Installing and Administering Internet Services

Chapter 9 397
Configuring mrouted
Configuring mrouted
on the given interface or tunnel, and is used primarily to influence the
choice of routes over which the datagram is forwarded; the larger the
value, the higher the cost. Metrics should be kept as small as possible
since mrouted cannot route along paths with a sum of metrics greater
than 31. In general, you should use a metric value of 1 for all links unless
you are specifically attempting to force traffic to take another route. In
this case, the metric of the alternate path should be the sum of the
metrics on the primary path + 1. The default value is 1.
The threshold is the minimum IP time-to-live (TTL) required for a
multicast datagram to be forwarded to the given interface or tunnel. It
controls the scope of multicast datagrams. If the TTL value in the
datagram is less than the threshold value, the datagram is dropped; if
the TTL is greater than or equal to the threshold, the packet is
forwarded. The default threshold is 1.
The TTL of forwarded packets is only compared to the threshold, it is not
decremented by the threshold. The TTL is set by the application that
initiates the IP multicast datagram and typically represents the number
of subnets, or hops, that the datagram will need to traverse to reach its
destination. Every multicast router decrements the TTL by 1. We
recommend that you use the default threshold value unless you have a
specific need to set it otherwise.
In general, all interfaces connected to a particular subnet or tunnel
should use the same metric and threshold values for that subnet or
tunnel.
The rate_limit option allows the network administrator to specify a
certain bandwidth in Kbits/second which would be allocated to multicast
traffic. The default value is 500Kbps on tunnels and 0 (unlimited) on
physical interfaces.
The boundary option allows an interface to be configured as an
administrative boundary for the specified boundary-name or
scoped-addr (scoped address). More than one boundary option can be
specified in phyint and tunnel commands. Packets belonging to the
scoped address, which is an IP multicast group address, will not be
forwarded on this interface. mask-len indicates the number of leading
1s in the mask applied (that is, bitwise logically ANDed) to the scoped
address. For example, the statement boundary 239.2.3.3/16 would
result in the mask 255.255.0.0 being logically ANDed with 239.2.3.3 to
isolate the first two octets, 239.2, of the scoped address. Therefore, all IP
multicast addresses beginning with 239.2 will not be forwarded on the
specified interface.