Re: prefix advertisement in bpg

David's right, not necessarily IGP, but the prefix has to be in your
routing table before BGP can advertise it to its neighbors. Simply put u
are advertising a /16 to ur neighbours but that /16 may not be on in ur
routing table. So u use the null0 static route, sometimes also called a
"pull route." This has nothing to do with NAT.

On Mon, Apr 15, 2013 at 12:42 PM, David Rothera <david.rothera_at_gmail.com>wrote:

> No, it will be using a null route so that it can advertise a summary route
> such as the /16 in your example.
>
> The null route is used so that when a packet arrives it checks to see if
> there is a route matching the destination that is more specific than the
> /16 null route, if there isn't then the null route is used and the traffic
> is dropped.
>
>
> Regards
>
> David Rothera
> CCIE #38338
> Sent from my iPhone
> Please excuse any mistakes and brevity.
>
> On 15 Apr 2013, at 08:16, Imran Ali <immrccie_at_gmail.com> wrote:
>
> so far i did not see any logical reason to point it to null 0 ,
>
> in case servers are using private ip, and they do nat to translate
> public block to private block . but since this public subnet is non
> existant we need a route for nat to work,
>
> is my mind correct ?
>
>
> On Mon, Apr 15, 2013 at 9:40 AM, David Rothera <david.rothera_at_gmail.com>wrote:
>
>> It doesn't have to be in an IGP (unless you are using IGP sync) but
>> simply the routing table itself, either by a static route or from an IGP.
>>
>> Regards
>>
>> David Rothera
>> CCIE #38338
>> Sent from my iPhone
>> Please excuse any mistakes and brevity.
>>
>> On 15 Apr 2013, at 06:14, Ahmed Hussain <engine10_at_gmail.com> wrote:
>>
>> > Before BGP can advertise the prefix it has to be in IGP. thats what the
>> > null route is doing.
>> >
>> >
>> > On Fri, Apr 12, 2013 at 10:01 PM, Imran Ali <immrccie_at_gmail.com> wrote:
>> >
>> >> why some bgp implementations adds a null route for advertised
>> prefixes
>> >>
>> >> ex
>> >>
>> >> router bgp 2
>> >> bgp log-neighbor-changes
>> >> network 128.16.16.0 mask 255.255.255.0
>> >> network 130.130.0.0
>> >> neighbor 10.10.10.1 remote-as 1 * neighbor 10.10.10.1 advertise-map
>> >> ADVERTISE non-exist-map NON-EXIST***neighbor 10.10.20.3 remote-as 3
>> >> !
>> >> ip route 130.130.0.0 255.255.0.0 *Null0*
>> >>
>> >>
>> >>
>> >> is has something to do with NAT ?
>> >>
>> >>
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Received on Mon Apr 15 2013 - 14:20:06 ART