Concept Guide

Dell Networking OS Behavior: Virtual router redundancy protocol (VRRP) hellos and internet group management protocol
(IGMP) packets are not affected when you enable egress ACL filtering for CPU traffic. Packets sent by the CPU with the
source address as the VRRP virtual IP address have the interface MAC address instead of VRRP virtual MAC address.
Counting ACL Hits
You can view the number of packets matching the ACL by using the count option when creating ACL entries.
1. Create an ACL that uses rules with the count option. Refer to Configure a Standard IP ACL Filter.
2. Apply the ACL as an inbound or outbound ACL on an interface. Refer to Applying an IP ACL.
3. show ip accounting access-list
EXEC Privilege mode
View the number of packets matching the ACL.
IP Prefix Lists
IP prefix lists are supported to control routing policy.
An IP prefix list is a series of sequential filters that contain a matching criterion (examine IP route prefix) and an action (permit
or deny) to process routes. The filters are processed in sequence so that if a route prefix does not match the criterion in the
first filter, the second filter (if configured) is applied. When the route prefix matches a filter, the system drops or forwards the
packet based on the filter’s designated action. If the route prefix does not match any of the filters in the prefix list, the route is
dropped (that is, implicit deny).
A route prefix is an IP address pattern that matches on bits within the IP address. The format of a route prefix is A.B.C.D/X
where A.B.C.D is a dotted-decimal address and /X is the number of bits that should be matched of the dotted decimal
address. For example, in 112.24.0.0/16, the first 16 bits of the address 112.24.0.0 match all addresses between 112.24.0.0 to
112.24.255.255.
The following examples show permit or deny filters for specific routes using the le and ge parameters, where x.x.x.x/x
represents a route prefix:
● To deny only /8 prefixes, enter deny x.x.x.x/x ge 8 le 8.
● To permit routes with the mask greater than /8 but less than /12, enter permit x.x.x.x/x ge 8.
● To deny routes with a mask less than /24, enter deny x.x.x.x/x le 24.
● To permit routes with a mask greater than /20, enter permit x.x.x.x/x ge 20.
The following rules apply to prefix lists:
● A prefix list without any permit or deny filters allows all routes.
● An “implicit deny” is assumed (that is, the route is dropped) for all route prefixes that do not match a permit or deny filter in
a configured prefix list.
● After a route matches a filter, the filter’s action is applied. No additional filters are applied to the route.
Implementation Information
Prefix lists are used in processing routes for routing protocols (for example, router information protocol [RIP], open shortest
path first [OSPF], and border gateway protocol [BGP]).
NOTE: It is important to know which protocol your system supports prior to implementing prefix-lists.
Configuration Task List for Prefix Lists
To configure a prefix list, use commands in PREFIX LIST, ROUTER RIP, ROUTER OSPF, and ROUTER BGP modes.
Create the prefix list in PREFIX LIST mode and assign that list to commands in ROUTER RIP, ROUTER OSPF and ROUTER BGP
modes.
The following list includes the configuration tasks for prefix lists, as described in the following sections.
● Configuring a prefix list
● Use a prefix list for route redistribution
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Access Control Lists (ACLs)