Link aggregation is a long and difficult process
Link aggregation is a process of "bundling" multiple links between two devices to form an aggregation group. The multiple links in the aggregation group can now be regarded as a logical link.
At this time, the traffic between the two devices can be distributed among the links in the aggregation group.
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So, what benefits can link aggregation bring to the network?
Increased network bandwidth
Bundle multiple links between A and B into a logical link. The bandwidth after bundling is the sum of the bandwidths of all links.
For example, there are 3 links between A and B for link aggregation, and the bandwidth of each link is 10 Gbps. The maximum bandwidth of this aggregation group can reach 30 Gpbs.
Improves the reliability of network connections
If one link between A and B fails and is interrupted, the traffic will be automatically redistributed among the remaining links, and the traffic between A and B will not be interrupted.
Achieve traffic load balancing
Link aggregation can evenly distribute the traffic between A and B to all member links, minimizing the risk of traffic blocking the link on each member link.
Avoid the generation of a Layer 2 loop
When link aggregation is used for the link between A and B, these links are no longer a single link working independently, but become a logical link to the outside world. Therefore, even if STP (Spanning Tree Protocol) is not used, no loop will be generated, effectively avoiding the risk of a Layer 2 loop between A and B.
Based on the above advantages, link aggregation has been widely used in IP networks.
MC-LAG is a more reliable "bundle"
In the era of mobile Internet, the Internet is becoming more and more closely related to our daily lives. While we enjoy the convenience of the Internet, it also brings a large amount of data interactive processing to the Internet, which puts higher requirements on the bandwidth and reliability of the network.
However, the traditional link aggregation technology is limited to one-to-one between two devices, and cannot achieve one-to-many device link aggregation.
Therefore, in order to provide a more reliable network, MC-LAG (Multi-Chassis Link Aggregation Group) was created.
When an access device (which can be a server or a switch) is connected to two upper-layer network devices A and B, MC-LAG technology can be used to form a cross-device link aggregation group.
The basic idea of MC-LAG is to allow two network devices A and B to link aggregate with the access device in the same state. From the perspective of the access device, it is as if a link aggregation relationship has been established with the same network device. In this way, the link aggregation technology is expanded from one-to-one device docking to being able to connect to two devices at the same time, and form a dual-active system.
Let's take a look at how this active-active system works.
MC-LAG working process
Before understanding the MC-LAG working process, let's first learn some basic concepts involved in MC-LAG technology.
MC-LAG master/slave devices: Like stacking, MC-LAG also selects master/slave devices. However, under normal circumstances, the master and backup devices forward business traffic at the same time. Only in the failure scenario will the behavior of the master and backup devices be different.
ICBG synchronization: ICBG (Inter-Chassis Backup Group) is used to synchronize information such as MAC and ARP entries between MC-LAG dual-homing devices. In ICBG, you can specify the interface that needs to be synchronized. The roles of devices A and B are divided into master and backup. Under normal circumstances, the master and backup devices forward business traffic at the same time.
Peer-link: A directly connected Layer 2 link between two MC-LAG devices A and B, used to negotiate packet interaction and partial traffic transmission. Peer-link uses multiple links for link aggregation.
Keepalive: A heartbeat link between two MC-LAG devices, carrying heartbeat packets. Its main function is to send dual-active detection messages between the active and standby devices to perform dual-active detection and prevent devices A and B from being dual-active.
MC-LAG member interface: An interface connecting access devices on two network devices A and B. The status of the two MC-LAG member ports needs to be synchronized. To increase reliability, it is recommended that the MC-LAG member port link aggregation be configured in LACP mode.
After understanding the basic concepts of MC-LAG, we will further understand the process of establishing MC-LAG, which includes the following 4 steps.
After the configuration of the two devices of MC-LAG is completed, they send link-building packets through the Keepalive link to negotiate with the other end in order to establish the MC-LAG system with the other end.
The two ends first select the master and backup according to the MC-LAG priority, and the one with the higher priority is the master; if the MC-LAG priority is the same, the system MAC of the two devices is compared, and the one with the smaller MAC is the master.
The master and backup devices synchronize information in real time or regularly through the Peer-link link. The synchronized information includes MAC, ARP, member interface status, etc.
The master and backup devices send heartbeat detection packets through the Keepalive link, which is mainly used for dual-master detection when the Peer-link fails.
After completing the above establishment process, MC-LAG can operate normally.
MC-LAG traffic forwarding
MC-LAG is mainly used in dual-access scenarios, that is, the access side device C uses MC-LAG technology to access the network side devices A and B. During normal operation, the upstream and downstream traffic is forwarded through devices A and B in a load balancing manner.
If the above network fails, how does MC-LAG protect the normal operation of the network?
Member interface link failure
If a member interface of MC-LAG fails, for example, a member interface of device B fails. Access-side device C senses the failure of the member interface of device B and transmits all upstream traffic to device A, which then forwards it.
When device B receives traffic sent from the network side to access-side device C, it will hand over the traffic to the normally functioning device A through peer-link and forward it to access-side device C.
MC-LAG device failure
If one MC-LAG device fails, for example, device B fails. At this time, device B cannot forward traffic, and all traffic is forwarded by device A.
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Peer-link failure
If a peer-link failure occurs, devices A and B cannot forward traffic at the same time, otherwise it will cause a series of problems such as broadcast storms and MAC drift, so only one device is allowed to forward traffic.
At this time, the backup device of MC-LAG (here is device B) will process all its physical interfaces except the peer-link interface, keepalive interface and management network port in error-down mode. At this time, all traffic will only be forwarded through the MC-LAG master device.
Uplink failure
If an uplink failure occurs, the uplink traffic of device C will be sent to device B through peer-link after reaching device A for forwarding (you can also set the uplink and downlink ports to be linked, so that all uplink traffic is sent directly from device C to device B for forwarding).
Through the previous introduction, we can understand that MC-LAG technology has more advantages than traditional link aggregation technology. While enhancing network reliability, it simplifies networking and realizes high-availability redundant protection and multi-path forwarding at the device level.
In addition, the two MC-LAG network devices operate independently and can be upgraded separately. During the upgrade process, as long as one device is ensured to operate normally, there will be almost no impact on the running business. At present, MC-LAG technology is being widely used in new IP metropolitan area networks and data centers. While adopting the Spine-Leaf network architecture, MC-LAG can be deployed to ensure network reliability. In the evolution of 5G, MC-LAG technology will surely provide more reliable protection for IP bearer networks.
