Routing
Routing is the process of selecting the best path for data packets to travel across one or more networks from a source to a destination. Over time this category will include posts for static routing, routing protocols, preference/administrative distance, redistribution and many more.
Different routing subcategories to explore:
Stay tuned for more.
BGP – as-path-prepend
1. Intro You can prepend one or more autonomous system (AS) numbers at the beginning of an AS path. The AS numbers are added at the beginning of the path after the actual AS number from which the route originates has been added to the path. Prepending an AS path makes a shorter AS path […]
BGP – loops
1. Intro When configured, loops will allow the local device’s AS number to be in the received AS paths, and specify the number of times detection of the local device’s AS number in the AS_PATH attribute causes the route to be discarded. There are 2 ways of configuring loops: As we can see, we can […]
BGP – as-override
1. Intro Compare the AS path of an route with the AS number of the BGP peer in the group, and replace all occurrences of the peer AS number in the AS path with its own AS number before re-advertising the route to the peer. This is essentially an alternate solution to the advertise-peer-as and […]
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. […]
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: […]