[j-nsp] hidden route

[Chris Grundemann]

On Fri, Nov 7, 2008 at 06:36, Nalkhande Tarique Abbas
<ntarique at juniper.net> wrote:
>
> Try Removing as-overide from R2.

Not if  you want to advertise  routes both from P2 to P3 and from P3 to P2.

>
>
> -----Original Message-----
> From: juniper-nsp-bounces at puck.nether.net
> [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of Tomasz Opala
> Sent: Friday, November 07, 2008 3:30 PM
> To: juniper-nsp at puck.nether.net
> Subject: [j-nsp] hidden route
>
> Hi gurus,
>
> Imagine following topology:
>
> P2(AS11)------R2(AS100)-------R3(AS100)-------P3(AS11)
>
> In order to advertise route 199.199.0.0/16 (static, redistriuted to BGP)
> from router P2 to P3 (and some routes form P3 to P2), as-override has
> been
> configured on both R2 and R3:
>
> write at R2# show group p2
> type external;
> peer-as 11;
> as-override;
> neighbor 192.168.1.2;
>
> [edit protocols bgp]
> write at R2#
>
> write at R3# show group p3
> type external;
> peer-as 11;
> as-override;
> neighbor 192.168.2.2;
>
> {master}[edit protocols bgp]
> write at R3#
>
>
> Route 199.199.0.0/16 is present in P3 routing table:
>
> p3 at P3> show route protocol bgp terse 199.199/16
>
> inet.0: 5 destinations, 5 routes (5 active, 0 holddown, 0 hidden)
> + = Active Route, - = Last Active, * = Both
>
> A Destination        P Prf   Metric 1   Metric 2  Next hop        AS
> path
> * 199.199.0.0/16     B 170        100            >192.168.2.1     100
> 100 I
>
>
> Apparently the same route is advertised back, from R2 to P2 (it can be
> seen
> as a hidden route):
>
> p2 at P2> show route hidden extensive
>
> inet.0: 8 destinations, 10 routes (8 active, 0 holddown, 1 hidden)
> 199.199.0.0/16 (2 entries, 1 announced)
> TSI:
> KRT in-kernel 199.199.0.0/16 -> {}
> Page 0 idx 0 Type 1 val 89768b8
>    Nexthop: Self
>    AS path: [11] I
>    Communities:
> Path 199.199.0.0 Vector len 4.  Val: 0
>         BGP
>                Next hop type: Router
>                Next-hop reference count: 1
>                Source: 192.168.1.1
>                Next hop: 192.168.1.2 via ge-1/2/1.2, selected
>                State: <Hidden Ext>
>                Inactive reason: Unusable path
>                Local AS:    11 Peer AS:   100
>                Age: 13:52:22
>                Task: BGP_100.192.168.1.1+60006
>                AS path: 100 100 I
>                Router ID: 10.0.1.2
>
> p2 at P2>
>
>
> Do you have any explanation of hidden route in P2?

When using as-override like this, you will see hidden routes.  What
happens is that R2 receives and installs 199.199.0.0/16, then ebgp
defaults to sending all installed routes to all ebgp peers and because
as-override is configured, it strips AS11 from 199.199.0.0/16 before
advertising it to P2.  In the case of routes that R2 learned from P3
(via R3), this is good but for 199.199.0.0/16 it is not as good.  P2
sees the route learned from R2 with a bgp/protocol next-hop of
192.168.1.2 (it's own address) and a physical/forwarding next-hop of
interface ge-1/2/1.2 -- P2 knows that it can't reach itself out of
that interface, so the route is hidden (Unusable path).  This does not
hurt anything though because R2 also has a good copy of that route
[199.199.0.0/16 (2 entries, 1 announced)].

>
>
> Thanks,
> Tomasz
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--
Chris Grundemann
www.chrisgrundemann.com