[j-nsp] load balancing in Route reflector scenario

[Robert Raszuk]

Hi Harry,

 > default, differences in route preference cause a JUNI to prefer an
 > IGP route while ios prefer the bgp routs over IGP.

Let's make a clear distinction between preferring eBGP route versus iBGP 
route. Talking CSCO here eBGP admin distance is as you say 20 while iBGP 
as even the URL provided by yourself says it is 200.

So keeping in mind that usually hot potato routing is a desired 
behaviour preferring EBGP learned path is highly recommended for a given 
prefix.

If you say that JUNI is to prefer IGP route over BGP one I am sure you 
must be referring to IBGP and not EBGP, but this is exactly the same in 
both vendors.

 > W/o this knob replacing a cisco with a juniper can result in
 > previously advertised bgp routes no longer being advertised.

I can rest assure you that this was not the main intention of this knob :)

Cheers,
R.




> I always thought that advertise-inactive was to make a juniper act
> like a cisco with regard to BGP route announcements, when, by
> default, differences in route preference cause a JUNI to prefer an
> IGP route while ios prefer the bgp routs over IGP.
>
> In junos, only the active route is readvertised/subject to export
> policy. With advertise-inactive you can make a juniper router, whose
> active route is an IGP route, advertise into BGP the "best bgp path",
> which here is inactive due to the igp route being preferred.
>
> W/o this knob replacing a cisco with a juniper can result in
> previously advertised bgp routes no longer being advertised.
>
> From:
> http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml
>
>  eBGP  20
>
> . . .
>
> OSPF  110
>
>
> From:
> http://www.juniper.net/techpubs/software/junos/junos64/swconfig64-routing/html/protocols-overview4.html
>
>  OSPF internal route  10
>
> IS-IS Level 1 internal route 15  . . .
>
> BGP 170
>
> HTHS.
>
>
>
>
> -----Original Message----- From: juniper-nsp-bounces at puck.nether.net
> [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Keegan
> Holley Sent: Wednesday, August 10, 2011 2:48 PM To:
> robert at raszuk.net Cc: juniper-nsp at puck.nether.net Subject: Re:
> [j-nsp] load balancing in Route reflector scenario
>
> 2011/8/10 Robert Raszuk<robert at raszuk.net>
>
>> Hi Keegan,
>>
>>
>> I think the advertise inactive knob turns that off, but I don't
>> know for
>>> sure because I've never tried it.  I know it's not supported on
>>> cisco routers.  The reason for it is the size of the BGP table.
>>> So if the table is 400k routes and you have 5 different ISP's and
>>> you advertise every route that would be 2M routes in the table.
>>> Since BGP doesn't allow multiple version of the same route in the
>>> routing table (separate from the BGP table where incoming routes
>>> are stored) you would still only use the original 400K the other
>>> 1.8M routes would just go unused unless you manipulated them some
>>> how.
>>>
>>
>> Advertise inactive is not about what get's advertised - it is about
>> if the best path is advertised or not. And if is decided based on
>> the check if the BGP path to be advertised is inserted in the
>> RIB/FIB or not.
>>
>
> Oh I see.  I have never used that command so thanks.  Most of the
> above example was what would happen if BGP advertised everything it
> learned instead of just the best path or the path in the routing
> table btw.
>
>>
>> By default Junos and IOS-XR advertise only those best path in BGP
>> which actually are installed into forwarding. Advertising inactive
>> knob will overwrite it.
>>
>
> Wouldn't this lead to traffic being blackholed?  If all the routes
> for a given destination are inactive would this still cause BGP to
> advertise a route for them?
>
>>
>> IOS classic/XE (for historical reasons) advertises all best paths
>> from BGP table and to enforce it not to advertise what has not
>> been installed into RIB/FIB there is knob called "suppress
>> inactive".
>>
>> Cheers, R.
>>
>>
>>
>>
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