Information Hiding in Routing Systems

One of the most effective methods to build stable and scalable routing designs is to summarize topology and reachability information in routing protocols. Summarization can help networks converge faster and limit the number of routers that need to perform route calculations when an event such as a link flap occurs. However, there are tradeoffs to

Faster EIGRP Feasible Successor Failover

The feasible successor is EIGRP predetermining an alternate next hop to reach a destination for fast failover. The feasible successor route is stored in the EIGRP topology table but is not installed in the RIB/FIB by default. The variance command can be used to promote the feasible successor to the routing table and traffic will

OSPF in Phase 1 DMVPN Networks

When designing hub and spoke networks, most architects will opt for distance or path vector routing protocols when given the choice. I don’t blame them, distance/path vector provides several benefits that work well in hub and spoke topologies. EIGRP based hub and spoke networks can scale to thousands of spokes without much work. Route summarization

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