How to Configure ISP Redundancy and Load Balancing
Symptom
Definitions
- ISP Load Balancing is used when more than one internet provider is connected to the firewall. Policy-Based Forwarding (PBF) is used to forward traffic based on the source subnet.
- ISP Redundancy is used when one service provider is down and all traffic needs to be routed to the remaining service provider.
Environment
- Normally, the firewall uses the destination IP address in a packet to determine the outgoing interface.
- The firewall uses the routing table associated with the virtual router to which the interface is connected to perform the route lookup.
- Policy-Based Forwarding (PBF) allows the user to override the routing table, and specify the outgoing or egress interface based on specific parameters such as source or destination IP address, or type of traffic.
The following topology includes:
Two internal subnets
- Subnet1: 192.168.1.0/24
- Subnet2: 172.16.1.0/24
Two ISP gateways
- ISP1: 10.30.6.254
- ISP2: 10.30.1.254
Cause
Two important items to remember:
- PBF rules are applied either on the first packet (SYN) or the first response to the first packet (SYN/ACK). Application-specific rules are not recommended for use with PBF.
- Address translation (NAT) rules are not applied unless a security rule matched the connection, which is why security rules need to be in place for the address translation to work.
Resolution
Configuring Redundancy
Primary ISP configuration:
- Create a PBF rule that forwards traffic to the default gateway.
- Attach a tunnel monitoring profile and set the action as "disable on failure."
Monitoring Profile:
This configuration forces all traffic coming from the 192.168.1.0/24 subnet to egress out of Ethernet 1/3.
A Monitor Profile is set up to monitor an IP address. In the test config, monitor profile "multiple isp" is used to monitor a public DNS 8.8.8.8.
When the monitor can no longer reach this IP address, the defined action (fail-over), takes place. The PBF rule is disabled and the firewall falls back to the static route created in the virtual router, as shown below. Path monitoring verifies connectivity to an IP address so the firewall can direct traffic through an alternate route. The firewall uses ICMP pings as heartbeats to verify that the specified IP address is reachable.
A monitoring profile allows specifying the threshold number of heartbeats to determine whether the IP address is reachable. When the monitored IP address is unreachable, the user can either disable the PBF rule or specify a fail-over or wait-recover action. Disabling the PBF rule allows the virtual router to take over the routing decisions.
Secondary ISP configuration
- Create a static route with a normal metric
Configuring Load Sharing
Example 1: Load balancing with no backup
In this case, PBF is used to force traffic from different subnets through the respective ISP. In this scenario, all traffic from subnet 192.168.1.0/24 is forwarded out of Ethernet 1/3, and subnet 172.16.1.0/24 is forced out of Ethernet 1/4.
Rules:
- Rule 1: Subnet 192.168.1.0/24 going to 0.0.0.0/0 next hop is ISP 1
- Rule 2: Subnet 172.16.1.0/24 going to 0.0.0.0/0 next hop is ISP 2
Example 2: Load balancing and redundancy
In this case, PBF is used to forward traffic out of a particular interface based on the source
A backup is configured if the ISP goes down.
Rules:
- Rule 1: Subnet 192.168.0.0/24 going to 0.0.0.0/0 next hop is ISP 1
- Rule 2: Subnet 172.16.0.0/24 going to 0.0.0.0/0 next hop is ISP 2
- Backup for Rule 1: Subnet 192.168.0.0/24 going to 0.0.0.0/0 next hop is ISP 2
- Backup for Rule 2: Subnet 172.16.0.0/24 going to 0.0.0.0/0 next hop is ISP 1
Rule 1 and Rule 2 perform the same action as Example 1.
The backup rules allow traffic to go through the ISP that has connectivity in case either were to fail.
If VPNs are configured (IPSec or GlobalProtect), refer to the following documents for information on how to configure the VPNs: