Revision as of 15:57, 12 June 2011

Multicast

IPv6 Address Type:	Multicast
Prefix:	ff00::/8
Local Routeable:	Yes and No
Global Routeable:	Yes and No
Global Unique:	Yes and No
Example:	FF0E::101
IPv4 Equivalent:	224.0.0.0/4
Described in:	rfc4291 section 2.7

	# (Optional)

Explanation

These addresses are used to identify multicast groups. They should only be used as destination addresses, never as source addresses.

IPv6 Address Types

Multicast Address Format

The IPv6 Multicast addresses contains two fields describing which kind of Multicast it is. Flag and Scope.

Flag Field

The flag field consists of four flags.


0	R	P	T

Flags
Flag	Explanation
T	T = 0 indicates a permanently-assigned well-known multicast address assigned by IANA. T = 1 indicates a non-permanently-assigned ("transient" or "dynamically" assigned) multicast address. See rfc4292
P	P = 0 indicates a multicast address that is not assigned based on the network prefix. P = 1 indicates a multicast address that is assigned based on the network prefix. See rfc3306
R	R = 0 indicates a multicast address that does not embed the address of the RP R = 1 indicates a multicast address that embed an RP (Rendezvous Point) See rfc3956
0	The high-order flag is reserved, and must be initialized to 0.

Scope Field

The Scope field is used to limit the Scope of the Multicast Group.

Multicast Address Scope Value in use
Value	Scope	Explanation
1	Interface-Local	Interface-Local scope spans only a single interface on a node and is useful only for loopback transmission of multicast.
2	Link Local	Link-Local multicast scope spans the same topological region as the corresponding unicast scope.
4	Admin Local	Admin-Local scope is the smallest scope that must be administratively configured, i.e., not automatically derived from physical connectivity or other, non-multicast-related configuration.
5	Site Local	Site-Local scope is intended to span a single site.
8	Organization Local	Organization-Local scope is intended to span multiple sites belonging to a single organization.
E	Global	Global span.

Permanent Assigned Multicast Address

The "meaning" of a permanently-assigned multicast address is independent of the scope value. For example, if the "NTP servers group" is assigned a permanent multicast address with a group ID of 101 (hex), then

FF01::101 means all NTP servers on the same interface (i.e., the same node) as the sender.

FF02::101 means all NTP servers on the same link as the sender.

FF05::101 means all NTP servers in the same site as the sender.

FF0E::101 means all NTP servers in the Internet.

Well Known Multicast examples


Address	Scope	Meaning	Description
FF01::1	Node	All Nodes	All nodes on the interface-local scope
FF01::2	Node	All Routers	All routers on the interface-local scope
FF02::1	Link Local	All Nodes	All nodes on the local-link scope
FF02::2	Link Local	All Routers	All routers on the link-local scope
FF05::2	Site	All Routers	All routers in a site scope

Pinging FF02::2

Router IPv6 fastethernet 0/1 is Link-local with five other Routers.

IPv6#<input>ping ipv6 FF02::2 source fastEthernet 0/1 repeat 1</input>
Output Interface: <input>fastethernet0/1</input>
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to FF02::2, timeout is 2 seconds:
Packet sent with a source address of FE80::21B:D4FF:FE0F:8CA3
Reply to request 0 received from FE80::21B:D4FF:FE0F:D812, 4 ms
Reply to request 0 received from FE80::21B:D4FF:FE0F:8AA4, 4 ms
Reply to request 0 received from FE80::F225:72FF:FE3B:D6D2, 4 ms
Reply to request 0 received from FE80::A60C:C3FF:FEE3:E4E6, 4 ms
Reply to request 0 received from FE80::128C:CFFF:FE2E:916E, 4 ms
Success rate is 100 percent (1/1), round-trip min/avg/max = 4/4/4 ms
5 multicast replies and 0 errors.

Pinging FF02::1

Pinging nodes on vlan139

Campus1#<input>ping FF02::1 source vlan 139 repeat 1</input>
Output Interface: <input>vlan 139</input>
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to FF02::1, timeout is 2 seconds:
Packet sent with a source address of FE80::218:18FF:FE7C:B441

Reply to request 0 received from FE80::21B:21FF:FE52:917D, 0 ms
Reply to request 0 received from FE80::214:5EFF:FE67:614A, 0 ms
Reply to request 0 received from FE80::216:76FF:FE9F:FEF6, 0 ms
Reply to request 0 received from FE80::250:56FF:FE8B:A, 0 ms
Reply to request 0 received from FE80::4A5B:39FF:FE5A:A7F7, 0 ms
Reply to request 0 received from FE80::20C:29FF:FE4C:C0C, 0 ms
Reply to request 0 received from FE80::20C:29FF:FE36:45B1, 0 ms
Reply to request 0 received from FE80::9221:55FF:FEBC:BD97, 8 ms
Reply to request 0 received from FE80::20C:29FF:FE5A:65ED, 42 ms
Reply to request 0 received from FE80::200:74FF:FE8F:85F4, 780 ms
Success rate is 100 percent (1/1), round-trip min/avg/max = 0/83/780 ms
10 multicast replies and 0 errors.

RFC's

rfc4291 "IP Version 6 Addressing Architecture" defines new flags in rfc3306 "Unicast-Prefix-based IPv6 Multicast Addresses" and rfc3956 "Embedding the Rendezvous Point (RP) Address in an IPv6 Multicast Address"

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