Orhan Ergun 3 Comments

Is fate sharing bad thing in network design? Someone asked this question recently on my youtube channel and I want to share a post for the website followers as well.

First of all, what is fate sharing ? Below is the Wikipedia definition of fate sharing.

” Fate-sharing is an engineering design philosophy where related parts of a system are yoked together, so that they either fail together or not at all “

We use this term a lot of time when we suggest a design recommendation for Multi Service PE (Provider Edge) devices in the Service Provider networks. Multi Service PE provides more than one service in the Service Provider networks.

Many Service Providers follow this design philosophy. Main reason for SPs to use more than one service (Ex: Internet and VPN) on the same router is cost. In order to reduce cost by better utilizing the router resources is not a bad idea but there is a fate sharing in this case.

Internet based attack can bring down the Multi Service PE and VPN customers on the same device are affected as well. In this case fate sharing is harmful.

But is fate sharing always a bad thing ? Actually no. As always it depends.

I can hear the voice of Transport Engineers right now 🙂

When there is a connection between two routers for example, if link fails and only one side of the link goes down (This is the case when you have a transport equipment or routers connected via the Ethernet switch), for the failure detection upper layer protocol is used (BFD, Protocol hellos etc.).

But of course it is better if both sides of the link go down at the same time. This is called fate sharing as well and in this type of situation, Transport people want fate sharing. (In fact one of the capabilities of MPLS-TP is fate sharing through the inline OAM messages).

Thus we can’t say that fate sharing is always a bad thing but depends on the situation.

To keep this post as short and understandable I won’t give another example but if you would like me to provide more example let me know, share your thoughts in the comment box below.

— 3 Comments —

  1. Hi Orhan,
    Thanks for the post. Could you give a few more examples to distinguish between use cases
    favoring fate sharing and those that don’t?

    • Hi Sudarshan,

      Whenever there is intraction there will be fate sharing actually, IGP – BGP , IGP-MPLS – BGP-MPLS and even human to network there are inteactions and problem on the one side affect the other side as well.

      If you design Multi level IS-IS network, MPLS LSP may not come up unless you leak the remote loopbacks etc.

      Or if you don’t take device from the IGP path, in case of planned failure, traffic is blacholed. Even if you take it out from the dataplane, when it comes back IGP has to wait BGP and so on.

      If you terminate your site to site VPNs at the core of your network, instability at the VPN network affect your core network.

      I can give you many more examples but hope it is more clear now.

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