History of Neworking: Alia Atlas on Fast Reroute
On this episode of the History of Networking, we talk to Alia Atlas about the history of fast reroute and Maximally Redundant Trees (MRTs). Remember to send in your suggestions for guests and technologies.
DFS and Low Points
On a recent history of networking episode, Alia talked a little about Maximally Redundant Trees (MRTs), and the concept of Depth First Search (DFS) numbering, along with the idea of a low point. While low points are quickly explained in my new book in the context of MRTs, I thought it worthwhile to revisit the concept in a blog post. Take a look at the following network:
On the left side is a small network with the nodes (think of these as routers) being labeled from A through G. On the right side is the same network, only each node has been numbered by traversing the graph, starting at A. This process, in a network, would either require some device which knows about every node and edge (link) in the network, or it would require a distributed algorithm that “walks” the network from one node to another, numbering each node as it is touched, and skipping any node that has already been visited (again, for more details on this, please see the book).
Once this numbering has been done, the numbers now produce this interesting property: if you remove the parent of any node, and the node can still reach Continue reading
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:
- B will learn about the link failure
- B will send an updated router LSP or LSA towards D, with the A->B link removed
- 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
- 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 Continue reading
Engineering versus Metaengineering
In my latest short take over at the Network Collective, I explain the difference between engineering and metaengineering.
On Breaking Things
On this short take over at the Network Collective, I talk about the importance of breaking things.