Subject: How to map a packet to a VRF for route lookup?

From: [email-masked]
Date: Wed, 13 Dec 2000 16:21:39 -0800

Hi all,
     I am very confused by the description in RFC2547bis with regarding to the
association of VRFs and CE sites.  In section 1.3 of RFC2547bis, the following
description is given:
     "Each PE router maintains a number of separate forwarding tables.
     Every site to which the PE is attached must be mapped to one of those
     forwarding tables."

     Also in section 3, the following text is given:
     Each PE router maintains one or more "per-site forwarding tables."
     These are known as VRFs, or "VPN Routing and Forwarding" tables.
     Every site to which the PE router is attached is associated with one
     of these tables.  A particular packet's IP destination address is
     looked up in a particular VRF only if that packet has arrived
     directly from a site which is associated with that table.

     From these descriptions,  one would conclude that a CE site is associated
with exactly one VRF.  But the description in another paragraph of section 1.3
seems to indicate otherwise:
     A PE router is attached to a site by virtue of being the endpoint of
     an interface or "sub-interface" (PVC, VLAN, GRE tunnel, etc.) whose
     other endpoint is a CE device.  If there are multiple attachments
     between a site and a PE router, all the attachments may be mapped to
     the same forwarding table, or different attachments may be mapped to
     different forwarding tables.  When a PE router receives a packet from
     a CE device, it knows the interface or sub-interface over which the
     packet arrived, and this determines the forwarding table used for
     processing that packet.  The choice of forwarding table is NOT
     determined by the user content of the packet.

     The above description seems to associate an interface or subinterface  with
a VRF.
     Am I missing somethin here?

-- James Huang

---

From: Robert Raszuk <[email-masked]>
Date: Fri, 15 Dec 2000 00:18:01 -0800

James,

[Disclaimer: The draft describes today's behaviour so I will limit my
comments to today's behaviour and only in the shipping implementation I
am familar with.]

> The above description seems to associate an interface or subinterface  with

Yes that is correct. CE "box" is connected to PE via logical or physical
link or a set of links. Each such a link can be associated with only one
vrf. The CE itself can be connected by multiple links to a PE and
therefor associated with multiple vrfs.

I think we should change the below first two quotes from the rfc to read
instead of "mapped to one" & "with one of these" into "mapped to at
least one" & "with at least one of these".

R.

> [email-masked] wrote:
> 
> Hi all,