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New Member

OSPF area route summary

Hello

I have a question about OSPF area route summarization using "area range " command. what if ip address is discontinuous, then how can I achive this route summarization

for example

1.1.4.17/30

1.1.5.0/24

1.1.6.0/24

1.1.7.0/24

in this case, how can i acheive area route summary for this address range?

Thanks

8 REPLIES
Purple

Re: OSPF area route summary

Since your ranges are discontiguous, the best you can do is two use 3 'area range' statements as such:

area 1 range 1.1.4.16 255.255.255.252

area 1 range 1.1.5.0 255.255.255.0

area 1 range 1.1.6.0 255.255.254.0

The only range you can summarise is: 1.1.6.0/24 and 1.1.7.0/24, giving you 1.1.6.0/23

Hope that helps... pls rate the post if it does.

Regards,

Paresh.

New Member

Re: OSPF area route summary

Thanks for answer,,

I need to summarize as 1 route....

VIP Purple

Re: OSPF area route summary

Hello,

if you summarize as one route, you can configure it as following:

area 1 range 1.1.4.0 255.255.255.0

This, obviously, will result in other networks being advertised as well, but it is the only way to get all your routes into one single summary...

HTH,

GP

Gold

Re: OSPF area route summary

It might still be possible to summarize to 1.1.4.0/22, depending on other factors in the network. For instance, if the address space between 1.1.4.0 through 1.1.4.16, and the address space between 1.1.4.18 through 1.1.5.255, aren't used anyplace in the network, and you don't project using them anyplace, then you could aggregate to 1.1.4.0/22 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 1.1.4.4/30 exists, and it's not behind the aggregation point you're considering. Suppose you aggregate to 1.1.4.0/22 anyway, what will happen? Some router "in the middle" will get 1.1.4.0/22, and 1.1.4.4/30, 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.

HTH.

:-)

Russ.W

New Member

Re: OSPF area route summary

Hi

The very idea of the area range command is to group many advertisements into one and this can happen only if a whole chunk of ip subnets can be grouped which means they have to be contiguous.

Hope this answers your question.

Also, you can group 1.1.4.0/24 through 1.1.7.0/24.

To do this, use the command,

area x range 1.1.4.0 255.255.252.0 [cost ..]

HTH

cheers

Arav

Re: OSPF area route summary

Hello,

in principle there are many solutions to your question. The given

A) area x range 1.1.4.0 255.255.252.0 [cost ..]

would do it, but also the command

B) area x range 1.1.0.0 255.255.0.0 [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:

1.1.4.0/30

1.1.5.0/24

1.1.6.0/24

1.1.7.0/24

gives in bit notation:

00000001.00000001.00000100.00000000

00000001.00000001.00000101.00000000

00000001.00000001.00000110.00000000

00000001.00000001.00000111.00000000

So the common part is

00000001.00000001.00000100.00000000

and the mask is

11111111.11111111.11111100.00000000

and recalculating this into decimal gives:

1.1.4.0 255.255.252.0 - exactly what you have seen in the previous post.

Did this help? Rate all posts please.

Martin

P.S.: decimal to binary and vice versa can f.e. be done with windows calculator in scientific mode.

New Member

Re: OSPF area route summary

Hi guys, thanks for attention..

actually, this was one of my last CCIE lab questions.

i configured as "area x range 1.1.4.0 255.255.252.0" and ABR advertise 1.1.4.0/22 and 1.1.5.0/24 1.1.6.0/24, 1.1.7.0/24(without 1.1.4.17/30) 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...

or just using "distribut-list" is best?

Purple

Re: OSPF area route summary

Hi,

There is a reasonably new feature that you can use to do this - OSPF ABR Type 3 LSA filtering. The URL that describes it is:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a00800b5d4f.html

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.

Hope that helps.

Paresh.

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