ISP Redundancy on a Cluster

Introduction

ISP Redundancy lets you connect Cluster Members to the Internet through redundant Internet Service Provider (ISP) links.

ISP Redundancy monitors the ISP links and chooses the best current link.

Notes:

  • R80.40 supports two ISPs.

  • ISP Redundancy is intended for traffic that originates on your internal networks and goes to the Internet.

Important:

  • You must connect each Cluster Member with a dedicated physical interface to each of the ISPs.

  • The IP addresses assigned to physical interfaces on each Cluster Member must be on the same subnet as the Cluster Virtual IP address.

IP addresses in the table below are only examples.

Item

Description

1

Internal network

2

Switches

3

Cluster Member A

3a

Cluster interface connected to the internal network (IP address 10.10.10.0/24)

  • Interface IP address 10.10.10.11
  • Virtual IP address 10.10.10.1

3b

Cluster interface (IP address 20.20.20.11) connected to the Sync network (IP address 20.20.20.0/24)

3c

Cluster interface connected to a switch that connects to ISP A

  • Interface IP address 30.30.30.11
  • Virtual IP address 30.30.30.1

3d

Cluster interface connected to a switch that connects to ISP B

  • Interface IP address 40.40.40.11
  • Virtual IP address 40.40.40.1

4

Cluster Member B

4a

Cluster interface connected to the internal network (IP address 10.10.10.0/24)

  • Interface IP address 10.10.10.22
  • Virtual IP address 10.10.10.1

4b

Cluster interface (IP address 20.20.20.22) connected to the Sync network (IP address 20.20.20.0/24)

4c

Cluster interface connected to a switch that connects to ISP B

  • Interface IP address 40.40.40.22
  • Virtual IP address 40.40.40.1

4d

Cluster interface connected to a switch that connects to ISP A

  • Interface IP address 30.30.30.22
  • Virtual IP address 30.30.30.1

5

ISP B

6

ISP A

7

Internet

ISP Redundancy Modes

ISP Redundancy configuration modes control the behavior of outgoing connections from internal clients to the Internet:

Mode

Description

Load Sharing

Uses the two links to distribute load of connections.

Connections coming in are alternated.

You can configure best relative loads for the links (set a faster link to handle more load).

New connections are randomly assigned to a link.

If one link fails, the other link takes the load.

In this mode, incoming connections can reach the application servers through either ISP link because the Cluster can answer DNS requests for the IP address of internal servers with IP addresses from both ISPs by alternating their order.

Primary/Backup

Uses one link for connections.

It switches to the Backup link if the Primary link fails.

When the Primary link is restored, new connections are assigned to it.

Existing connections continue on the Backup link until they are complete.

In this mode, incoming connections (from the Internet to application servers in the DMZ or internal networks) also benefit, because the Cluster returns packets using the same ISP Link, through which the connection was initiated.

Best Practice:

  • If both ISPs are basically the same, use the Load Sharing mode to ensure that you are making the best use of both ISPs.

  • You may prefer to use one of your two ISPs that is more cost-effective in terms of price and reliability. In that case, use Primary/Backup mode and set the more cost-effective ISP as the Primary ISP link.

Outgoing Connections

  • In ISP Redundancy Load Sharing mode, outgoing traffic that exits the Cluster on its way to the Internet is distributed between the ISP Links. You can set a relative weight for how much you want each of the ISP Links to be used.

    For example, if one link is faster, it can be configured to route more traffic across that ISP link than the other.

  • In ISP Redundancy Primary/Backup mode, outgoing traffic uses an active primary link.

    Hide NAT is used to change the source address of outgoing packets to the address of the interface, through which the packet leaves the Cluster. This allows return packets to be automatically routed through the same ISP link, because their destination address is the address of the correct link. Hide NAT is configured by the administrator.

Incoming Connections

For external users to make incoming connections, the administrator must give each application server two routable IP addresses, one for each ISP. The administrator must also configure Static NAT to translate the routable addresses to the real server address.

If the servers handle different services (for example, HTTP and FTP), you can use NAT to employ only two routable IP addresses for all the publicly available servers.

External clients use one of the two addresses. In order to connect, the clients must be able to resolve the DNS name of the server to the correct IP address.

Note - In the following example, the subnets 172.16.0.0/24 and 192.168.0.0/24 represent public routable addresses.

In the following example, the Web server www.example.com is assigned an IP address from each ISP:

  • 192.168.1.2 from ISP A

  • 172.16.2.2 from ISP B

If the ISP Link A is down, then IP address 192.168.1.2 becomes unavailable, and the clients must be able to resolve the URL www.example.com to the IP address 172.16.2.2.

An incoming connection is established, based on this example, in the following sequence:

  1. When an external client on the Internet contacts www.example.com, the client sends a DNS query for the IP address of this URL.

    The DNS query reaches the Cluster. The Cluster has a built-in mini-DNS server that can be configured to intercept DNS queries (of Type A) for servers in its domain.

  2. A DNS query arriving at an interface that belongs to one of the ISP links, is intercepted by the Cluster.

  3. If the Cluster recognizes the name of the host, it sends one of the following replies:

    • In ISP Redundancy Primary/Backup mode, the Cluster replies only with the IP addresses associated with the Primary ISP link, as long as the Primary ISP link is active.

    • In ISP Redundancy Load Sharing mode, the Cluster replies with two IP addresses, alternating their order.

  4. If the Cluster is unable to handle DNS requests (for example, it may not recognize the host name), it passes the DNS query to its original destination or the DNS server of the domain example.com.

  5. When the external client receives the reply to its DNS query, it opens a connection. Once the packets reach the Cluster, the Cluster uses Static NAT to translate the destination IP address 192.168.1.2 or 172.16.2.2 to the real server IP address 10.0.0.2.

  6. The Cluster routes the reply packets from the server to the client through the same ISP link that was used to initiate the connection.