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Theory of Route Redistribution

jc-lemaire
Level 1
Level 1

I am working in a hub-spoke 300 node network. The hub (HQ) is OSPF, the spokes (branch ofices) are OSPF, and the WAN links are BGP / MPLS. 

I understand how to configure, etc, of route redistribution, but I don't understand the theory, in other words, the why.

In my case, do I redistribute BGP into OSFP or vice versa? 

 

3 Replies 3

Jon Marshall
Hall of Fame
Hall of Fame

Are you talking about DMVPN with tunnels running over an MPLS network or do you simply mean you have multiple sites connected to an MPLS network ?

If the former you may not need to do any redistribution, if the latter you would need two way redistribution (or a variation on that).

Can explain more but could you clarify what exactly your setup is ?

Jon

Hi

My understanding is you are using MPLS L3VPN where you have MG-BGP on your PE routers, is that correct?

You are using VRF assigned to your customers, this VRF have been created under an Address-family under the BGP process to isolate the clients networks, as you have already created a VPN tunnel using BGP, it will transport the routes redistribute into de address-family to the other PEs through MPLS, the idea of the mutual redistribution is enable the communication between the different CE routers in both ways.. 

For example if you only redistribute the BGP into the OSPF on CE (A) router, the CE (A) will be able to see the routes advertised from other CE (B) router through the MPLS / VPN tunnel but the other CE (B) will not be able to know the routes from CE (A), the communication will be in 1 way only, the packets will not able know how return to the source. 

You can see the routes received through the VPN tunnels using the following command:

show bgp vpnv4 unicast all summary




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technthusiast
Level 1
Level 1

Why? Because the routes are created by each protocol specified by each routing protocol in their own unique way which is not understood by other routing protocols. But thankfully you can configure 'redistribution' which will convert an OSPF route into a BGP route and convert a BGP route into an OSPF route. But the catch is you need to manually specify what the route is worth in the protocol you are redistributing to (e.g. if you're redistributing a BGP route into OSPF, will these routes be type 1 or 2? Well you need to tell the router that). 

To answer your last question: You have to redistribute the route both ways. 

Routing protocols have their own completely unique ways of measuring and comparing one route from another. 

OSPF will measure routes based on the 'cost' to get to that remote route. BGP uses a whole different world of measuring and comparing routes, it uses things like how many Autonomous Systems (AS) are between itself and the remote route, the local preference, weight, whether the route was internal or external BGP, and etc and so forth. 

If you want a BGP network to process OSPF routes, you need to convert those OSPF routes into BGP routes by 'redistribution'. And vice versa. If you didn't do that each protocol would have no idea what the opposite routing protocol's routes are worth or mean.

Try spending US dollar bills in Europe, sure they can see that it's money but they won't do anything with it until you convert it first into something they use and understand.

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