Tech Library
BGP – advertise-peer-as
1. The default In the Juniper documentation we have the following statement: by default, Junos OS does not advertise the routes learned from one EBGP peer back to any EBGP peers that are in the same AS as the originating peer. Let’s explore this feature and put it to the test. The text book scenario for […]
ISIS – Adjacency Requirements
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 […]
ISIS – MTU and Hello Padding
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 […]
ISIS Authentication
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:
ISIS Attached Bit
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 […]
ISIS L1/L2 redistribution and summarization
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. […]
LDP deaggregate
When an LDP egress router advertises multiple prefixes, the prefixes are bound to a single label and aggregated into a single forwarding equivalence class (FEC). By default, LDP maintains this aggregation as the advertisement traverses the network. On a random PE, we can see how multiple prefixes are bound to one single label: Apply the […]
LDP track-igp-metric
1. Intro In this post we will demonstrate the impact of LDP track-igp-metric. The setup is the one from the diagram below, where R1 receives the 3.3.3.0/24 prefix via BGP from R9 and R6. Depending on whether track-igp-metric command is configured or not, R1 will prefer either R9 or R6 to reach 3.3.3.0/24. 2. Initial […]
LDP Synchronization
1. Intro In this post we will look at the LDP Synchronization feature, and the impact it can create in the network. We will work with the following setup, where between R1 and R3, the LDP protocols was not activated in the configuration the R1 router. Because of this, the IGP protocol (OSPF in this […]
OSPF Sham Link
1. Intro We will demonstrate the Sham Link usage with the following setup. We have CE1 and CE2 router that have a backdoor link between them. The main path between CE1 and CE2 should be via the MPLS L3VPN, so we configured a high metric between CE1 and CE2, as depicted in the next figure: […]