Summarization addressing

[jayantpatankar]

Hi,

Can you please clarify how excatly to configure a summary mask/address?

Say 4 consecutive and contigous networks are given like

192.168.3.16/28

192.168.3.32/28

192.168.3.48/28

192.168.3.64/28

And they are to be summarized, then as the mask should be /28-2 = /26.

so the summary address should be;

192.168.3.16/26

Am I right? Please confirm.

Thanks in advance,

[gpulos]

your subnets are as follows:

.16 = 00010000

.32 = 00100000

.48 = 00110000

.64 = 01000000

SNM = 11100000

with the above subnets you cannot summarize on the last/fourth octet as shown above.

you could summarize on the third octet:

192.168.3.0/24

but this will provide for a problem since this covers all subnets under 192.168.3

this may not be what you want and if your network has other subnets under this range, it will create a routing problem; ie: 192.168.3.128/28 would be included in that summary and if this subnet existed elsewhere in the network, you'll have a routing problem to/from this subnet.

please see these links for more summary configuration info:

eigrp -

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#summarization

ospf -

http://www.cisco.com/warp/public/104/3.html#12.0

[Richard Burts]

Jayant

Greg is correct that trying to summarize the addresses that you gave is problematic. The issue is that to summarize them into a single block you will include more addresses than you specify in your example. It is important not only that the addresses be contiguous, and that they are a number of addresses that can be summarized (4 /28s can make 1 /26), but that the addresses begin on a binary boundary. Part of the issue is that if you configure 192.168.3.16/26 what you really have is 192.168.3.0/26 and the addresses included in the block 192.168.3.16/26 are 192.168.3.0 through 192.168.3.63.

Greg suggests that you could summarize as 192.168.3.0/24 which is true but includes more addresses. If you want a single more efficient block you could summarize 192.168.3.0/25. But that includes quite a few addresses that are not in your original specification.

To do effective summarization we must consider how many addresses we are trying to summarize (summarizing 64 addresses is good), where the address block begins, and where the address block ends.

HTH

Rick