ClassicLink overview - User Guide| Alibaba Cloud Documentation Center

Limits

Note the following before using the ClassicLink function:

Up to 1,000 ECS instances of the classic network can be connected to the same VPC.
An ECS instance of the classic network can be connected to only one VPC (belong to the same account and the same region).

For cross-account connection such as connecting an ECS instance of account A to a VPC of account B, the ECS instance can be transferred from account A to account B.

To enable the ClassicLink function of a VPC, the following conditions must be met:


VPC CIDR block	Limitations
172.16.0.0/12	There is no custom route entry destined for 10.0.0.0/8 in the VPC.
10.0.0.0/8	There is no custom route entry destined for 10.0.0.0/8 in the VPC. Make sure that the CIDR block of the VSwitch to communicate with the ECS instance in the classic network is within 10.111.0.0/16.
192.168.0.0/16	There is no custom route entry destined for 10.0.0.0/8 in the VPC. Add a route entry, of which the destination CIDR block is 192.168.0.0/16 and the next hop is the private NIC, to the ECS instance of the classic network. You can use the provided script to add the route. Click Here to download the route script. Note Before running the script, read the readme file in the script carefully.

Connection scenarios

The following table lists the scenarios of connecting an ECS instance in the classic network to a VPC network.


Network type of the initiator	Region/account	Network type of the receiver/intranet communication
		Classic network	VPC
Classic network	Same region Same account	Add a same-account authorization rule in the security group.	Build a ClassicLink connection.
	Same region Different accounts	Add an across-account authorization rule in the security group.	Solution A: Migrate the ECS instance of the classic network to the VPC network Connect the VPCs Solution B: Transfer the ECS instance of the classic network to the account of the VPC Build a ClassicLink connection
	Different regions Same account	Migrate both ECS instances to the VPC network. Connect the two VPCs.	Migrate the initiator ECS instance to the VPC network. Connect the two VPCs.
	Different regions Different accounts
VPC	Same region Same account	Build a ClassicLink connection	Connect the VPCs
	Same-region Cross-account	Solution A: Migrate the ECS instance of the classic network to the VPC Connect the VPCs Solution B: Migrate the ECS instance of the classic network to the account of the VPC. Build a ClassicLink connection
	Cross-region Same-account	Migrate the receiver ECS instance of the classic network to the VPC Connect the VPCs
	Cross-region Cross-account

Introduction to ClassicLink

The bottom layer implementation of the intercommunication between a classic network and VPC is consistent with that of the intercommunication between two different classic networks. Therefore, the intranet latency and bandwidth limit remain unchanged. Operations like downtime migration, hot migration, stopping, starting, restarting, and system disk replacement will not change the established ClassicLink link.

The classic network and VPC are two different network planes. ClassicLink establishes a private communication channel between these two network planes through routing. Therefore, to use the ClassicLink function, you must plan the network properly to avoid network conflicts.

The IP address range of the classic network in Alibaba Cloud is 10.0.0.0/8 (excluding 10.111.0.0/16). As long as the IP address range of the VPC does not conflict with 10.0.0.0/8, you can use ClassicLink to establish a private communication. The IP address ranges of the VPC that can communicate with the classic network through ClassicLink are 172.16.0.0/12, 10.111.0.0/16 and 192.168.0.0/16.

Principle of ClassicLink

After an ECS instance of the classic network is connected to a VPC through ClassicLink:

The ECS instance in the classic network can access the cloud resources in the VPC.

For example, an ECS instance in the classic network is connected to a VPC of the IP address range 10.0.0.0/8, and the VPC has a VSwitch of the IP address range 10.111.1.0/24. If you have deployed cloud resources such as ECS instances and RDS in the VSwitch, then the ECS instance in the classic network can access these resources through ClassicLink.
After the ClassicLink connection is successfully established, ECS instances in the VPC can only access the ECS instances in the classic network that is connected to the VPC, and cannot access the ECS instances in the classic network that is not connected to the VPC or other cloud resources in the classic network.