2 Quick BGP Questions

Answered Question
Jun 10th, 2008
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1.) What is the difference between summarizing routes using the aggregate-address command and simply advertising a supernet using the network command?


Example:


bgp 64513

network 10.32.80.0 mask 255.255.254.0


--OR --


aggregate-address 10.32.80.0 255.255.254.0



2.) If I redistribute static routes into BGP according to a route map, is it true that, barring any other filters, all BGP neighbors will receive the static routes?


If the answer is yes, why would anyone have the following config?


router bgp 65xxx

redistribute static route-map ADV.REMOTE.SUMMARY


neighbor 10.x.x.x route-map ADV.REMOTE.SUMMARY out


route-map ADV.REMOTE.SUMMARY permit 10

match ip address REMOTE.SUMMARY.NETS


ip access-list extended REMOTE.SUMMARY.NETS

permit ip host 172.22.0.0 host 255.255.240.0

permit ip host 172.27.0.0 host 255.255.240.0


Thanks in advance...


Victor

Correct Answer by royalblues about 8 years 10 months ago

If the requirements are met, then i dont see a difference except that the prefix could be adverised with as-set

But most of the time you would have a set of subnets downstream whereas you want only the summary to be advertised. In this case to use the network statement, you would actually require some sort of static route (say to null0) for the supernet to get advertised.


2. you are right. With the configurations listed, the route-map seems redundant and may have been added just as a precautionary measure so that no one accidentally advertises another prefix via network statement or other means.


Narayan

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royalblues Tue, 06/10/2008 - 06:52
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Victor,


1. the network command would advertise only the /23 network if it finds a valid route in its routing table whereas the aggregate command would send the supernet along with more specific prefixes (unless u use summary-only option) if at least one of the

more-specific addresses belonging to the aggregate is their in the BGP table


2. Redistributed static routes into BGP would be advertised to all neighbors as per the filters


In your example, the neighbor would only receive the redistributed static routes and not any other prefix which may have been through the use of network statements. He is actually redistributing as well as controlling what needs to be sent



Narayan

lamav Tue, 06/10/2008 - 07:02
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Hi, Narayan:


I should have asked question number 1 with the caveat that the requirements are met for using the network command. Assuming they are, there really is no qualitative difference between using the network command and using the aggregate-address command (with the summary-only keyword afterward). Correct? I guess I am saying that you dont absolutely have to use the aggregate address command to advertise a summarized route. Correct?


2.) I see what you're saying, but it seems that what they are doing is pointless/extraneous/not necessary because the redistribute static statement calls the route-map named ADV.REMOTE.SUMMARY, which is the exact same route map that is called in the neighbor statement. So, even without the route-map being added to the neighbor statement, that neighbor would have received the exact same static routes. Correct?


Thanks


Victor

Correct Answer
royalblues Tue, 06/10/2008 - 07:11
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If the requirements are met, then i dont see a difference except that the prefix could be adverised with as-set

But most of the time you would have a set of subnets downstream whereas you want only the summary to be advertised. In this case to use the network statement, you would actually require some sort of static route (say to null0) for the supernet to get advertised.


2. you are right. With the configurations listed, the route-map seems redundant and may have been added just as a precautionary measure so that no one accidentally advertises another prefix via network statement or other means.


Narayan

lamav Tue, 06/10/2008 - 07:24
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Thanks, Narayan.


I just wanted to make sure I wasn't missing something...


Victor

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