OSPFv2 Link State Advertisements

Link State Advertisements (LSAs) are one of hardest things to understand while learning OSPF. I believe that having a basic understanding of the LSA types and being able to interpret them is crucial to understanding OSPF operation. LSAs are flooded to populate the Link State Database (LSDB) on routers participating in OSPF. The content of

Back to the Books

Back in mid-February, I sat and passed Cisco’s 350-401 Enterprise Core exam. The final crunch before the exam wore me thin so I took a few days of rest and celebration. The current CCNP structure requires passing a core exam and 1 concentration exam within the same track. During my ENCOR studies I was torn


On the surface Cisco’s HSRP and the IETF’s VRRP appear to be the same thing. They both provide IP next hop redundancy by using an election process to determine which router should host a virtual IP address. The router that wins the election will host the VIP and respond to ARP requests for the VIP.

Does the Internet Still Matter?

I know, it seems like a silly question to ask. Of course, the internet matters. Probably more than ever! I’d argue that for most people the only thing that actually matters is the ISP they’re connected to and the big content providers. It turns out that efficiently fetching content over long distances is a hard

It works! (kinda)

We finally have spoke to spoke traffic working in our phase 1 DMVPN! After fixing the BGP third party next hop issue described in the last post we still had reachability issues between spokes. The spokes could reach the hub and the hub could reach the spokes just fine. Traffic transiting the hub to route

Home Lab DMVPN Lessons Learned

My last post was about the home-to-home DMVPN we’ve been working on. The design intent was to build a phase 1 DMVPN so spoke to spoke traffic should use the hub as a transit node. It wasn’t until we tried to forward traffic from spoke to spoke that we realized we have issues. Spoke to

eBGP, TTL and Connected-Check

It is well known that eBGP packets default to having a Time to Live (TTL) value of 1. This has caused confusion for many network practitioners who wish to run eBGP between loopback addresses of directly connected routers. This misunderstanding sometimes leads to ebgp-multihop being configured when it is not necessary. This also gave me

Dead Peer Detection in Multipath Networks

When I learned about Dead Peer Detection, I assumed it to be some generic IPsec keepalive and didn’t put much thought into it. A few weeks ago, I became more interested in DPD and started reading RFC 3706. I learned that DPD is a feature of IKE and I started considering how the protocol may

Next Hop Recursive-Looped

In this morning’s lab exercise I intentionally designed and configured a network to cause recursively looped routing lookups. When I started the experiment, I did not know this is the term for this condition. I simply wanted to see what would happen if the route to the destination was also the route to the next