Operating Manual 6175.4760.02 – 01 5.85
M3SR Series 4100 Editing the Multicast Routing Table
When two radios are out of reach, an intermediate radio must forward incoming multicast
packets (coming over the radio channel) to the radio channel, thus AIR AIR. Directions
involving the PPP interface are only active if PPP is enabled in the SRC setting of the IP port.
The TTL threshold defines the TTL value that incoming multicast packets must exceed in or-
der to be routed. Packets with a TTL equal to or smaller than the configured threshold will be
dropped.
• PC1 with IP 192.168.101.2 sends a multicast packet to the group 224.0.5.1. Radio1 re-
ceives the packet on the Eth IP interface. A lookup in the MC routing table finds an entry
with matching sender and group IP address. According to the routing direction the packet
is passed to the air IP interface for transmission. The packet is transmitted over the air
network and is received by Radio2 to 4 on its air IP interfaces. Radio2 finds a matching
entry in its multicast routing table (sender 192.168.101.2, group 224.0.5.1). The routing
direction defines the Eth IP interface as outgoing interface and the packet is transmitted
on the LAN (local area network).
• According to its multicast routing table, Radio4 forwards the received packet to the PPP
IP interface. PC4 receives the packet over the established PPP link.
• Usefulness of the TTL threshold:
Each IP packet (and so multicast packets) contains a TTL field. The initial value set by
the sender is decremented by each router (hop) the packet passes. If the packet's TTL
reaches the value 1, the packet is not forwarded anymore by the router, i.e. the packet
will be dropped.
Radio1 Air IP: 192.168.200.1
MC Routing Table Sender IP Group IP Direction TTL
192.168.101.2 224.0.5.1 ETH AIR 2
Radio2 Air IP: 192.168.200.2
MC Routing Table Sender IP Group IP Direction TTL
192.168.101.2 224.0.5.1 AIR ETH 1
Radio3 Air IP: 192.168.200.3
MC Routing Table Sender IP Group IP Direction TTL
192.168.101.2 224.0.5.1 AIR ETH 1
Radio4 Air IP: 192.168.200.4
MC Routing Table Sender IP Group IP Direction TTL
192.168.101.2 224.0.5.1 AIR PPP 1
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