Re: prefix advertisement in bpg

Ahmed ,

Synchronisaiton is legacy rule and is off by default , so you dont need any
route in IGP to adveritse a bgp route.

On Mon, Apr 15, 2013 at 12:20 PM, Ahmed Hussain <engine10_at_gmail.com> wrote:

> 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 - 12:29:24 ART