Single/Dual Homed and Multi-homed Designs

When talking about ISPS, BGP, and connections, sometimes you will hear terminology like “single homed”, “dual homed”,”single multi-homed” or “dual multi-homed”. These are different design topologies where we describe how a customer is connected (using BGP) to one or more ISPs.

Let’s take a look at some examples.



Single Homed

The single homed design means you have a single connection to a single ISP. With this design, you don’t need BGP since there is only one exit path in your network. You might as well just use a static default route that points to the ISP.

The advantage of a single-homed link is that it’s cost effective, the disadvantage is that you don’t have any redundancy. Your link is a single point of failure but so is using a single ISP.

single homed connection

Dual Homed

The dual homed connection adds some redundancy. You are still only connected to a single ISP, but you use two links instead of one. There are some variations for this design. Here’s the first one:

dual homed connection single routers

With this design, we use a single router on both ends, but we do have redundant links.

To increase redundancy, we can add a second router:

dual homed connection two isp routers

In the example above, the ISP has a second router. We also could have used a second router at the customer’s side and a single router at the ISP. For even more redundancy, add a second router at both sides:

dual homed router redundancy

The example above offers the most redundancy when you are connected to a single ISP. We have two links and two routers on both ends. One disadvantage of this design is that we are still using a single ISP.

Single Multi-homed

Multihomed means we are connected to at least two different ISPs. The most simple design looks like this:

single multihomed connection two isps

Above you see that we have a single router at the customer, connected to two different ISPs. The single point of failure in this design is that you only have one router at the customer. When it fails, you won’t be able to connect to any ISP. We can improve this by adding a second router:

single multihomed connection two isps redundancy

This is a pretty good design, we only use single links, but we are connected to two different ISPs using different routers.

Dual Multihomed

The dual multihomed designs means we are connected to two different ISPs, and we use redundant links. There are some variations, here’s the first one:

dual multihomed two isps

Above you can see that we are connected to two different ISPs, using one router and two links to each ISP. We have redundant ISPs and links, but the router is still a single point of failure. We can improve this by adding a second router:

dual multihomed connection two enterprise routers

The design above is better; it has two customer routers. One disadvantage, however, is that once one of your router fails, you will lose the connection to one of the ISPs. Using the same number of routers and links, the following design might be better:

dual multihomed connection dual routers

This design has redundant ISPs, routers, and links. Both customer routers are connected to both ISPs. This design does offer the highest redundancy but it’s also an expensive option.

Conclusion

You have now learned what the different (BGP) connection options to an ISP are:

  • Single homed: you are connected to a single ISP using a single link.
  • Dual homed: you are connected to a single ISP using dual links.
  • Single multi-homed: you are connected to two ISPs using single links.
  • Dual multi-homed: you are connected to two ISPs using dual links.

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Forum Replies

  1. Hello Srinivasan

    The Dual homed method 2 involves two separate ISP routers from the SAME ISP. This would most likely increase the cost of the solution compared to method 1 because of the fact that you require your ISP to provide you with two pieces of equipment on their end.

    Method 3 would most likely be even more expensive, because you are purchasing connectivity to two DIFFERENT ISPs, so you can’t take advantage of any discounts the single ISP may give you for multiple links.

    What I’m describing is the most probable case concerning cost. Ultimately it depen

    ... Continue reading in our forum

  2. Hello Alexander

    In order to run a redundancy gateway protocol on two edge routers with a single ISP router, you’d need some infrastructure between the edge routers and the ISP routers like a switch. It is possible, but this would introduce a new single point of failure and this is not desired especially on the edge of the network.

    //cdn-forum.networklessons.com/uploads/default/original/2X/a/a3aba10cfedb22063e8a63aaac5774f1408f0fad.png


    That would also mean that if your edge routers are performing NAT, you would require three public IP addresses, one for each ro... Continue reading in our forum

  3. Wow, a great and detailed response. I see my Problem in understanding now.
    So any of these are not recommended :smiley:

    So better redundancy with 2 own Routers and 1 Provider Router would be via having a link for each own router to the Provider Router and using AS Path for advertising the backup path via the standby router?

  4. Hello Jason

    If I understood the configuration correctly, then the 4500 VSS devices are customer devices which connect to each individual ISP, correct? If this is the case, then the configuration that you are looking at is Single Multihomed:

    //cdn-forum.networklessons.com/uploads/default/original/2X/b/b49900c322119893ce8199c7ce17c884b96e5031.png


    Even though you are using two devices, they are still viewed as one. The limitation here is that the VSS as a whole cannot belong to two ASs. Each router can only be assigned to a single AS. So if these VSS configure... Continue reading in our forum

  5. Hello Dakshinamurthy

    Based on your description, we’re looking at something like this:

    https://cdn-forum.networklessons.com/uploads/default/original/2X/1/14efdc53420f752ad17f96c1445ca93ff16cc65f.png

    Now the first thing I’d say is in such a situation, you wouldn’t have the servers connected just to CE1 but you would find a way to have them connect to both CE1 and CE2 either via internal routing or using some first hop redundancy protocol like HSRP or VRRP.

    However, if you want to maintain only a single physical connection from the servers to CE1, then you would a

    ... Continue reading in our forum

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