Ip Multicast Address Range Restrictions - Avaya 8800 Planning And Engineering, Network Design

Multicast flow distribution over MLT
MultiLink Trunking distributes multicast streams over a multilink trunk based on the source-subnet
and group addresses of the packets. You can choose the address parameters that the distribution
algorithm uses. As a result, you can distribute the load on different ports of the MLT and achieve an
even stream distribution.
To determine the egress port for a particular Source, Group (S,G) pair, the number of active ports of
the MLT is used to MOD the number generated by the XOR of each byte of the masked group
address with the masked source address. (The MOD function returns the remainder after a number
is divided by divisor; the XOR [or exclusive-or function] operates such that a XOR b is true if a is
true, or if b is true, but not if both are false, or both are true.)
Flow distribution and stream failover considerations
This section describes a traffic interruption issue that can occur in a PIM domain that has the
multicast MLT flow redistribution feature enabled. The following figure illustrates a normal scenario
where multicast streams flow from R1 to R2 through an MLT. The streams are distributed on links
L1, L2, and L3.
Figure 83: Multicast flow distribution over MLT
If link L1 goes down, the affected streams are distributed on links L2 and L3. However, with
redistribution enabled, the unaffected streams (flowing on L2 and L3) also start distributing. Because
the switch does not update the corresponding RPF (Reverse Path Forwarding) ports on switch R2
for these unaffected streams, this causes the activity check for these streams to fail (because of an
incorrect RPF port). Then, the switch improperly prunes these streams.
To avoid this issue, the activity check is set to 210 seconds. If the activity check fails when the (S,G)
entry timer expires (210 seconds), the switch deletes the (S,G) entry. The (S,G) entry is recreated
when packets corresponding to the (S,G) pair reach the switch again. There can be a short window
of traffic interruption during this deletion-creation period.

IP multicast address range restrictions

IP multicast routers use D class addresses, which range from 224.0.0.0 to 239.255.255.255.
Although subnet masks are commonly used to configure IP multicast address ranges, the concept of
subnets does not exist for multicast group addresses. Consequently, the usual unicast conventions
—where you reserve the all 0s subnets, all 1s subnets, all 0s host addresses, and all 1s host
addresses—do not apply.
Addresses from 224.0.0.0 through 224.0.0.255 are reserved by the Internet Assigned Numbers
Authority for link-local network applications. Packets with an address in this range are not forwarded
by multicast-capable routers. For example, OSPF uses 224.0.0.5 and 224.0.0.6, and VRRP uses
224.0.0.18 to communicate across local broadcast network segments.
June 2016 Planning and Engineering — Network Design
Comments on this document? infodev@avaya.com
General multicast considerations
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