[j-nsp] load balancing in Route reflector scenario

[Harry Reynolds]

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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