><span>metric 100</span></span>? ๐ฅ This article uses real terminal output examples to analyze the meaning of each parameter line by line and reveals how to diagnose network issues using them!๐ At the end of the article, we include ใTuning Tipsใ, a must-read!
1. Full Display of Output Examples
oneh@ubuntu-server-24-04:~$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host noprefixroute
valid_lft forever preferred_lft forever
2: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 00:0c:29:cd:d6:f1 brd ff:ff:ff:ff:ff:ff
altname enp2s1
inet 192.168.2.150/24 metric 100 brd 192.168.2.255 scope global dynamic ens33
valid_lft 75842sec preferred_lft 75842sec
inet6 fe80::20c:29ff:fecd:d6f1/64 scope link
valid_lft forever preferred_lft forever
2. Line-by-Line Breakdown: In-Depth Parameter Analysis
1. Loopback Interface <span>lo</span> Detailed Explanation
-
<span>LOOPBACK</span>: Identifies the loopback interface (used only for internal communication on the local machine). -
<span>mtu 65536</span>: Maximum Transmission Unit (a very large value, suitable for high-speed local communication). -
<span>qdisc noqueue</span>: No queue scheduling (no queuing, direct mode). -
<span>state UNKNOWN</span>: Special state identifier for the loopback interface (normal phenomenon).
inet 127.0.0.1/8 scope host lo
<span>scope host</span>: Accessible only from the local machine, cannot be connected from external networks.<span>valid_lft forever</span>: IP address is permanently valid (static configuration).
2. Physical Network Card <span>ens33</span> Detailed Explanation
2: ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
Interface Identity:
<span>2</span>: Interface number (a unique ID assigned by the system).<span>ens33</span>: Interface name (physical network cards commonly use<span>ethX</span>, virtual network cards are generally<span>ensXX</span>or<span>vethX</span>).
Status Tags: Vital Signs of the Network
<span>BROADCAST</span>: Supports broadcast communication (a typical Ethernet feature).<span>MULTICAST</span>: Supports multicast (e.g., video conferencing, cluster communication).<span>UP</span>: Software layer is enabled (<span>ip link set ens33 down</span>can disable it).<span>LOWER_UP</span>: Physical layer is connected (cable is plugged in/WIFI signal is normal).
Performance Triad: MTU, Queue, and Scheduler
-
<span>mtu 1500</span>: Maximum Transmission Unit. For example, the maximum size of a parcel. The default for LAN is<span>1500</span>, VPN/virtual machines can set<span>1450</span>to avoid fragmentation. Reference modification command:<span>sudo ip link set ens33 mtu 1450</span> -
<span>qdisc fq_codel</span>: Fair queue algorithm (automatically optimizes network latency and throughput).<span>mq(Multi-Queue)</span>: Multi-queue mode, suitable for multi-core CPUs. -
<span>state UP</span>: Interface is enabled (software layer activated). -
<span>qlen 1000</span>: Send queue length. It affects performance; if the value is too small, packets may be lost under high traffic (queue overflow); if the value is too large, latency increases (queuing time becomes longer).
link/ether 00:0c:29:cd:d6:f1 brd ff:ff:ff:ff:ff:ff
altname enp2s1
<span>link/ether</span>: MAC address of the network card (hardware unique identifier).<span>altname enp2s1</span>: Alternate name of the interface (compatible with different naming conventions).
inet 192.168.2.150/24 metric 100 brd 192.168.2.255 scope global dynamic ens33
<span>metric 100</span>: Routing priority (the smaller the value, the higher the priority).<span>scope global</span>: Accessible from external networks (e.g., public IP).<span>dynamic</span>: IP is dynamically assigned by DHCP (<span>valid_lft 75842sec</span>indicates the remaining lease time).
inet6 fe80::20c:29ff:fecd:d6f1/64 scope link
<span>fe80::</span>: IPv6 link-local address (valid only within the same local area network).<span>scope link</span>: Cannot communicate across routers.
3. Key Parameter State Changes
1. Interface State Combinations
| Status Tag | Meaning | Typical Scenario |
|---|---|---|
<span>UP + LOWER_UP</span> |
Interface is enabled and physical link is normal | Normal network connection state |
<span>UP</span> (without <span>LOWER_UP</span>) |
Software layer is enabled but physical link is disconnected | Cable not plugged in/WIFI not connected |
<span>DOWN</span> |
Interface is manually disabled | Network is actively turned off |
2. <span>valid_lft</span> and IP Lifecycle
-
<span>dynamic</span>Type of IP:valid_lft 75842sec # Remaining about 21 hours - DHCP lease will automatically renew before expiration.
-
<span>static</span>Type of IP:valid_lft forever # Permanently valid
4. Practical Tuning Tips
1. Optimize Network Latency (<span>qdisc</span>)
# Enable fair queue algorithm (default optimized)
sudo tc qdisc replace dev ens33 root fq_codel
# View current queue configuration
sudo tc -s qdisc show dev ens33
2. Adjust Routing Priority (<span>metric</span>)
# Modify interface routing priority (the smaller the value, the higher the priority)
sudo ip route modify default via 192.168.2.1 metric 50 dev ens33
3. Temporarily Increase Queue Length (<span>qlen</span>)
# To cope with high traffic scenarios (prevent packet loss)
sudo ip link set dev ens33 txqueuelen 2000
4. High-Concurrency Web Server
sudo ip link set ens33 txqueuelen 200 # Increase send queue
sudo ethtool -L ens33 combined 4 # Enable 4 transmission queues
sudo ip link set ens33 mtu 9000 # Enable jumbo frames, requires switch support
# Create load balancing group
sudo ip link set ens33 group lb
sudo ip link set ens34 group lb
# Batch configuration
sudo ip link set group lb mtu9000 txqueuelen 2000
5. Quick Troubleshooting Scripts
1. Check Physical Link Status
ip a | grep -A2 "ens33" | grep "LOWER_UP"
# Output โ Cable is connected; no output โ Physical link issue
2. View Remaining DHCP Lease Time
journalctl -u NetworkManager | grep "DHCPv4 address"
# Example output: DHCPv4 address 192.168.2.150/24 ... expires in 75842 seconds
3. Verify IPv6 Connectivity
ping6 fe80::20c:29ff:fecd:d6f1%ens33
# Other devices in the same local area network can respond
Further Learning
- Advanced Commands:
<span><span>ip route</span></span>(routing table),<span><span>ip neigh</span></span>(ARP table) - Permanent Configuration Modifications: Edit for Ubuntu systems
<span><span>/etc/netplan/</span></span><code><span>, for CentOS systems edit</span><code><span><span>/etc/sysconfig/network-scripts/</span></span><code>
Core Parameter Quick Reference Table
| Parameter | Function | Key Command |
|---|---|---|
<span>mtu</span> |
Controls packet size | <span>ip link set ens33 mtu 9000</span> |
<span>qdisc</span> |
Controls network queue scheduling algorithm | <span>tc qdisc show</span> |
<span>metric</span> |
Routing priority decision | <span>ip route show</span> |
<span>valid_lft</span> |
Determines if IP is dynamically assigned | <span>journalctl -u NetworkManager</span> |
<span>LOWER_UP</span> |
Diagnoses physical link issues | <span>ip a | grep LOWER_UP</span> |
Open your Linux terminal, type “ip a”, and refer to this article to analyze your network configuration! If you encounter any issues, feel free to leave a comment for discussion! ๐
Previous Reviews: Download links for open-source operating systems (Red Hat series) – CentOS, Rocky, Alma, openEuler, please bookmark for future use. Step-by-step installation of Ubuntu 24.04 Server version.
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