Subject: BGP/MPLS VPN

From: "Sakthivadivu.S.S" <[email-masked]>
Date: Thu, 19 Jun 2003 16:50:51 +0530

Hi,

    When two sites namely site1 and  site2 are connected to the same PE
router (PE1), is it required to run two instances of BGP in PE
router.Suppose if both site1 and site2 are having same route (110.10/16) and
EBGP is running between CE router and PE router, how will PE router  handle
this routes. since IPV4 routes will be same which are received from CE
router.Before converting to VPN-IPV4 routes, the decesion process will be
run and only one route will be installed in BGP.

  Please help me to handle this situation without using multiple instance of
BGP in PE router.

Regards,
Sakthi

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---

From: Siddharth Aanand <[email-masked]>
Date: Thu, 19 Jun 2003 09:34:53 -0400 (EDT)

In your example, do site1 and site2 belong to the same VRF or not?

-Sidd Aanand

---

From: David Charlap <[email-masked]>
Date: Thu, 19 Jun 2003 10:02:40 -0400

> run and only one route will be installed in BGP.

Whether or not to run two instances is an implementation detail.  You 
could run two instances, or you could have one instance that is designed 
to track routes from multiple VRFs.  Either way is doable.  The best way 
will depend on your router's architecture.

As for what the decision process is, it will depend on if the two sites 
are in the same VRF or not.

If they're in the same VRF, then all the usual BGP rules for 
aggregation, policy, etc. will apply.  There is little difference 
between this an a non-VPN environment.

If they're in separate VRFs, then the routes will be kept separate 
(either in different BGP instances, different tables within a single BGP 
instance, or in a single table using a per-VRF key as a discriminator.) 
  They will be exported to other VRFs and to the core according to 
whatever route targets are configured.

> Please help me to handle this situation without using multiple instance of

If you don't want to create multiple BGP instances, then you must be 
able to track which routes are learned from which VRF.  Which means 
either using separate per-VRF tables, or changing the keys of your 
existing table in order to discriminate between identical prefixes 
learned from different VRFs.

-- David