A brief talk about link aggregation, have you learned it?

Speaking of airport expressways, people often use them to go to and from the airport, and they often encounter traffic jams. In fact, some cities will build a second airport expressway, which is equivalent to widening the highway lanes to the airport, thus alleviating traffic jams. Even if one of the expressways is congested, drivers who know in advance can take another expressway. There is also a way similar to multiple airport expressways in the network - link aggregation. Today, I will talk about link aggregation.

1. What is link aggregation?

Link aggregation refers to "bundling" physical link segments with the same transmission medium type and the same transmission rate together, so that they appear to be one link logically. Link aggregation, also known as trunking, allows peer-to-peer physical links between switches or between switches and servers to increase bandwidth exponentially at the same time.

Link aggregation modes are divided into static aggregation and dynamic aggregation.

1. Static aggregation

Static aggregation is also called static Trunk (On mode). Users need to manually configure the aggregation group number and port members, and directly add multiple physical ports to the aggregation group to form a logical port.

Static aggregation does not run the LACP protocol. Since the status of the peer port of the link cannot be detected, if the peer port is down, as long as the local port is up, traffic will still be forwarded to the peer port, which may cause some business interruption.

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2. Dynamic aggregation

Dynamic aggregation runs on the LACP protocol based on IEEE802.3ad. LACP (Link Aggregation Control Protocol) is the link aggregation control protocol, which is a standard protocol described by IEEE 802.3ad.

LACP is a protocol that implements dynamic link aggregation. Simply put, it dynamically aggregates multiple physical ports into a Trunk group to improve bandwidth and redundancy.

LACP allows switches to negotiate through messages to determine which physical links can be bundled together and set corresponding parameters, such as priority, activity status, etc. When multiple physical links are bundled into a logical link, they will share the same IP address and MAC address, thus forming a logical high-speed channel.

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2. How does LACP work?

1. LACP dynamic mode aggregation group establishment process

LACP exchanges information with the peer through LACPDU (Link Aggregation Control Protocol Data Unit). After the LACP protocol of a port is enabled, the LACPDU message contains information such as the device's system priority, MAC address, interface priority, interface number, and operation key. After receiving this information, the peer compares it with the information saved by other ports to select the port that can be aggregated. Both parties reach an agreement on whether the port joins or exits a dynamic aggregation group and determines the link that bears the service traffic.

• The two ends send LACPDU messages to each other. Create a link aggregation group on the two devices, configure the LACP mode, and configure the member interface. At this time, the LACP protocol is enabled on the member interface, and the two ends send LACPDU messages to each other. The LACPDU message contains the device's system priority, MAC address, interface priority, interface number, and operation key.
• When a member interface joins a link aggregation group, both devices will receive LACPDU messages from the other end. Both ends check and record the information of the other end, and compare this information with the information saved by other member interfaces of the aggregation group to select the link aggregation group that can be joined.
• Determining the active interfaces in the link aggregation group The active interfaces in the link aggregation group are selected based on the interface priorities, and data is forwarded from these active links in a load-sharing manner.

2.LACP working mode

LACP working modes can be divided into active mode and passive mode.

• In active mode, the network device actively sends LACPDU and waits for the response from the peer device. If the peer device also supports LACP and is configured in passive mode, it will respond to LACPDU to establish link aggregation.
• In passive mode, the network device only receives LACPDU and responds accordingly to the request. Devices in passive mode are usually not active.

If the LACP working mode of the dynamic aggregation group member port is passive mode and the peer end is also passive mode, neither end will send LACPDU. If the LACP working mode of either end is active mode, LACPDU can be exchanged between member ports.

3.LACP timeout mode

LACP timeout modes include long timeout and short timeout.

• The long timeout mode sends LACPDUs slowly (30-second cycles) and the long timeout is 90 seconds.
• The short timeout mode sends LACPDU quickly (1 second period) and the short timeout is 3 seconds.

The default LACP timeout mode is long timeout, that is, the sending period is 30 seconds to send an LACP protocol message, and if the LACP protocol message from the other end is not received for more than 90 seconds, the negotiation is considered to have failed.

3. What benefits does link aggregation bring?

• Increase link bandwidth Link aggregation can increase network bandwidth. Link aggregation aggregates multiple physical ports to form a logical interface. The maximum bandwidth of the link aggregation interface can reach the sum of the bandwidths of each member interface, thereby achieving the simultaneous increase of bandwidth of peer physical links.
• Improve network reliability Link aggregation achieves link redundancy. When link aggregation is configured, when an active link fails, traffic can be switched to other available member links, thereby improving the reliability of the link aggregation interface and achieving link redundancy. For example, in the case of cross-slot link aggregation, service interruption caused by a single slot failure can be avoided.
• To achieve traffic load sharing within a link aggregation group, traffic can be shared among all member links based on certain rules. For example, a five-tuple includes source IP, destination IP, etc., thereby achieving load sharing of inbound/outbound loads on each member link and reducing the pressure on a single link.

4. What are the usage scenarios of link aggregation?

1. Fixed-line home broadband & campus network access scenarios

With the increase of fixed network services including home broadband and IPTV, the access bandwidth pressure of OLT is gradually increasing. Link aggregation can achieve fast and convenient broadband multiplication. In addition, the increase of campus network access traffic is also growing rapidly. Link aggregation is also widely used in campus network switch connections.

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2. Mobile bearer access scenario

With the rapid growth of 5G services such as data and high-definition video services, the requirements of mobile bearer networks for large bandwidth and high reliability have also increased significantly. In fact, link aggregation technology is widely deployed in mobile backhaul IPRAN networks, which not only realizes 10GE/100GE link bandwidth expansion, but also ensures link redundancy and realizes high-reliability service assurance.

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3. BRAS network scenario

BRAS is the core device for fixed network service authentication and has very high requirements for both the increase in access user traffic and link security and reliability. The deployment of link aggregation in the BRAS network has the flexibility of bandwidth expansion. As the access user traffic increases, link aggregation group members can be added to dynamically adjust the bandwidth of the link aggregation link; at the same time, through cross-board bundling of link aggregation, the reliability of tens of thousands of users on each single board is guaranteed.

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4. Data center scenario

Data centers, such as 5G telecom cloud networks, are the most core and critical networks for mobile services. The extensive application of link aggregation ensures the bandwidth expansion and link reliability of mobile services in data centers. At the same time, the load sharing of link aggregation can effectively utilize the member links of the link group to ensure efficiency.

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