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New Member

Route Distinguisher operation


I need help with the operation of RD.

I Know that the RD (64 bits) is attached to the prefix when is send into the MP-BGP cloud to other PE?s, being converted in a VPNv4 packet.

My question is: What is the route decision process in the receiving PE?.

For example:

PE1 is configured to import routes with IMPORT RT 1:100 and RD 2:100.

To PE1 arrives 2 updates both with EXPORT RT 1:100 and for network /24.

- (RD)2:200 - /24

- (RD)2:300 - /24

1- Both networks are inserted in the MP-BGP table? (I believe that yes).

2- What network is imported into the VRF, both, only one? (I don?t know)

Thanks for all.


Re: Route Distinguisher operation

1) Case 1: Assuming both the routes belong to the same VPN the PE1 will pass on both the routes to the directly connected CE, as it see both the routes as two different routes, and if the CE is configured to load balance it will load balance over these two paths.

2) Case 2: If they belong to two different VPN's then you RT numbering is wrong, as the traffic will be blackholed. As just different RD numbers wont prevent the route being imported into a certain VPN site.

As the import and export is totally goverened only by RT. RD takes care of keeping the route unique across.



New Member

Re: Route Distinguisher operation

Thank very much for your answer, my doubt is resolved.

Re: Route Distinguisher operation

I received some unicasts upon my last reply to this post. The confusion created

was, how will two paths be passed to the CE.

Well its quite evident that if the CE has only one physical path to the PE then it

cannot receive two paths. Only if the CE has two paths (dual homed) then only the CE

loadbalance or else the PE will do tha loadbalancing for the same end destination.

But to do that we have to use different RD values for the same routes from two different

PE's. So that we get the route till our PE in its MPBGP table and then we have to get

it into the forwarding table that is the VRF table by using maximum-paths ibgp 2.

Just wanted to clear the confusion. :-)

And to answer more crisply rather than generically to the original post,

so as to avoid confusion.


1- Both networks are inserted in the MP-BGP table?

A - Yes both the routes will be there in the MPBGP table as it wont be

filtered by your RR.

Can be verified by using "show ip bgp vpnv4 all"


2- What network is imported into the VRF, both, only one?

Both the routes would be installed in the CE VRF table by using maximum-paths ibgp 2.

Can be verified by using "show ip route vrf x"



Cisco Employee

Re: Route Distinguisher operation

Route Distinguisher (RD) is only used to make the routes unique within the provider network (MP-BGP) it is not used once those routes are put into a VRF and sent to the customer. RD is added to a route when it is advertised to another PE via MP-BGP. It is then removed from the route by the receiving PE when it is placed into the VRF.

Route Target (RT) is an extended community attribute and this is what is actually used to determine which routes actually get into a VRF. Routes will only be placed in a VRF if the route has an RT value (ext community) attribute on it that matches the import value configured on the VRF. The export value configured on the VRF is the extcomm attribute that is added to the route when it is advertised from the VRF. If a PE has no VRFs configured on it with import RT values matching an of the RT attributes on the routes, then the route is silently discarded when recieved by the PE.

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