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What Is ISP Link Selection?

As services expand and diversify, enterprises usually deploy multiple links at the network egress to improve egress link bandwidth and reliability. ISP link selection is also called ISP address library link selection. When a device functions as an egress gateway and connects to multiple ISP networks, we can enable ISP link selection on the device so that it forwards the traffic to a specific ISP network from the corresponding outbound interface. This ensures that the traffic is forwarded on the shortest path, improving the utilization of link resources and user experience.

Why Do We Need ISP Link Selection?

Currently, network connections are needed everywhere. All enterprises need reliable links to access the Internet. A failure on a single link may cause network interruptions, bringing immeasurable loss to enterprises. Insufficient bandwidth resources also affect service experience. Therefore, the stability of Internet access is critical to users, making link redundancy an inevitable choice. As services expand and diversify, enterprises usually lease Internet links and bandwidth resources from multiple ISPs.

The interconnection of ISP links in China is a complex historical problem. For users, it is difficult to ensure cross-ISP access experience. For example, an enterprise that leases China Telecom's private lines has a good experience in accessing services deployed on China Telecom's network, while an enterprise that leases the private lines of another ISP has a poor experience. If a single ISP link is leased, users may experience issues when accessing resources of other ISPs. They may even experience these issues if multiple ISP links are leased. As such, enterprise network administrators need to consider how to select proper links to forward user traffic and how to improve link resource utilization while ensuring access experience. For example, enterprises require that China Telecom links be used for China Telecom services, and China Unicom links for China Unicom services.

When a device functions as an egress gateway and connects to multiple ISP networks, we can enable ISP link selection on the device so that ISP routes to the carrier network can be generated in batches and the device can forward the traffic to a specific ISP network from the corresponding outbound interface. This ensures that the traffic is forwarded on the shortest path, improving the forwarding efficiency and ensuring an optimal user experience.

How Does ISP Link Selection Work?

ISP link selection, also called ISP address library link selection, is implemented based on ISP routes. Each ISP has its own well-known public network segment. ISP link selection integrates these well-known network segments of ISPs into the device. When a device functions as an egress gateway and connects to multiple ISP networks, we can specify the outbound interface and next hop to deliver static routes in batches so that the traffic to a specific ISP network can be forwarded through the corresponding outbound interface. This ensures that the traffic is forwarded over the shortest path based on ISP routes, improving the forwarding efficiency.

Assuming that the device has two egress links that belong to different ISP networks, when intranet users access Server2 on the network of ISP2, if the device has equal-cost routes, the device can forward traffic to Server2 through two different paths: path 1 and path 2 in the figure. Apparently, path 2 is not the best path, and path 1 is the most desired path. With ISP link selection configured, when intranet users access Server1 or Server2, the device selects an outbound interface based on the ISP network where the destination address resides to forward the traffic over the shortest path to the server: path 3 or path 1 in the figure.

ISP link selection
ISP link selection

Application Scenarios of ISP Link Selection

ISP link selection can be used independently or together with other link selection policies such as those of policy-based routing (PBR).

Table 1-1 Application scenarios of ISP link selection

Category

Usage Scenario

ISP link selection

To enable the traffic to a specific ISP network to be forwarded through the corresponding outbound interfaces without needing to traverse the networks of other ISPs, configure ISP link selection.

ISP link selection + PBR

To select an outbound interface based on the ISP network to which the destination address of packets belongs and implement link backup or load balancing based on the multi-egress policy, configure ISP link selection and PBR.

ISP link selection + transparent DNS proxy

This can be used when intranet users access the web server through a domain name. The DNS proxy enables intranet users to obtain different ISP addresses in a balanced manner and then perform ISP link selection. The device selects an outbound interface based on the ISP network to which the destination address of the packets belongs.

ISP Link Selection of Huawei Firewalls

Huawei firewalls provide the ISP link selection service for enterprises. When multiple links are deployed at the network egress, the link selection process is that shown in the following figure.

ISP link selection process
ISP link selection process
  1. If the health check function is configured on DeviceA, the device periodically sends probe packets to the probed device to check whether the link between the local end and the destination network is normal. When ISP uplink selection is required, the device will query real-time link status using the health check. Based on the link status, the device determines the outbound interfaces participating in link selection. If the health check function is not configured on DeviceA, all links are considered normal by default.
  2. When a service request packet from a client arrives at the device, the device looks up ISP routes in the routing table.

    Administrators need to download the latest ISP address file from Huawei security center (isecurity.huawei.com), modify the ISP address file as required, and upload the ISP address file to DeviceA. Then, an ISP route is added to DeviceA and the outbound interface and next hop of the ISP route are set. After the configuration is complete, all IP address segments in the ISP address file are converted into static routes with the protocol type being UNR.

  3. When traffic matches an ISP route (static route) and multiple outbound interfaces are available to forward the traffic, the device needs to perform ISP link selection to determine the optimal outbound interface.
  4. Before ISP link selection, the device checks whether each outbound interface link is available. Faulty links will not participate in ISP link selection.
  5. The device obtains the link selection result.
  6. The device forwards service request packets through the corresponding outbound interface based on the ISP link selection result.
  7. The service server sends reply packets to the client.
About This Topic
  • Author: Zhu Wenjuan
  • Updated on: 2024-09-13
  • Views: 1059
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