ISIS

1. Intro Often times, troubleshooting an ISIS adjacency between two routers is not a smooth process. Well, it can be if you don’t clearly know all requirements. In this post we will see all ISIS adjacency requirements and how to troubleshoot when the neighbors are not UP. As a network engineer we must know all […]

1. Intro ISIS has a rather indirect method of checking the interface MTU to establish if it meets the adjacency requirements. As an OSI Layer 2 protocol, IS-IS does not support data fragmentation. Therefore, maximum packet sizes must be established and supported between two routers. During adjacency establishment, the IS-IS protocol makes sure that the […]

1. Intro All IS-IS protocol exchanges can be authenticated to guarantee that only trusted routing devices participate in the autonomous system (AS) routing. By default, IS-IS authentication is disabled on the routing device. We have 2 ISIS Authentication options: 2. ISIS Interface Authentication 3. ISIS Level Authentication External Resources:

1. Intro In Junos, the IS-IS Attached Bit (ATT-bit) is a flag automatically set by Level-1/Level-2 (L1/L2) routers in their Level-1 Link State Packets (LSPs). It signals to pure Level-1 (L1) routers in an area that the L1/L2 router is connected to another area or the backbone, allowing the L1 routers to install a default […]

1. Intro We will use the following topology to see the redistribution rules in action. As a reminder, find next the default L1/L2 redistribution rules: Note: with wide-metrics-only configured, the External L1 routes become Internal L1 routes. So, Rule 3 above essentially becomes Rule 1. This use case is fully covered in the wide-metrics-only post. […]

ISIS – Interior Gateway Protocol (IGP) Type: Link State Algorithm: Dijkstra  Metric: 10 for any interface0 for Loopback interface Metric Type: Narrow and Wide by default. But, ISIS silently limits metric to 63.In modern ISIS deployments we should enable wide-metric-only. Preference (AD): 15 / 18 (L1/L2 Internal)160 / 165 (L1/L2 External) PDU Types IIH IS-IS […]

1. Intro In this part we will focus on the ISIS IPv6 metric, more specifically, the differences between IPv4 and IPv6. But before that, let’s remind ourselves that this post is part of a 3 part series about the impact of ISIS Narrow and Wide metrics: 2. The test topology The topology is the same, […]

1. Intro In this part we will focus on the use unexpected impact of the wide-metrics-only knob. But before that, let’s remind ourselves that this post is part of a 3 part series about the impact of ISIS Narrow and Wide metrics: 2. The test topology We will update our test topology to demonstrate the […]

1. Intro The ISIS Metric topic will have 3 parts: 2. The ISIS metric defaults A quick review of the ISIS metric defaults: 3. The unexpected impact of using both Narrow and Wide by default As already mentioned, Junos uses a combination of both Narrow and Wide metrics. But why use both? Well, the advantage […]

1. Intro First of all, lets quickly review the what the official documentation is saying about the ISIS overload timeout knob: You configure or disable overload mode in IS-IS with or without a timeout. Without a timeout, overload mode is set until it is explicitly deleted from the configuration. With a timeout, overload mode is […]