[nsp] bgp - aggregates and specific routes

[Roger]

joshua sahala wrote:

>> 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.   
>
>
> prevent what?  you will no longer learn the /24, so you will no
> longer advertise the /24, only the /19.
>  
>
......?

> you can disagree if you like, but routing works on the most specific
> prefix - so /24 is more specific than /19, which is more specific
> than /16, ad nauseum
>  
>
Yes - the more specific the route the better.  That part I know.

> <> you don't need anything conditional, so long as they are advertising
> the /24 to another provider, the traffic for the addresses in that
> block WILL go to them. why the desire to complicate things?


Ok - now I see your point....

> <>
>
> if they are advertising the /24 to you, and you are in turn
> advertising that /24 to your providers, you will.  if they stop
> advertising the /24, you will stop advertising the /24
>  
>
Ok - your saying advirtise 2 networks - the /19 and the /24.  Thats it.

If the /24 link goes down, the aggregate will still be sent out BUT 
other BGP peers - say in Timbuktu - will use the more specific /24 if 
said downstream customer has other eBGP peers up...
Ok, basically your post is what Timothy.Hall said.  However Tim's post 
was a lot easier to follow...

I'll offer up the following setup for conditional advertisements..  
Refer to my original post to get the BGP setup.  Yes this is a bit 
overkill..  And would require getting permission from our upsteam 
provders to advertise lots of smaller networks...

Basically I feel I have 2 options from this excercise..  A simple 
elegant setup and this .... thing...  While the below would work from a 
design point its just ugly.

(config)#router bgp 1
(config-router)#neighbor 1.2.3.4 remote-as 1234
(config-router)#neighbor 1.2.3.4 advertise-map MAP1 non-exist-map MAP2
(config-router)#neighbor 5.6.7.8 remote-as 5678
(config-router)#neighbor 5.6.7.8 advertise-map MAP1 non-exist-map MAP2
(config)#route-map MAP1 permit 10
(config-route-map)#match ip address 1
(config)#route-map MAP2 permit 10
(config-route-map)#match ip address 2
(config)#access-list 1 permit 192.168.0.0 0.0.15.255
(config)#access-list 1 permit 192.168.17.0 0.0.0.255
(config)#access-list 1 permit 192.168.18.0 0.0.1.255
(config)#access-list 1 permit 192.168.20.0 0.0.3.255
(config)#access-list 1 permit 192.168.24.0 0.0.7.255
(config)#access-list 2 permit 192.168.16.0 0.0.0.255


Anyhow Josh - you were right - just took a round about way to understand 
what you were getting at.  Couldn't see the forest for the trees...

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


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