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List:       isp-bgp
Subject:    [isp-bgp] BGP Loop Detection Mechanism doubt.
From:       "Murilo Antonio Pugliese" <mpugliese () diveo ! net ! br>
Date:       2005-11-08 18:45:55
Message-ID: B676B3073BBDCB4DAB33C62D4F09A46D198CF0 () exch ! corp ! diveo ! net ! br
[Download RAW message or body]

Hi folks.

As my first message to this list, I may introduce myself.
My name is Murilo Pugliese, I'm an electrical engineer, and work for a \
telecommunications company at Brazil that among  other services (as those one related \
to IDCs) provide Internet access for companies, MPLS-based VPN, and VoIP services.  I \
have one doubt regarding the BGP Loop Detection that I'm expecting to get it clear \
through this list.

Talking about MPLS-VPN deployments, I have a scenario (common request) were a VPN \
customer has a site with 2 Wan  connections & wants that this site receive its \
downstream traffic load-shared.  At first we just associated both wan connections to \
the same VRF, issued the load-sharing command at interfaces level,  and created \
parallel routes toward the site LAN prefix (each one pointing to one of its wan \
connection).

We could perceive that this deployment worked just fine for traffic originated at \
VRFs present @ different/remote PEs; but for traffic originated at a VRF present @ \
the same PE the source VRF would choose only one adjacency and send all  its traffic \
just through this chose adjacency, so the downstream load-sharing request wasn't \
being provide as required.  
To overcome this for sites with this kind of need, we decided to established eBGP \
between PE and CE, though the devices  loopback addresses. In this way, doesn't \
matter if traffic is originated from a VRF at the same PE or from a remote PE, the  \
downstream traffic is always load-shared.

Now comes my doubt. If couples sites of the same VPN are configure using this \
solution, and both sites establish BGP using the  same AS #, when one VRF gets to \
import a prefix propagated through my MPLS Domain from the remote VRF, it's possible \
that the BGP Loop Detection Mechanism interpret it as a loop and discard the prefix ? \


Be advised that there is no BGP establishment between both VRFs, only between each \
VRF and its peer CE.

The command below was issued at a VRF that imports a prefix from a remote VRF that \
uses AS # 6500 to establish eBGP with its  peer CE. I know there is no AS Path, but \
there is a reference of the AS # associated to the routing information.

	Router#sh ip cef vrf 55123456789 0.0.0.0 0.0.0.0
	0.0.0.0/0, version 757, epoch 0, per-destination sharing
	0 packets, 0 bytes
	Flow: AS 65000, mask 0
	tag information set, all rewrites owned
	local tag: VPN route head
	fast tag rewrite with
	Recursive rewrite via A.B.C.D/32, tags imposed {944}
	via A.B.C.D, 0 dependencies, recursive
	next hop A.B.X.Y, FastEthernet5/0/0 via A.B.C.D/32 (Default)
	valid adjacency
	tag rewrite with
	Recursive rewrite via A.B.C.D/32, tags imposed {944}
	Recursive load sharing using A.B.C.D/32.
	Router#

Willing to hear from you folks ASAP.
Yours Truly.
Murilo Pugliese.


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