Comtech EF Data CDM-570 Installation And Operation Manual page 211

CDM-570/570L Satellite Modem with Optional IP Module
Ethernet IP Module Interface
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Managed Switch Mode Multicast Option – Multicast packets in Managed Switch Mode
are identified using multicast MAC address. These identified multicast packets are either
routed or dropped based on the Managed Switch Mode multicast option.
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Managed Switch Mode uses Layer 2 (MAC) addresses to learn where to send packets. In
comparison, router mode uses the destination IP address in the packet in conjunction with
the route table to determine where to forward the packets.
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The Ethernet IP Module Interface in Managed Switch Mode is configured to be in
promiscuous mode with a data rate of 10/100BaseT Half Duplex/Full Duplex. The IP
Module needs to be in promiscuous mode in order to learn the attached networking
devices.
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Since Managed Switch Mode does not use a routing table, the determination of where to
send a packet is made by a learning process. When the system is powered-up, all packets
from each subnet (local and remote) will be sent over the satellite interface. However, as
each IP Module learns which devices are attached to their local Ethernet interfaces, the IP
Module begins to filter packets which it has learned are locally attached to its Ethernet
interface.
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The Managed Switch Mode learning/forwarding algorithm is as follows:
o If the packet is destined for the IP Module, then process it locally.
o If the packet is from the Ethernet IP Module Interface, then send it to the Satellite
interface; OR if the destination Layer 2 (MAC) address of the packets matches the
source Layer 2 address for a packet we have already seen, then the destination MAC
address of this packet is on our local subnet; so why send it over the satellite
interface. In this case, the IP Module will drop the packet.
o If the packet is from the satellite interface, then send it out the Ethernet interface.
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Managed Switch Mode also supports VLAN's and when VLAN Mode is enabled, will
forward traffic as follows:
o If an untagged packet arrives at the IP Module, it will egress as a tagged packet with
the defined Native VLAN ID.
o VLAN entries can be added into the VLAN Table and to forward VLAN's over the
satellite. A VLAN entry is also needed on the receiving CDM-570 to receive the
VLAN traffic and they will egress as tagged VLAN's.
o If a tagged packet arrives at the Ethernet port, it will only be forwarded if there is a
matching VLAN entry in the VLAN Table and it will egress as tagged. If there is no
matching VLAN in the VLAN Table, the tagged packet will be dropped.
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VLAN operation in the CDM-570 also includes full 802.1Q VLAN QoS support. With
the QoS Option enabled, the IP Module will read the 802.1Q VLAN Priority (7 through
0) that is in the VLAN Header in each VLAN frame received and will prioritize traffic
accordingly.
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Revision 12
MN/CDM570L.IOM