The MPLS WG Archive

[Wojciech Dec <wdec@cisco.com>]

Hi,

I'd say there is a better way to look at this:
You need a RT to figure out into which VRF to import the route to, and I'd 
say that it's a property of a VPN rather than of a particular PE - thinking 
of it in other terms (ie a property of a PE) would lead to increasingly 
complex RT import-export configurations which would not scale, since a new 
PE would imply a config change on all existing PEs
IP addresses need to be made VPN wise unique - The RD makes the ip address 
unique (thereby forming a vpnv4 address).
I guess that so far the above is no news.
Now on top of the above config scaling issues and to explain further 
consider the case of a Route Reflector in the picture and a multi-PE homed 
site (prefix);
the RT for this prefix is the same (belongs to the same VPN), and if there 
were to be no unique RD (or no RD at all) then an RRs would only reflect 
the best prefix (ie only one of the PEs would be the 'target' to reach that 
VPN prefix).

Regards,
Woj.

At 14:19 21/06/2001 +0530, sailendra mahanty wrote:
>Hello Juerg,
>
>At the remote PE, when BGP receives the route update, it won't mantain two
>routes to same IP address (Overlapped IP addresses) if RD is not used. Here
>i agree that RD is needed for BGP regular processing like selecting routes
>from RIB-In to Loc-RIB.
>
>But by some means (say by using RT value) if one can populate appropriate
>VRF with routes to overlapped addresses, RD may not be reuired.
>
>Please let me know if i am missing something ?
>
>Regards,
>-sailendra
>
>-----Original Message-----
>From: owner-mpls@uu.net [mailto:owner-mpls@uu.net]On Behalf Of
>Juerg.Fankhauser@swisscom.com
>Sent: Thursday, June 21, 2001 12:44 PM
>To: mpls@uu.net
>Subject: RE: Use of RD in BGP/MPLS VPN
>
>
>Hi Sailendra,
>
>I think that's because an advertized route can be associated with any number
>and combination of RTs (depending on your VPN routing scheme), while it
>carries exactly one RD as part of the address.
>
>That means: Even if it would be possible in theory to resolve the routing
>with RT alone, thanks to the RD it can be handled by regular (multiprotocol)
>BGP processes and tables.
>
>btw - P routers are not even BGP speakers in MPLS/VPN.
>
>Regards,
>-----------------------------
>Juerg FANKHAUSER
>swisscom AG, Bern
>juerg.fankhauser@swisscom.com
>-----------------------------
>
>
>
>-----Original Message-----
>From: sailendra mahanty [mailto:sailendram@future.futsoft.com]
>Sent: Donnerstag, 21. Juni 2001 07:01
>To: mpls@UU.NET
>Subject: Use of RD in BGP/MPLS VPN
>
>
>Hi All,
>
>draft-rosen-rfc2547bis-03.txt clearly distinguish between RD and RT. The aim
>of the RD is to resolve the ambiguity of IP4 address for IP address
>overlapping situation.
>
>IMHO when there is a BGP session between two PEs, the route advertise
>information in UPDATE msg is not used by other BGP speaker ( P routers
>specifically ).
>If i am missing something in saying this please let me know.
>
>If i am right in saying the last statemnet, the address overlapping can be
>resolved at the peer PE by using RT.
>
>Putting the question in other way, what way the RD value associated with a
>route is used in the remote PE ?
>
>Can anybody please help me to get clear picture of RD.
>
>Regards,
>-sailendra

_________________________________________________________
Wojciech Dec   CCIE# 5563      Phone:  +31 (0)20 342 3034
Network Consulting Engineer    E-Mail: wdec@cisco.com
Cisco Systems
PO Box 22514,
1100 DA Amsterdam
The Netherlands