[j-nsp] BGP strange Next hop behavior (in JNCIP)

[Ivan Ivanov]

Hi,

You probably have the lo0.0 of R5 in level-2. So the IBGP session is using
the attached default route on R6 from R5 to reach the IBGP peer. When R6
gets the aggregate it contains the lo0.0 also. And the router is falling in
the situation where it has to use this agg for keeping the IBGP session
UP instead of the default, because the agg is more specific. In other words
to resolve the NH via the route itself, recursive. So JUNOS behavior is to
put the route as hidden and keep the IBGP session via the default.

To resolve this you can advertise the agg from R5 with the NH IP address of
the interface towards the R6.

The explanation is on page 537 in the Study guide. Several pages before is
described the problem.

HTH,


On Wed, Mar 16, 2011 at 08:09, medrees <medrees at isu.net.sa> wrote:

> Hi Expertise
>
>      I’m wondering from strange behavior for two IBGP session included in
> JNCIP_StudyGuide, I have one router make aggregation for the whole network
> 10/8(R5 in case study of EBGP) and where the routers inside the other ISIS
> are level-1 routers (R1 & R6) so they reach the Level1-2 router R5 Lo0
>  through the default route.
>
> We have two scenarios:
>
> 1-   when this route inserted in their BGP table the R1 which receive this
> aggregated subnet (with Protocol NH of the router which created the
> aggregated route R5) through another Level1-2 router R3 will simply choose
> this subnet more preferred than the default route and the session of the
> BGP
> between R1 & R3 will flap and it is normal behavior .
> 2- The abnormal behavior appear in the other Level-1 area which has direct
> physical connection with the aggregated router R5 the route won’t be
> inserted in the BGP active table but it will be inserted as Hidden route
> which cause the Level-1 router R6 keep the IBGP session up.
>
> The two issues solved by use static route for the next hop of this
> aggregated route toward the neighbor while sent this update to be preferred
> more than even the aggregated BGP route or the Default ISIS route.
>
> But the question is why the remote router choose this route as active one
> while the direct connected router (Route-reflector client ) flagged this
> route as hidden route??
>
>
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-- 
Best Regards!

Ivan Ivanov