Looking at some of the best configuration practices for Cisco IOS. Cisco's default is for all routers to be L1L2, and this is suboptimal because all we need is for routers to be an L2 only. Once IS-IS is started, other configuration is also required for Cisco IOS. "log-adjacency-changes" captures adjacency changes in the system log and of course can be exported to a log collector for further monitoring. The two types of metrics supported in iOS: we need metrics to be wide which gives us a 24-bit range. The default metric in IS-IS, as in IOS is only 6 bits which is not much scope at all for networks today. We set the "is-type" to be "level-2-only", this is a router-wide configuration, and then finally we set to NET address as I've described earlier. To activate IS-IS from an interface, we go to the actual physical interface and, add in the command line, "ip router isis" and the process ID this will tell IS-IS to look for neighbors on that interface to form adjacencies with. And it also puts the interface subnet address into the Link State Database. If we don't want to have IS-IS running on interface, we go to the actual process command and mark that interface as passive. That will still put the IP subnet used on the interface into IS-IS, but will not look for any adjacencies. If there's no IS-IS configuration for any interface at all then CLNS will not be run on that interface and no subnet will go into Link State Database. All interfaces in IS-IS have a default metric of 10. This is fine for a uniform backbone, with equal sized links. But most backbones have different link capacities between the routers and the PoPs. And today many operators will set the default metric to be a very large number, for example 100,000. And following that, many operators develop their own interface metric strategy. The higher the bandwidth, lower the metric. And you can set, in Cisco IOS, the metric simply by going to the interface and using the "isis metric" command. Care needs to be needed as the sum of the metrics determines the best path through the network. IS-IS will choose the lowest cost path from end to end. And will load balance across paths with equal total cost to the same destination. In the example here, The best path/lowest cost is 11. We go from the user to the content the user is observing. One path has a total cost of 15. The other path has a total cost of 11. In the second example, both paths have a cost of 15. So, IS-IS will load balance over the two paths. Even though from the user perspective one path starts off with 2 megabits uplink and other one with 10 megabits. Operators need to be very careful when calculating the costs across the network. As there could be unintentional load balancing happening in parts of the infrastructure.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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