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


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.

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

OSPF Autoconfiguration with Python

I find that sometimes when I want to lab up a specific scenario I have to do the same “router ospf 1234” and “network 198.51.blah.blah area blah” commands over and over just to get basic routing configured. Awhile ago I wrote an EIGRP auto configuration script but at that time I didn’t know about the

Soft Reconfiguration Inbound

This morning I set up an eBGP session between routers with a switch in the middle configured for SPAN. The SPAN session sent the output to my laptop for Wireshark analysis. Of course, I would need prefixes in BGP to have something to observe. I took inspiration from Free Range Routing’s sharpd feature and wrote