Anycast Addresses
Anycast routing is a mechanism for addressing multiple interfaces, usually on different nodes, with the same IP address. Traffic destined to the address gets routed to the nearest node. The anycast functionality is discussed later, in the section "The Anycast Process." Tin anycast address has the same format as a unicast address. No special FP defines an anycasi address.
Anycast addresses are assigned from the unicast address space. In fact, the addresses are taken from the Interface ID field. The Subnet-Router anycast address is predefined, and all router interfaces connected to a link must be assigned this address. It is a unicast address with an all-zero interface identifier. Figure 8-7 illustrates the Subnet-Router anycast address.
Figure 8-7 Subnet-Router Anycast Address n bits
Subnet Prefix
128-n bits
0000000000000000
An example of a Subnet-Router anycast address is FEC0:0:0:A:: on a router interface assigned with the unicast address FEC0:0:0:A:200:CFF:FE0A:2C51.
The highest 128 interface IDs are reserved for assigned subnet anycast addresses. A reserved subnet anycast address is an anycast address that is available on every IPv6 subnet, regardless of the type or format of the prefix. Figure 8-8 illustrates the construction of reserved anycast addresses.
Figure 8-8 Anycast Address Construction
EUI-64 Interface ID
|
64 bits |
57 bits |
7 bits |
|
Subnet Prefix |
1111110111...111 |
Anycast ID |
|
i |
\<-Interface identifier field-►! |
|
|
n bits |
121-n bits |
7 bits |
|
Subnet Prefix |
1111111111...111 |
Anycast ID |
|
i |
\<- Interface identifier field-► ! |
|
Figure 8-8 shows that the reserved anycast address space is taken completely from the Interface ID field. A reserved anycast address is reserved for every subnet. The top 128 interface IDs are reserved for anycast address allocation. The last 7 bits of the address identify the specific anycast address.
At the time of this writing, the only specified anycast address is the mobile IPv6 home-agents. It has an anycast identifier of binary 1111110. That is, on every IPv6 subnet, the address associated with the EUI-64 interface ID FDFF:FFFF:FFFF:FFFE is reserved for the mobile IPv6 home-agent anycast address. The mobile IPv6 home-agent on prefix FEC0:0:0:A::/64 uses the anycast address FEC0:0:0:A:FDFF:FFFF:FFFF:FFFE.
All the remaining anycast IDs, hexadecimal values 0-7D and 7F, are reserved for future use.
Because anycast addresses are syntactically indistinguishable from unicast addresses, an interface must be explicitly configured to recognize that its address is an anycast address.
Continue reading here: Multicast Addresses
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