From: Bard Lowry (bard.lowry@xxxxxxxxxxxxxxxx)
Date: Tue Jan 01 2002 - 17:57:12 GMT-3
If anyone could help clarify the MP-iBGP route selection process for
VPN-IPv4 prefixes, it would be greatly appreciated. I am reading through the
* MPLS and VPN Architectures * book (Pepelnjak & Guichard), pages 194-196
for reference. I am paraphrasing below...
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The Enhanced BGP Decision Process for VPN-IPv4 Prefixes section states:
After routes are learned from other PE-routers across MP-iBGP sessions, but
BEFORE these routes are imported into any VRFs, the first step is to group
all relevant (those in the same VPN) routes so that they can be compared
(for subsequent best path selection)...the process followed by the PE-router
to group 'comparable' VPN-IPv4 routes is as follows:
1. Takes all routes with the same route target as any of the import
statements within the VRF.
2. Considers all routes that have the same route distinguisher as the one
assigned to the VRF being processed.
3. Creates new BGP paths with a Route Distinguisher that is equal to the
Route Distinguisher configured for the VRF that is being processed.
All the routes are now comparable and, at this point, the (standard) BGP
selection process is executed.
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My questions:
Item # 1 in the list above is straightforward to me, i.e., that the
receiving MP-iBGP peer looks at the route target extended community
attribute to group routes for subsequent insertion into a VRF. No problem
here.
Item # 2 is what has thrown me. On page 244 of the same book, it states, "
The BGP selection process applies to all routes that must be imported into
the same VRF, plus all routes that have the same route distinguisher as this
VRF."
So, two sets of routes are examined, those with the relevant ROUTE TARGET
values (as defined in the import policies of the VRF) and those with the
same ROUTE DISTINGUISHER as "this" VRF. This is where my confusion
begins....
My understanding is that you can create an MPLS/VPN by using a unique ROUTE
DISTINGUISHER for each VRF, so it is not necessary (and often not desirable)
for the ROUTE DISTINGUISHERs used on various PE LSRs to be the same value.
In this case, no routes would meet the condition described in item # 2.
However, if you decided to use a common ROUTE DISTINGUISHER for the VRFs
across all PE LSRs, what types of routes would meet condition # 2 but not
also meet condition # 1 ? I need an example of a route where these two
separate conditions would not always be true, i.e., the ROUTE DISTINGUISHER
matches that configured for the local VRF but the ROUTE TARGET does not
match the import rule(s)? If such a route existed in an MPLS/VPN domain, is
the author trying to communicate that the MP-iBGP selection process uses
these routes for best-path-selection for the VRF anyway? That would seem to
defeat the intent of the route target import policies (?)
Also, concerning item # 3, am I correct that this is an intermediate step
needed to allow MP-iBGP to consider the route distinguishers as identical
(i.e., stripping whatever original route distinguisher arrived with the
route?)
Thank you.
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