What is any-cost address ?

[Yasir Iqbal]

Dear All,

Can someone explan the ipv6 anycost address in details with example becuase i am little confused with this term.

[Seb Rupik]

Do you mean anycast?

An anycast address has the same format as a unicast address, however on a network different hosts can have the same address on their interfaces, these interfaces MUST be configured as anycast.

As unicast can be thought of as one-to-one, multicast as one-to-many, anycast is ono-to-one-of-many. What this means is a packet sent to an anycast address will be routed to the closest (as in routing cost) host.

cheers,

Seb.

[Yasir Iqbal]

Dear Rupik,

Thanks for the reply, please can u further elaborate the sentence "on a network different hosts can have the same address on their interfaces".

[Seb Rupik]

Differnet hosts on the same subet can have the same IPv6 unicast address assigned to their interfaces; typically a loopback interface.

[James Leinweber]

anycast is a router concept; of the available pool of unicast destinations for a particular address, some upstream router will eventually pick the "nearest" one by some criterion, and the unicast packet ends up at that specific host.  Different clients from different originating LAN's could end up at different destinations if they take different paths through the routers.

In general, anycast is used with hosts which are on different LAN's, no two on-link with each other.  A typical use is to increase redundancy for DNS servers.  The soon to be deprecated IPv6 6to4 relays also share anycast address 192.88.99.1/24 on the v4 side.  Anycast is not a v6-specific concept; it can be done with both v4 and v6.

Having the same IPv6 unicast address assigned to multiple interfaces would tend to result in duplicate address detection failures, either on-host or on-link.

-- Jim Leinweber, WI State Lab of Hygiene

[Seb Rupik]

Hi Jim,

When using the 'anycast' keyword on an IPv6 address declaration:

ipv6 address ipv6-address/prefix-length anycast

...DAD checks on that interface are not carried out, allowing for duplicate addresses to exist on the same subnet.

cheers,

Seb.

[Peter Koltl]

Any-cost address means that some enterprises will soon be willing to pay any high price for a new IPv4 block.

[Harold Ritter]

Good one Peter

[fred]

Take a look at Section 2.6 of RFC 4291.

An anycast address is best thought of as a single address that will get you to the nearest server offering a specified service. For example, one of the DNS root servers, F Root, is in fact about over 50 servers in locations worldwide (http://www.isc.org/f-root/). However, for the purposes of DNS access (not for maintanence; they each have an individual address for purposes that require them to be individual machines), they each use the same two addresses: F-root answers queries over IPv4 on 192.5.5.241, and over IPv6 on 2001:500:2f::. When you send a DNS request to one of those two addresses, it goes to whichever of the two happens to be online at the time and, from a routing perspective, nearest to you.

[Yasir Iqbal]

Dear All,

Thanks for your help. It make sense.