Helpful Hints For Creating Ethernet Services Access Lists; Source And Destination Addresses; Ethernet Services Access List Entry Sequence Numbering; Sequence Numbering Behavior - Cisco ASR 9000 Series Configuration Manuallines

Implementing of Layer 2 Access Lists
• If no conditions match, the software drops the packet because each access list ends with an unwritten or
• The access list should contain at least one permit statement or else all packets are denied.
• Because the software stops testing conditions after the first match, the order of the conditions is critical.
• Inbound access lists process packets arriving at the router. Incoming packets are processed before being
• Outbound access lists process packets before they leave the router. Incoming packets are routed to the
• An access list can not be removed if that access list is being applied by an access group in use. To remove
• An access list must exist before you can use the ethernet-services access-group command.

Helpful Hints for Creating Ethernet Services Access Lists

Consider these when creating an Ethernet services access list:
• Create the access list before applying it to an interface.
• Organize your access list so that more specific references appear before more general ones.

Source and Destination Addresses

Source MAC address and destination MAC address are two of the most typical fields on which to base an
access list. Specify source MAC addresses to control packets from certain networking devices or hosts. Specify
destination MAC addresses to control packets being sent to certain networking devices or hosts.

Ethernet Services Access List Entry Sequence Numbering

The ability to apply sequence numbers to Ethernet services access-list entries simplifies access list changes.
The access list entry sequence numbering feature allows you to add sequence numbers to access-list entries
and resequence them. When you add a new entry, you choose the sequence number so that it is in a desired
position in the access list. If necessary, entries currently in the access list can be resequenced to create room
to insert the new entry.

Sequence Numbering Behavior

These details the sequence numbering behavior:
• If entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and
implicit deny statement. That is, if the packet has not been permitted or denied by the time it was tested
against each statement, it is denied.
The same permit or deny statements specified in a different order could result in a packet being passed
under one circumstance and denied in another circumstance.
routed to an outbound interface. An inbound access list is efficient because it saves the overhead of
routing lookups if the packet is to be discarded because it is denied by the filtering tests. If the packet is
permitted by the tests, it is then processed for routing. For inbound lists, permit means continue to process
the packet after receiving it on an inbound interface; deny means discard the packet.
outbound interface and then processed through the outbound access list. For outbound lists, permit means
send it to the output buffer; deny means discard the packet.
an access list, remove the access group that is referencing the access list and then remove the access list.
successive entries are incremented by 10. The maximum sequence number is 2147483646. If the generated
sequence number exceeds this maximum number, this message is displayed:
L2VPN and Ethernet Services Configuration Guide for Cisco ASR 9000 Series Routers, IOS XR Release 6.3.x
Helpful Hints for Creating Ethernet Services Access Lists
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