Overview

IPv6 route summarization allows for a group of addresses to be summarized into one summary address. This simplifies administration, reduces the size of the routing table(s) and reduces demands on network resources (i.e. RAM, CPU, Bandwidth).

It’s important to be accurate with summarization:

  • Under-summarizing can cause us to leave out desired networks.
  • Over-summarizing can cause us to include undesired networks.

How it Works

Let’s use the following topology:

IPv6 Route Summarization - 62 Bit Example
IPv6 Route Summarization - 62 Bit Example

If we wanted to include the three network addresses on the other side of R-2 in R-1’s routing table we can combine them into one summary address using these four steps:

  1. From left to right, discover the first bit where the values differ.
  2. Convert all values in the differing hextet from hexadecimal to binary.
  3. From left to right, count all of the bits that match to discover the prefix value. To speed things up remember:
          A hextet is 16 bits
          A nibble is 4 bits
  4. Add all zeros after matching bits.

Example 1

Step 1

Using the above topology, from left to right discover the first bit where the values differ:

2001:DB8:CAFE:1::/64
2001:DB8:CAFE:2::/64
2001:DB8:CAFE:3::/64

Step 2

Convert all values in the differing hextet from hexadecimal to binary:

2001:DB8:CAFE:0000 0000 0000 0001::/64
2001:DB8:CAFE:0000 0000 0000 0010::/64
2001:DB8:CAFE:0000 0000 0000 0011::/64

Step 3

From left to right, count all of the bits that match in all addresses to discover the prefix value:

2001:DB8:CAFE:0000 0000 0000 0001::/64
2001:DB8:CAFE:0000 0000 0000 0010::/64
2001:DB8:CAFE:0000 0000 0000 0011::/64
  16     16     16       4        4        4     2        /62

Step 4

  1. All bits after the matching bits are given a value of zero.
  2. Convert binary to hexadecimal.

2001:DB8:CAFE::/62

Example 2

Let's add 3 more networks:

IPv6 Route Summarization - 61 Bit Example
IPv6 Route Summarization - 61 Bit Example

Step 1

From left to right discover the first bit where the values differ:

2001:DB8:CAFE:1::/64
2001:DB8:CAFE:2::/64
2001:DB8:CAFE:3::/64
2001:DB8:CAFE:4::/64
2001:DB8:CAFE:5::/64
2001:DB8:CAFE:6::/64

Step 2

Convert all values in the differing hextet from hexadecimal to binary:

2001:DB8:CAFE:0000 0000 0000 0001::/64
2001:DB8:CAFE:0000 0000 0000 0010::/64
2001:DB8:CAFE:0000 0000 0000 0011::/64
2001:DB8:CAFE:0000 0000 0000 0100::/64
2001:DB8:CAFE:0000 0000 0000 0101::/64
2001:DB8:CAFE:0000 0000 0000 0110::/64

Step 3

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