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i_noc
Posts: 20
Registered: ‎07-08-2009
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BGP - Originating Networks

  Let's say I have a BGP connection to an ISP, and I want to advertise or "originate" a network to my ISP.   Let's say the network is 5.5.5.0/24.  In cisco I would use a command like "network 5.5.5.0 mask 255.255.255.0" to ensure I advertise that route to my BGP neighbors.

 

  In Juniper, I read that the default "export" policy for BGP is "Accept and export active BGP routes".  My router doesn't have any "active BGP routes" (lets say it just gets a default route from the ISP via BGP).  So by default my router wouldn't advertise any routes to the ISP via BGP correct?

 

  Let's say I have a prefix list that just contained 5.5.5.0/24.  If I have a policy statement that states "if from my prefix list, then accept", that would then initiate advertisement of the 5.5.5.0/24 network correct?

Super Contributor
davidjdv
Posts: 113
Registered: ‎02-26-2008

Re: BGP - Originating Networks

Hello,

 

In order to advertise a route in BGP that has not been learned via BGP, two conditions must be reunited:

 1/ The route needs to be active in the routing table

 2/ You need to match and accept this route in a policy that is applied as "export" in BGP

 

A prefix-list is just that: a list of prefixes... it is just a handy way of referring to one or more ranges of addresses (say your core backbone...) but this does not imply these routes exist in the routing table. So a prefix-list addresses 2/ but not 1/.

 

Typically, you define an aggregate-route (5.5.5/24) to make it active in the routing-table (with a next-hop of discard or reject) and then match this in the export policy.

 

Regards,

/david 

Contributor
SvenH
Posts: 34
Registered: ‎02-19-2009

Re: BGP - Originating Networks

Much like David said, the route needs to be active in the routing table and then you just need to write an export policy for it.

 

In my case, what I did was I put in a static route on my border router that routed the IP addresses I wanted to originate to the inside of our network:

 

set routing-options static route 5.5.5.0/24 next-hop ....

 

Afterwards I went ahead and created the prefix list and the export policy (I used exact here because I did not want anything else leaking out on accident.  You can use different switches that would also export shorter or longer routes)

 

policy-options {
    prefix-list your-ips {
        5.5.5.0/24;
    }
    policy-statement your-ips-to-advertise {
        term fgcu-ips {
            from {
                prefix-list-filter your-ips exact;
            }
            then accept;
        }
        term reject-all-else {
            then reject;
        }
    }

}

 

Then the export policy goes into your protocols statement:

 

protocols {
    bgp {
        group your-bgp-peers {
            export your-ips-to-advertise;
            neighbor your-isps-ip-here {
                peer-as XXXX;
            }
        }
    }
}

 

Hope that helps!

 

SH

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