Acls And Multicast Packets - Cisco WS-C3550-12G Software Configuration Manual

Chapter 19 Configuring Network Security with ACLs
Using VLAN Maps with Router ACLs

ACLs and Multicast Packets

Figure 19-9 shows how ACLs are applied on packets that are replicated for IP multicasting. A multicast
packet being routed has two different kinds of filters applied: one for destinations that are other ports in
the input VLAN and another for each of the destinations that are in other VLANs to which the packet
has been routed. The packet might be routed to more than one output VLAN, in which case a different
router output ACL and VLAN map would apply for each destination VLAN.
The final result is that the packet might be permitted in some of the output VLANs and not in others. A
copy of the packet is forwarded to those destinations where it is permitted. However, if the input VLAN
map (VLAN 10 map in Figure 19-9) drops the packet, no destination receives a copy of the packet.
Figure 19-9 Applying ACLs on Multicast Packets
Catalyst 3550 switch
with enhanced
multilayer switch image
Input Output
VLAN 10 router router VLAN 20
map ACL ACL map
Frame
Host A Host B
(VLAN 10) (VLAN 20)
Routing function
Host C
(VLAN 10)
VLAN 10 VLAN 20
Packet
Catalyst 3550 Multilayer Switch Software Configuration Guide
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