Introduction
ACL Use Cases
In RBFS, Access Control Lists (ACL) serve multiple purposes:
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Provide security by traffic filtering. This applies to both host and transit traffic. ACLs for traffic filtering are user-defined by configuration.
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Redirecting control traffic to the CPU. Such protocol ACLs, also referred to as trap rules, are automatically created by the respective protocol, and do not need to be configured.
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Classifying traffic for differentiated QoS treatment. This is a special form of ACLs referred to as a multi-field (MF) classifier. For more information about MF classifiers, please refer to the HQoS Configuration Guide.
ACL Components and Processing
User-defined ACLs consist of rules and ordinals. In case of multiple matching ACL rules, you can use priorities to define the result of the ACL.
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Rules - A rule is a named ACL entry that typically contains one or multiple match criteria and an action.
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Ordinals - An ordinal is solely a numbered configuration object. A rule can consist of multiple ordinals. Ordinals help to structure the configuration. In RBFS, it makes no difference if you configure one rule with multiple ordinals or multiple rules with one ordinal each. Please note ordinals do not define the order of processing.
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Scope - ACLs generally apply globally. In particular, they are not applied to interfaces. You can however configure an interface as a match criteria.
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Priorities - ACL entry priorities are used to define the processing of multiple matching ACL rules. In RBFS, by default, all ACL entries have the same priority, and there is no specific order. For example, if one ACL rule shall permit ICMP traffic from a specific prefix, and another rule shall deny any other ICMP traffic, it will by default result in a conflict as an ICMP packet matches both rules. To ensure that the more specific rule matches first, you can set its priority to higher. When the ACL priority value is set to a lower number, priority is higher.
Prefix Lists
A prefix list is a named list of prefixes. Instead of listing multiple individual prefixes in a match rule of the ACL itself, you can reference a list that contains the prefixes, and thereby apply a common action to all matching prefixes. This helps to maintain lists and reuse them in multiple ACL rules.
Prefix lists can be used in Access Control List (ACL) for traffic filtering, as well as in Multifield (MF) classifier ACLs. This guide describes how to configure prefix lists and apply them to user-defined ACLs for traffic filtering. For applying prefix lists to MF classifiers, please refer to the HQoS configuration guide.
When a prefix list is configured and referenced in an ACL, it is internally first added to an intermediate ACL configuration table. For each prefix, one separate rule is added to the final ACL configuration table. This is different from a prefix match in the ACL rule itself that is directly added to the ACL configuration table. A dedicated range of ordinals (200001-4294967295) is reserved to expand ACL rules when using prefix lists. If configured, the priority will be copied from the prefix list ACL configuration to all the expanded ACL rules.
When using prefix lists, the following restrictions apply:
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You cannot configure the same prefix-list name to match the source prefix list and destination prefix list.
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You cannot configure both the source prefix and source prefix list on the same ACL configuration.
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You cannot configure both the destination prefix and destination prefix list on the same ACL configuration.