[nsp] bgp - aggregates and specific routes

[Timothy.Hall at alltel.com]

You'll have to advertise both the /19 and the /24 (vs suppressing the more specific route). Routing decisions are first and foremost made by longest prefix match. If you are not advertising the /24, the default path will always be through the customer's other ISP. If the customer just wants to use the "other" ISP as a backup to you, then they just need to prepend their AS on the backup ebgp link.

Then - you will be the best path for traffic to your customer. When the link between you and them goes down, the backup path will be used since you will no longer be advertising the /24, just the /19.

hth

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net]On Behalf Of Roger
Sent: Wednesday, July 14, 2004 1:35 PM
To: joshua sahala; cisco-nsp at puck.nether.net
Subject: Re: [nsp] bgp - aggregates and specific routes


joshua sahala wrote:

>>A customer of ours, who's range is say 192.168.16.0/24 will be using our 
>>numbers and advertising said route to other eBGP peers.
>>    
>>
>
> your numbers?
>
>  
>
Yes - the 192.168.16.0/24 falls within the 192.168.0.0/19 range.  This 
was mentioned in the previous post.  The /24 is downstream from us and 
they're getting their own ASN. 

In real life we have a /19 delgated to us by ARIN.

> how are you learning the /24?  if you are learning it from your
> customer, then when the link goes down, you will stop learning that
> prefix and will subsequently stop advertising it.  if you are
> learning it via some other means, then i'd need to know how that is
> to answer this.
>  
>
We are learning the /24 via eBGP.  Now if the eBGP session between us 
and said downstream customer(16.0/24) goes down connectivity to the 
16.0/24 will still occur because it will be lumped in w/ our 
aggregate...  We need to prevent this when the eBGP session goes down. 


> /24 is more specific than /19, so for addresses in that /24, traffic
> will go towards your customers other providers.  all other traffic
> for the /19 will come to you
>
>  
>
I disagree.  While yes the /24 is more specific we are only advertising 
our aggregate /19 to upstream providers.  If the eBGP link between us 
and our downstream customer, using our numbers, goes down  connectivity 
will appear to normal because the /19 is still shown as up.

I'm looking for a route-map statement or some conditional advertisement 
statement.

>>router bgp 1
>>no synchronization
>>bgp log-neighbor-changes
>>network 192.168.0.0 mask 255.255.224.0
>>neighbor 1.2.3.4 remote-as 1234
>>neighbor 1.2.3.4 description WAN Link 1
>>neighbor 1.2.3.4 send-community
>>neighbor 5.6.7.8 remote-as 5678
>>neighbor 5.6.7.8 description WAN Link 2
>>neighbor 5.6.7.8 send-community
>>neighbor 192.168.16.254 remote-as 2
>>neighbor 192.168.16.254 description downstream customer
>>neighbor 192.168.16.254 send-community
>>!
>>ip route 192.168.0.0 255.255.224.0 Null0
>>    
>>
>
>this looks good - make sure that you are using some prefix filters
>and/or as path filters to prevent readvertising prefixes that you do
>not want to provide transit for ;-)
>  
>

No - the problem is we WANT to be a transit AS for our downstream 
customer 16.0/24 only if a eBGP link between us and them is in working 
order.  I'll continue to look.

I'll look over my original post - I think you missed the point or I 
didn't explain things clearly..

-- 
Rock River Internet                          Roger Grunkemeyer
202 W. State St, 8th Floor                grunky at rockriver.net
Rockford, IL 61101                           815-968-9888 x102

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