It might still be possible to summarize to 220.127.116.11/22, depending on other factors in the network. For instance, if the address space between 18.104.22.168 through 22.214.171.124, and the address space between 126.96.36.199 through 188.8.131.52, aren't used anyplace in the network, and you don't project using them anyplace, then you could aggregate to 184.108.40.206/22 with no problems.
The trickier option is to aggregate anyway, and assume the longer prefix matches of the "missing" networks are going to route traffic correctly. For instance, suppose 220.127.116.11/30 exists, and it's not behind the aggregation point you're considering. Suppose you aggregate to 18.104.22.168/22 anyway, what will happen? Some router "in the middle" will get 22.214.171.124/22, and 126.96.36.199/30, which is two different routes of two different prefix lengths. This router would route all the traffic correctly, since the longer prefix wins.
It's worth taking a look to see if you can aggregate because the "holes" simply aren't used, or if all the routers impacted would actually receive the longer prefix advertisements for the "holes," and routing would work anyway.
in principle there are many solutions to your question. The given
A) area x range 188.8.131.52 255.255.252.0 [cost ..]
would do it, but also the command
B) area x range 184.108.40.206 255.255.0.0 [cost ..]
would create one summary route to be announced. So both would fulfil the requirement you gave. But in most cases the smaller summary would be preferable, because with it the ABR does not claim to represent a large IP address space, which in reality is not reachable behind the ABR.
Generally you can calculate the summary by looking at the common bits of all routes you like to summarize. Example:
gives in bit notation:
So the common part is
and the mask is
and recalculating this into decimal gives:
220.127.116.11 255.255.252.0 - exactly what you have seen in the previous post.
Did this help? Rate all posts please.
P.S.: decimal to binary and vice versa can f.e. be done with windows calculator in scientific mode.
actually, this was one of my last CCIE lab questions.
i configured as "area x range 18.104.22.168 255.255.252.0" and ABR advertise 22.214.171.124/22 and 126.96.36.199/24 188.8.131.52/24, 184.108.40.206/24(without 220.127.116.11/30) to the Backone. so i thought just not advertising all /24 routes on ABR then can acheive the goal. does is make sense?
and the reason i posted this issue is i just wondering is there any fancy way to acheive this, seem like no,,,
another question...so is there any fancy way to not advertise particular routes on ABR...
With this feature, you use the 'area filter-list prefix [out|in]' to specify which routes you want to advertise. All prefixes that match the prefix-list will be advertised. Any that don't will not be advertised. This is a really handy feature that provides you granularity in filtering prefixes between areas.
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