There are three types of OSPF authentication. Type 1 – no authenticationType 2 – clear textType 3 – cryptographic (MD5 or SHA) Lets explore each type in a lot more detail and then look at the router configuration for some real world examples. Authentication can be configured on a per area or per interface, but […]
Should you run OSPF over DMVPN?
You may have heard that you should not run OSPF over DMVPN but do you actually know why? There is actually no technical reason why you cannot run OSPF over DMVPN but it does not scale very well. So why is that? The reason is that OSPF is a link state protocol so each spoke […]
How to read and understand the OSPF Database
Just a quick post to point you all over to a fantastic resource on How to Read and Understand the OSPF Database Written by Alfred Lumbis who I believe is a Cisco employee it goes through step by step what you need to do to fully understand the OSPF database Extract: OSPF, being a link-state […]
OSPF Anatomy of an Internet Routing Protocol
Following a recommendation from Brian Dennis on a recent INE Vseminar. I decided to purchase a book every network engineer should read. OSPF Anatomy of an Internet Routing Protocol by John T Moy, the man who wrote the RFC for OSPF. The RFC for OSPFv2 is here RFC2328 Although this book was written many years […]
CCIE Troubleshooting OSPF Hello Mismatch
Whilst reading the RFC for OSPFv2 – RFC 2328 I noticed an interesting sentence in section 10.5 about Receiving Hello Packets. The generic input processing of OSPF packets will have checked the validity of the IP header and the OSPF packet header. Next, the values of the Network Mask, Hello Interval, and Router Dead Interval fields […]
OSPF cost calculation
How does OSPF work out the interface cost? By default the ospf cost calculation is based on a reference bandwidth of 100Mb The calculation is Reference BW / Configured BW So for a 100Mb interface using the default reference bandwidth of 100Mb The calculation is 100Mb / 100Mb =1 However with modern networks 100Mb is […]
OSPF LSA Types Cisco for IPv4 Explained
This post will explain the Cisco OSPF LSA Types hopefully in an easy to understand format. These are my study notes for the CCIE R&S Lab exam. The main LSA types are: LSA type 1 (Router) Every router generates a type 1 LSA with the state of each of their links, there are 3 types […]