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Understanding How to Use Standard Firewall Filters

Using Standard Firewall Filters to Affect Local Packets

On a router, you can configure one physical loopback interface, lo0, and one or more addresses on the interface. The loopback interface is the interface to the Routing Engine, which runs and monitors all the control protocols. The loopback interface carries local packets only. Standard firewall filters applied to the loopback interface affect the local packets destined for or transmitted from the Routing Engine.


When you create an additional loopback interface, it is important to apply a filter to it so the Routing Engine is protected. We recommend that when you apply a filter to the loopback interface, you include the apply-groups statement. Doing so ensures that the filter is automatically inherited on every loopback interface, including lo0 and other loopback interfaces.

Trusted Sources

The typical use of a standard stateless firewall filter is to protect the Routing Engine processes and resources from malicious or untrusted packets. To protect the processes and resources owned by the Routing Engine, you can use a standard stateless firewall filter that specifies which protocols and services, or applications, are allowed to reach the Routing Engine. Applying this type of filter to the loopback interface ensures that the local packets are from a trusted source and protects the processes running on the Routing Engine from an external attack.

Flood Prevention

You can create standard stateless firewall filters that limit certain TCP and ICMP traffic destined for the Routing Engine. A router without this kind of protection is vulnerable to TCP and ICMP flood attacks, which are also called denial-of-service (DoS) attacks. For example:

  • A TCP flood attack of SYN packets initiating connection requests can overwhelm the device until it can no longer process legitimate connection requests, resulting in denial of service.

  • An ICMP flood can overload the device with so many echo requests (ping requests) that it expends all its resources responding and can no longer process valid network traffic, also resulting in denial of service.

Applying the appropriate firewall filters to the Routing Engine protects against these types of attacks.

Using Standard Firewall Filters to Affect Data Packets

Standard firewall filters that you apply to your router’s transit interfaces evaluate only the user data packets that transit the router from one interface directly to another as they are being forwarded from a source to a destination. To protect the network as a whole from unauthorized access and other threats at specific interfaces, you can apply firewall filters router transit interfaces .