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NSSA no-summary with multiple ABR failure scenario

Hi,

 

Please see diagram attached.

 

In an nssa no-summary area only a type-3 default route will be present the routing table of the routers. In the diagram IOU1 will receive Two ECMP type-3 default routes from IOU2 and IOU5.

 

In a failure scenario where the link between IOU3 to IOU5 and IOU3 to IOU2 goes down, traffic from IOU1 could be blackholed if ECMP decides to foward it to IOU3 having no valid links that are up and could forward outbound traffic.

nssa failure scenario.png

 

Question, is there a way to do a conditional default route on the ABR so that it will not cause this blackhole scenario. Right now im not sure whether using NSSA in this type of design is recommended.

 

 

On the ABR, area x nssa no-summary will automatically advertise a Type-3 default route whether there is no present default route in the RIB.

Default-information-originate that has the capability of doing a route-map is advertised as a Type-5 so naturally the Type-3 will be proffered.

 

Any recommendations on this setup or suggestions for a better design?

 

regards,

Jon

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Hall of Fame Expert

Re: NSSA no-summary with multiple ABR failure scenario

Hello @jon,

 

in CCIE evil prep LABs the ABRs nodes could use :

a virtual link

a GRE p2p tunnel

 

in reality all you need is to have two VLAN based subinterfaces between ABRs nodes one in in area 0.0.0.0 and the other in area NSSA.

 

 

This is well explained in a book written by Jeff Doyl and Jennifer De Haven Carrol  Routing TCP/IP Vol I.

 

 

see for example

 

https://www.amazon.com/Routing-TCP-IP-1-2nd/dp/1587052024

 

 

Hope to help

Giuseppe

 

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