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Aggregate Routes - when/why?

‎12-06-2016 05:03 PM

Okay - 

 

I'm not getting my itch scratched as far as answers go regarding 'when' aggregate routes are needed.

 

  • I understand that these are routes that are manually created, just like a static route.
  • I understand that these are routes which are not used for forwarding traffic, but simply for advertising (i.e. BGP)
  • I understand that the only action an aggregate route can take is to 'discard' or 'reject'.
  • I understand that there must be a contributing route for the aggregate route to become active.
  • I understand that these are routes which summarize a bunch of smaller routes, thus reducing the route table.

Let's say I have the four networks below

 

1.1.1.0/24 reth0.0

1.1.1.1/24 reth1.0

1.1.1.2/24 reth2.0

1.1.1.3/24 reth3.0

 

Couldn't I simply use a prefix-list to advertise 1.1.1.0/22 or MUST an aggregate route be used?  If so, WHY?

 

Or is this not a good use case for needing an aggregate route?

 

 

Thanks!

4 REPLIES 4
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Re: Aggregate Routes - when/why?

‎12-06-2016 05:39 PM

Your understanding is correct. To answer your questions:

No - using prefix list tells the peer you can deliver packets to ALL of the networks you advertise 1.1.1/22 which I suppose would be incorrect, so you could potentially black hole traffic if you cannot deliver, Aggregate route on the other hand allows you to advertise the whole subnet as indicated but deliver only to the ones you can deliver and send a reject/discard those you cannot deliver. Now in addition to reducing the size of the peer you are advertising the agg route to, you also hide instabilitiexs internally and reduce the likelyhood of flapping when a route goes down. 

Another use case that aggregate routes are typically used, on Border routers for advertising a default route in the reverse direction, i.e.  into the IGP domain. So that if the Border router does not have internet connectivity, based on not receiving a contributing route from the peer, then the default route will not become active and is not advertised so the internal routers will find a different route to remote destinations.

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Re: Aggregate Routes - when/why?

[ Edited ]
‎12-06-2016 06:04 PM

Thanks lyndidon -

 

Question - when you say "No -using prefix list tells the peer you can deliver packets to ALL of the networks you advertise 1.1.1/22 which I suppose would be incorrect, so you could potentially black hole traffic if you cannot deliver..."

 

Are you saying "no you 'can't' do that", or "Yes, you 'could' do that, but it's not recommended because you can potentially black-hole packets", and that the advantage the aggregate-route gives is the capability to send a 'rejects' back to the sender letting them know you cannot process the packet.

 

Correct?

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Re: Aggregate Routes - when/why?

‎12-06-2016 06:20 PM

 "Yes, you 'could' do that, but it's not recommended because you can potentially black-hole packets", and that the advantage the aggregate-route gives is the capability to send a 'rejects' back to the sender letting them know you cannot process the packet.

 

Correct!

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If this solution worked for you please flag my post as an "Accepted Solution" so others can benefit..]
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Re: Aggregate Routes - when/why?

‎12-06-2016 11:47 PM

Hi,

 

The prefix-list is used in the policy part of the configuration to indicate which routes to announce.

While a prefix-list can be defined without an active route, BGP still requires an active route to be present in the routing table for advertisement.

 

An aggregate route is useful when small prefix routes are available in the routing-table but not the larger prefix you would wish to advertise in the prefix-list. The aggregate route allows creation of the route in the routing-table which enables BGP to then announce the route.

 

You could also use generated routes which does have a next-hop and not discard action.

One of the use-cases is it allows implemenation of some best practices such as BCOP for internet transit, i.e announcing global aggregate prefixes to Internet transit.

 

Cheers,

Ashvin

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