Responding to Readers: Questions on Microloops

Two different readers, in two different forums, asked me some excellent questions about some older posts on mircoloops. Unfortunately I didn’t take down the names or forums when I noted the questions, but you know who you are! For this discussion, use the network show below.

In this network, assume all link costs are one, and the destination is the 100::/64 Ipv6 address connected to A at the top. To review, a microloop will form in this network when the A->B link fails:

1. B will learn about the link failure
2. B will send an updated router LSP or LSA towards D, with the A->B link removed
3. At about the same time, B will recalculate its best path to 100::/64, so its routing and forwarding tables now point towards D as the best path
4. D, in the meantime, receives the updated information, runs SPF, and installs the new routing information into its forwarding table, with the new path pointing towards E

Between the third and fourth steps, B will be using D as its best path, while D is using B as its best path. Hence the microloop. The first question about microloops was—

Would BFD help prevent the microloop (or perhaps make it last a shorter period of time)?

Consider what happens if we have BFD running between A and B in this network. While A and B discover the failure faster—perhaps a lot faster—none of the other timing points change. No matter how fast A and B discover the link failure, B is still going to take some time to flood the change to the topology to D, and D is going to take some time to compute a new set of shortest paths, and install them into its local routing and forwarding tables.

Essentially, if you look at convergence as a four step process—

1. Discovery
2. Reporting
3. Calculation
4. Installation

Then you can see the microloop forms because different routers are “doing” steps 3 and 4 at different times. Discovery, which is what BFD is aimed at, is not going to change this dynamic. The second question was—

Can microloops occur on a link coming up?

For this one, let’s start in a different place. Assume, for a moment, that the A->B link is down. Now someone goes in and configures the A->B link to make it operational. At the moment the link comes up, B’s shortest path to 100::/64 is through D. When the new link comes up, it will learn about the new link, calculated a new shortest path tree, and then install the new route through A towards 100::/64. E will also need to calculate a new route, using B as its next hop.

The key point to consider is this: who tells E about this new path to the destination? It is B. To form a microloop, we need D to install a route through B towards 100::/64 before B does. This is theoretically possible in this situation, but unlikely, because D is dependent on B for information about this new path. Things would need to be pretty messed up for B to learn about the new path first, but not recalculate its shortest path tree and install the route before D can. So—while it is possible, it is not likely.

Thanks for sending these terrific questions in.