Quantum and Evolution Series Installation and Operating Handbook
8-41
8.12.7.2 Ethernet Point-to-Multipoint Bridging
•
Uses a shared outbound carrier to multiple remote modems, with either direct returns
over satellite (to multiple Rx modems at the hub), indirect terrestrial returns or no return.
•
If no return channel exists then you can only ever send multicast traffic (since there is
no reply to any ARP requests so no other traffic will ever be sent to the modem from the
PC).
•
The hub Tx modem does not expect to receive replies to ARP requests (since replies
will be by a different path i.e. via hub Rx modems).
•
However, the ARP replies sent to the Rx-only hub modems still get back to the
originating devices, so these still continue to sent data to the modem, which passes it
indiscriminately over satellite except for data destined for devices it recognises as being
on the local terrestrial side of the network,
•
It does not matter what subnetting is used at the remotes – the devices can be on one
or many subnets unless…
o
TCP acceleration or header compression is also being used, in which case all
the modems need to be on a single subnet.
o
When routing is used, all the modems need to be on a single satellite subnet but
you can use different subnets on the terrestrial side.
•
Double hop is supported, where a remote can communicate with another remote via the
hub. Double hop is not wasteful of bandwidth because the hub Tx modem only
retransmits data over satellite if it cannot find the destination device on the local
terrestrial network.
•
Each remote sees the data intended for the other remotes and these unwanted packets
must either be filtered out by enabling VLAN tagging or be filtered by a router. A similar
problem exists in routing mode, in which case ‘black hole’ gateways can be specified at
the remotes to forward unwanted packets to a non-existent address, causing them to
be dropped by the remote modem.
