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subnet router anycast address clarification

binaryhat

I'm unclear on subnet router anycast address in IPv6. Is it the same thing as the subnet ID in IP4?

IPv6: 2001:0DB8:1111:0000:: from 2001:0DB8:1111::/48

IPv4: 192.168.1.0/24

Thanks

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Comments

Jon_Cisco

I don't really know what your asking here. I don't recall studying anything about subnet router anycast address. I'm just starting to review for the ICND2 so maybe I'll come across something on my second pass. Was there a specific question that is stumping you or are you just looking to expand your knowledge?

Jon

xnx

Anycast Addressing

An IPv6 anycast address is assigned to more than one interface, typically belonging to different nodes, with the property that a packet sent
to an anycast address is routed to the “nearest” interface having that
address, according to the routing protocol’s measure of distance.

Anycast addresses, when used as part of a route sequence, permits a
node to select which of several Internet service providers it wants to
carry its traffic. This capability is sometimes called “source selected
policies.”

Anycast addressing is implemented by configuring anycast
addresses to identify the set of routers belonging to internet service
providers (for example, one anycast address per Internet service
provider). The anycast addresses can be used as intermediate addresses
in an IPv6 routing header to cause a packet to be delivered by means of
a particular provider or sequence of providers.

Other possible uses of
anycast addresses are to identify the set of routers attached to a particular subnet, or the set of routers providing entry into a particular routing domain.

Jon_Cisco

Hopefully he comes back with some more info. It is not clear for me if he is trying to understand what an anycast address is or if it is something more specific he is asking about.

binaryhat

I am reading Odom's ICND1 book and he states:

The IPv6 subnet ID, more formally called the subnet router anycast address, is
reserved, and should not be used as an IPv6 address for any host.

So based on that I assume its like the IPv4 subnet ID or network subnet.

Jon_Cisco

I grabbed this off a Wiki. It seemed like a simple explanation.

[h=3]Reserved anycast addresses[/h] The lowest address within each subnet prefix (the interface identifier set to all zeroes) is reserved as the "subnet-router" anycast address. Applications may use this address when talking to any one of the available routers, as packets sent to this address are delivered to just one router.

I have not read up on this in a while so I don't recall how to determine which router gets the packet. I plan to start reading Odem's book next week so I don't have much more info for you right now.

Good Luck!

ruei0

I had this same problem when studying for my ICND1.

I am reading Odom's ICND1 book (100-101) 10th print; at the end of Chapter 26 it is suggested that you know the definition of "subnet router anycast address" even though it is left a little vague.

On page 722, Odom is explaining how an ISP may assign an engineer a 48bit prefix, and the engineer has 16 bits to play with for subnetting. This allows 65536 subnets, and he lists the "first 22 Possible subnets with a 16 bit subnet field" in Figure 26-11. (he also has a check mark next to four subnets that will be used in the next figure; i used an * instead)

2001:0DB8:1111:0000::
* 2001:0DB8:1111:0001::
* 2001:0DB8:1111:0002::
* 2001:0DB8:1111:0004::
* 2001:0DB8:1111:0005::
2001:0DB8:1111:0006::
.
.
.
2001:0DB8:1111:0015::

Shortly after this list he makes a highlighted note on page 723, making it seem important
"NOTE The IPv6 subnet ID, more formally called the subnet router anycast address, is reserved, and should not be used as an IPv6 address for any host."

The following figure, 26-12 needs to use four subnets... and it uses the 2nd, 3rd, 4th, and 5th from the above list~ it skips the 1st subnet (2001:0DB8:1111:0000) for some reason. Since that note was right in between these two figures, I thought maybe it is referring to the first subnet. Earlier in the book, it mentioned that historically, Subnet Zero was avoided (not now) due to confusion as to a network ID and subnet ID being the same address.

Although it may or may not be on the exam, the study guide suggests that this should be known; but it wasn't clearly explained in the book.