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I think this kind of topic may be a course basic, but I'm still have a lot of technical doubts on how to implement redundant links on Layer 3, for example:

-In order to use dual homing for a remote site, for example:

|Router 1|-----(link 1, ISP 1, etc.)-----|Router 2|

|________|-----(link2, ISP2, etc.)-------|________|
  • Is it always necessary to use different ISPs?
  • Regarding the protocol configuration, the new link should be in the same area (in OSPF) or as the same level (in IS-IS) or participate in any way on the existing process?
  • Should create a whole new process on the router for this specific link?
  • This will not cause a L3 loop? -Would this be avoided (L3 loop) by simply giving a higher cost to one of the links?

Many thanks in advance for your warm responses.

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  • First, it is unlikely that you would be peering with an ISP using OSPF or IS-IS. It would most likely be using eBGP. In any case, the routing protocols have built-in features to prevent routing loops. – Ron Maupin♦ Mar 1 '18 at 3:04
  • What's a type of links between routers? Is it L2 or L3? Is it private or public? After that, you'll choose protocols what you can use to implement redundancy. – Iliya Zubakin Mar 1 '18 at 8:50
  • @RonMaupin an ISP could provide a transparent connection between those routers over which you could run an iGP. – Teun Vink♦ Mar 1 '18 at 22:36
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin♦ Apr 1 '18 at 21:13
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This could get a little heavy so brace yourself.

  1. From a routing perspective, Understand the concepts of VRFs ( virtual routing forwarding tables ). Generally, a lot of ISPs use MPLS to route traffic across their core, and this would mean using either L3VPN or L2VPN for providing a circuit between your sites. Each customer could come into the ISP core on a different VRF ( research on 6PE and 6VPE ) and carried across to the other location. In case they don't use MPLS, they could still use VRFs with IP to route traffic. In Such cases, you would not have those routes leaking between the 2 ISPs, and you should not have any problems with loops ( unless your branch sites are connected via another backdoor link)

  2. From a redundancy perspective, If you need to think about redundancy only, you don't even need OSPF as such. Static routing with SLAs could help. An easy reference could be this IP SLA on NE

  3. In terms of loops, if you are really interested in understanding how loops could happen in OSPF , read this Loop prevention in OSPF and BGP Loop Prevention. Basically within the site, you could have transient loops but not permanent ones in the long run.

Hope this helps.

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