Dell FCX624-E Laptop User Manual


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890 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Overview
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The interfaces respond to general queries by sending a membership report containing one or more
of the following records associated with a specific group:
Current-state record - Indicates the sources from which the interface wants to receive or not
receive traffic. This record contains the source addresses of the interfaces and whether or not
traffic will be included (IS_IN) or excluded (IS_EX) from that source address.
Filter-mode-change record - If the interface changes its current state from IS_IN to IS_EX, a
TO_EX record is included in the membership report. Likewise, if an interface current state
changes from IS_EX to IS_IN, a TO_IN record appears in the membership report.
MLDv1 leave report - Equivalent to a TO_IN (empty) record in MLDv2. This record means that
no traffic from this group will be received regardless of the source.
An MLDv1 group report - Equivalent to an IS_EX (empty) record in MLDv2. This record means
that all traffic from this group will be received regardless of source.
Source-list-change record - If the interface wants to add or remove traffic sources from its
membership report, the report can include an ALLOW record, which contains a list of new
sources from which the interface wishes to receive traffic. The report can also contain a BLOCK
record, which lists current traffic sources from which the interface wants to stop receiving
traffic.
MLD protocols provide a way for clients and a device to exchange messages, and allow the device
to build a database indicating which port wants what traffic. Since the MLD protocols do not specify
forwarding methods, MLD Snooping or multicast protocols such as IPv6 PIM-Sparse Mode (PIM
SM) are required to handle packet forwarding. PIM SM can route multicast packets within and
outside a VLAN, while MLD Snooping can switch packets only within a VLAN. These devices do not
support PIM-SM routing.
If a VLAN is not MLD Snooping-enabled, it floods IPv6 multicast data and control packets to the
entire VLAN in hardware. When snooping is enabled, MLD packets are trapped to the CPU. Data
packets are mirrored to the CPU and VLAN flooded. The CPU then installs hardware resources so
subsequent data packets can be hardware-switched to desired ports without going through the
CPU. If there is no client report, the hardware resource drops the data stream. The hardware can
either match group addresses only (* G), or both source and group (S G) addresses in the data
stream. If MLDv2 is configured in any port of a VLAN, the VLAN uses an (S G) match, otherwise it
uses (* G). Because the hardware can match only the lowest 32 bits of a 128 bit IPv6 address, the
output interfaces (OIF) of a hardware resource are the superset of the OIF of all data streams
sharing the same lowest 32 bits. For example, if groups ff10::1234:5678:abcd and
ff20::5678:abcd share the same hardware resource, then the OIF of the hardware matching (*
5678:abcd) is the superset of these two groups.
Stackable devices allocate 16K of hardware resources for MAC learning, IGMP, and MLD snooping.
If a data packet does not match any of these resources, it might be sent to the CPU, increasing the
CPU burden. This can happen if the device runs out of hardware resources, or is unable to install a
resource for a specific matching address due to a hashing collision. Because the hardware hashes
addresses into 16K entries, some addresses may be hashed into the same entry. If the collision
number in an entry is more than the hardware chain length, the resource cannot be installed. The
chain length can be configured using the hash-chain-length command, as follows.
PowerConnect(config)#hash-chain-length 8
Syntax: [no] hash-chain-length <num>
The <num> parameter range is 4 to 32, in multiples of 4. If the input value is not a multiple of 4,
then it will be changed to the multiple of 4 lower than then the input value (e.g. 11 will be changed
to 8). The default hash chain length is 4. A chain length of more than 4 may affect line rate
switching.