Why the need for RD and RT ?

[michaeljmorris]

All,

I'm beginning my study of MPLS. I understand the difference between RD and RT. RD makes IPv4 routes unique inside the shared Layer3 network. RT determines which routes get imported into VRFs on PERs.

My question is, why use both RD and RT ? RD has to be unique inside each VPN. So why not just do the importing based on the RD. You could still "import" other VPNs if you had extranet needs, just import the extranet RD.

I know I missing something here. Thanks.

Michael J. Morris

[karief]

Basically IP Prefix from CE's is IPV4 (32bit) and PE Converts it in to VPN-IPV4 . I belevie having 8byte more nothing but RD(8byte). RD is used to generate VPN-IPV4 with VRF on the PE Routers.

regards

arief

Sr.Network Engineer

[michaeljmorris]

What ?

[karief]

I did'nt understand what is "what?" , Let it clear what is RD ?

1.RD is used to disambiguate IP addresses via VPN-IP addresses

2.RD is not used to constrain distribution of routing information (route filtering)

3.Identifier attached to routes that distinguishes to which VPN it belongs. Each routing instance must have a unique route distinguisher associated with it. If the instance type is vrf, the route-distinguisher statement is required.

The route distinguisher is a 6-byte value and teh format looks like

as-number:number -- where as-number is your assigned AS number (a 2-byte value) and number is any 4-byte value. The AS number can be in the range of 1 through 65535.

ip-address:number-- where ip-address is an IP address in your assigned prefix range (a 4-byte value) and number is any 2-byte value. The IP address can be any globally unique unicast address

Let me know further....

regards

arief

[michaeljmorris]

arief,

Yes, thanks, I understand what the RD is and it's format. My question is why is there a need for an RD and an RT ? Since RD could be set the same for an entire VPN, then why not make your importing decisions on RD and skip having RT ? Seems redundant to me.

Thanks.

Michael Morris

[Harold Ritter]

You need to decouple these two to cater for extranet applications, where two different VPNs (different RDs) need to share routes. You couldn't do that if RT and RD had to be the same.

[michaeljmorris]

Ok, but the same way you can import multiple RTs for extranets you could import multiple RDs... Since the RD in unique to the VPN, if you import RD you get an extranet.

So, I'm still wondering why the need for RD and RT ?

Thanks.

[Harold Ritter]

Decoupling the RD and the RT makes it a lot more flexible by not allowing you to import multiple RT but also to export multiple RT, which you could not do if you didn't have an extended attribute to carry these multiple values.

Another consideration is that some SPs like to assign a different RD for the same VRF on different PEs for loadbalancing purposes. Having only a RD would clearly force them to maintain a potentially long import list on each PE, whereas by decoupling the RD and RT you only have to configure one RT for both import and export.

[michaeljmorris]

Ok...that makes sense...thanks....

[ipotts]

Hello,

Can you please explain in more detail how this load balancing is achieved? Do you have a sample configuration.

Many Thanks

Ian

[Harold Ritter]

I should precise that this is loadsharing in a scenario where route reflectors are used and that byou have a customer site connected to two or more PE routers. If you used the same RD on both PEs, the VPNv4 prefixes propagated to the route reflectors would be consider to be the same. The route reflector would make a best path decision and propagate only that best path to the other PEs in the network, which by itself prevent loadsharing between the two PEs connected to the customer site.

I don't have any sample config but it should be pretty simple. Different RDs but same RT across all of the PEs connected to the same VPN.

[ipotts]

Hello Hritter,

Thank you for your high quality of responses. This has been very useful to me,

Regards

Ian