We're now going to look at two links to the same ISP but with load sharing it's a little bit unusual to buy a circuit or connection and leave it idle as we saw in the previous example. I mean usually what happens is we use the main link we're very happy with it until the day it breaks then we discovered in the backup doesn't actually work even though it's there and BGP may be working. So endsites tend to buy maybe two equal capacity circuits and balanced traffic over those two links much more common so let's have a look at the diagram the link from router a to C is one link the one from B to D is the second link again we're using a s 100 for the upstream provider NES six five five three four the primary s for the customer insight and router e again is removing the primary s and any customer sub prefixes from what s 100 is announcing to the internet so how would we configure this well as before we announce the slash nineteen aggregate on each link and in fact as we work through this series you find that's a common theme we always announce the aggregate on all external links it's one of the industry best practices it's the expectation from the regional registries when they delegate address space to an end and user or an site what we're going to do now though is take this slash 19 that we're announcing and divide it into two so we're going to announce two slash twenties as well one on each leg so the first slash 20 will go on the first link the second slash 20 will go on the second link and what will happen here is incoming traffic for the first slash 20 will use the first link and incoming traffic for the second slash 20 will use the second link now this is just where we start please don't think that right now this will give you perfect load balancing it's unlikely it will and user traffic throughout the day and evening can be quite variable but it's place where we start we divided into to see what happens and what we end up doing as we work through this is varying this split until we get the perfect load sharing that we are after it might be that we have to take one in the twenties and divided into two twenty once one twenty one and one length one twenty one on the other and so on we accept the default right from the upstream provider as before we're not going to do anything special here nearest exit is good enough for now we're going to the same upstream provider and as with most n sites the vast majority of the traffic will be incoming as the end users will be consumers and downloaders let's look at the configuration so in router a we originated the slash nineteen aggregate as before and we also now originated 1/20 so let's take the first slash twenty out of that nineteen if you look at the prefix list we're now allowing out there slash 90 negra gate as well as the slash twenty inbound it's the same as before just the default round if you look at router B again we originate the aggregate and we also originate this second slash twenty so that one begins at 164 dot 16.0 slash twenty the prefix list contains the 19 and the 20 and that is what gets announced to the upstream provider if we look in the upstream the configuration is very similar to what it was before we're doing the default originated so our customer gets the default right we have the ID band prefix list to let that default out to our customer and then inbound we have a prefix list which allows the customer prefixes in but notice the difference from the previous example we're now allowing the slash nineteen but we're allowing slash 20s as well we're not saying which of the two slash twenty side of that aggregate we would just allow either all this is more flexible for the upstream provider because if the end user decides to swap around the slash 20s they're announcing on the link the upstream provider doesn't have to make any changes so this scale is much better router D configuration is exactly the same if we go to router e which is a s1 hundreds links to their upstream provider we strip out the private es because as before we don't announce private ESS to the Internet and we have a night band filter that only allows the slash 19 aggregate out there is no need for the global internet to see the two slash 20s that the end site is announcing to a s 100 for traffic engineering this is local traffic engineering information and it's not useful or helpful for the global Internet to see it what about the default route for outbound traffic well we can originate the default route in the IGP on the border routers and rely on the IGP metrics for nearest exit BGP we'll just have one best path which means that all traffic from the network will just go one way what we want to try and do is maybe balance the outbound traffic a little bit better on the two outbound links to the upstream provider so what we do is we simply originate the default right into our chosen IGP the slide shows an example of how to do it in Cisco IOS using OSPF and using eius eius we'll look at this and a little bit more detail later on so again note the load sharing configuration is only on the two customer otters the upstream ISP simply has to remove customer prefixes from external announcements and remove the primary s from the external announcements as well as we'll see you later on we could use bgp communities and in fact these days and more developed networks network operators prefer the use of bgp communities for this type of traffic engineer

© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.

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