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OSPF LSA5 Forwarding-Address Field (FA)

Published on March 4, 2013 By Jose Miguel Huertas

The Forwarding-Address (FA) field and how it’s processed to make a routing decision when dealing with type 5 LSA is something very interesting. In this post I will try to bring some light to this feature. First of all, what’s the Forwarding-Address (FA) field? Well, it’s a field located in the type 5 and type [...]

Interpreting the OSPF Database::Part II

Published on March 16, 2011 By Jose Miguel Huertas

We already talked about the  LSAs type 1 and LSAs type 2 in the post Interpreting the OSPF Database::Part I, and found out how to get info from these LSAs in the OSPF Database. Let’s talk today about the LSA type 3, also called “Summary LSA”. The network which we will focus on for this [...]

Interpreting the OSPF Database::Part I

Published on February 25, 2011 By Jose Miguel Huertas

OSPF is probably the most common IGP in both the enterprise and service provider networks. It’s a pretty manageable protocol and easy to understand, although sometimes we may get confused with the different announcement types and the situations where they apply. So let’s see if we can put some light in all this. This is [...]

SPF calculation in OSPF and IS-IS

Published on February 16, 2011 By Jose Miguel Huertas

Today I would like to talk about Shortest Path First (SPF) calculation in OSPF and IS-IS. We already know that both OSPF and IS-IS are link state routing protocols and that they use the Dijkstra SPF algorithm to build a shortest path tree from a specific node acting as root to all other nodes in [...]

BGP Communities: OSPF Domain-ID and OSPF Route Type

Published on January 19, 2011 By Jose Miguel Huertas

On my last post I mentioned the BGP communities; OSPF Domain-ID and OSPF RT. My intention now is to go deeper into these BGP communities and show how we can work with them. When OSPF is being used as the PE-CE protocol, the PEs needs to inform other PEs how to interpret the routes they [...]

Troubleshooting OSPF network type mismatches against a Backbone device

Published on January 9, 2011 By Daniel Rodriguez

I’m going to start with an interesting troubleshooting scenario that can be very tricky to diagnose. Let’s imagine a ticket that is worded like this: “R2 lost the prefixes to the networks behind the backbone, find the problem and fix it. At the end of this ticket, R3 should be able to ping BB1 redistributed [...]

Using OSPF sham-links on a basic MPLS VPN

Published on January 8, 2011 By Jose Miguel Huertas

OSPF is used very often in MPLS/VPN environments. It’s common to see several sites communicate through a MPLS backbone with the routing protocol used between CEs-PEs being OSPF. The common behavior is that after prefixes were announced from the CE to the PE by OSPF, they are later redistributed into MP-BGP and the vice versa on the [...]