Apologies for how terrible the Topology is, I took an existing picture and put some purple arrows leading off the ABR's, indicating that this is actually where Type 4 LSA's are created. Now ASBR's create Type 5 LSA's which of course are External / Redistributed routes which are shared throughout the entire routing domain across … Continue reading OSPF: Type 4 LSA’s, who creates them, why they create them, a relatively shorter post for once!
OSPF Summarization is done only on ABR and ASBR routers in your OSPF domain, and use two completely different commands, but what if a router is an ABR and an ASBR? For example, did you know that using the command "default-information originate ..." you are telling the router to create a Type 5 LSA to … Continue reading OSPF: Important details regarding Summarization and Default Routes for exam day, it’s a long but worthwhile read!
For those who have forgotten how LSA's work between Area's, let us refresh your memory: Type 1 and 2 LSA's live within their own Areas to create the SPF Tree ABR's extract the Network / Subnet Mask information to create a Type 3 LSA which it then floods into the other Area (Summary LSA in … Continue reading OSPF: Route filtering between Areas (Inter-Area) configured and explained!
To calculate the best path to a route, first the router builds an LSDB that is then run against the SPF Algorithm, which then comes up with its best paths to subnets. Ultimately what makes it into the IP Route Table is decided by CEF, which views all protocols different tables to decide the best … Continue reading OSPF: Metric (Cost) Calculation for both Intra-Area and Inter-Area explained, and Metric fine tuning for path selection!
Sticking with this Topology in case I need to lab something for demonstration, it is clear what routers are where. To begin, every router within any given Area should have an identical copy of the LSDB, and that only Type 1 and Type 2 LSA's are flooded only within the Area they originated. The Database … Continue reading OSPF: Database Exchange Process including messages, neighbor states, and all things neighbor forming related!
When you first configure a router in an Area via an OSPF instance, you are creating a Link State Database, that already contains entries for different LSA Types. Each router considers itself to be the Root of what is called the "Shortest Path First Tree", which is a mapping via LSA's from neighbor routers, which … Continue reading OSPF: LSA Type 1 (why it’s unique), explanation of SPF Tree, and a handy show command for exam day!
I pulled this topology from my older posts when I took a deep dive into the world of LSA's, how to read the Topology table like a laundry list and under stand it, and what type of routers did what. First I want to list links that are absolutely vital to read and understand for … Continue reading A collection of IMPORTANT links to review and know for exam day, then a quick overview of LSA Types / OSPF Router Types!
The OSPF RID has a much more significant role in a neighbor formation than with EIGRP, obviously because that it what it uses to identify its neighbor rather than the IP address of the physical interface the neighbor was learned on. So what if two routers were to have the same Router ID? That question … Continue reading OSPF: Duplicate RID’s in the same Area and between Areas, and MTU Mismatches on OSPF links, explained!
This conversation really begins with the interface on which OSPF is operating on, and the Layer 2 encapsulation that it is being used on the interface, like above over my NBMA I am running Frame-Relay as the Layer 2 encapsulation. From this information alone OSPF knows whether it can Broadcast across the network, if it … Continue reading OSPF: Every OSPF Network Type, configuring them, and them explained to painful detail with a lot of output / config examples!
This will be our topology for most of Review, though I am currently leaving Area 34 and Area 15 out of the equation since we aren't really worried about those for review. You can create a process and an Area by configuring OSPF directly on an interface, rather than making the process and adding the … Continue reading OSPF: Creating OSPF from the interface, playing with Hello / Dead timers, intro to Hello-Multiplier and other good review!
One thing that was lacking from Chris Bryants training that I found in the INE series is explanations between when to use OSPF Filter-list vs a Distribute-list. I have modified the network as reflected in the Topology so Area 34 will no longer be a stub, and Area 15 will only be an NSSA Area. … Continue reading OSPF: DEEP Dive – Distribute-List vs Filter-List, and reviewing Prefix-Lists as Filter-Lists use Prefixes to filter!
Since I already have the lab configured with different OSPF Area's, and we had a riveting exploration into the world of LSA's and the LS DB of each router, I wanted to hit some other topics again thoroughly before I switch up protocols for more DEEP Dives into all things CCNP ROUTE related! Without further … Continue reading OSPF: DEEP Dives into Summarization methods, Authentication, Default-Information Originate (Always)!
To lead off where we left, we made Area 15 an NSSA Area which stopped the Type 4 and 5 LSA's from entering the Area, now we are going to do some Redistribution the other way and turn R5 in the NSSA Area into an ASBR. I've created networks (loopbacks) 126.96.36.199 /32 and 188.8.131.52 /24, … Continue reading Part 3: OSPF LSA DEEP Dive LSA Type 7, DEEP Dive into OSPF Stub and NSSA Areas, and how they impacts LSA traffic!
I will start this off by stating that we will start redistribution on the ASBR specified R2 to examine LSA types 4 and 5 (and where we can spot them), and R1 will eventually also be turned into an ASBR when we redistribute some loopback routes into the NSSA Area so we can review type … Continue reading Part 2: OSPF LSA DEEP Dive, LSA Types 4 / 5, and turn Area 15 to an NSSA to see what happens with the LS Database!
So we are back to OSPF, as I never really went into a deep dive with LSA types, or the LSA DataBase from different routers to show their perspective of the OSPF network caused by these Link State Advertisements. I've only configured base OSPF and RIP configurations in this lab so far, the above Topology … Continue reading Part 1: OSPF LSA DEEP dive, starting with LSA Types 1 / 2 / 3, and an Intro to all LSA Types and OSPF Routers types!