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Who does not like a good old tunnel? There are different tunnel types available for us to use when building our networks. The ones we will cover here are GRE and IPsec, as well as briefly mentioning DMVPN, GET VPN and FlexVPN. GRE Generic Routing Encapsulation (GRE) is one of the most versatile tools to keep in your networking tool belt. This type of tunnel provides encapsulation without any sort of encryption.

In the beginning, God created IPv4 When the internet was first created, large public IPv4 address blocks were handed out and assigned to organizations. When the usage of the internet grew it became quite apparent that the available IPv4 address space would not be enough. One of the most implemented solutions to this problem is Network Address Translation (NAT), which allows Layer 3 capable devices to rewrite source and/or destination addresses.

This post will be covering some useful, but less heavy technologies, namely NTP and FHRP. These are essential to building a resilient and well functioning network. It’s funny, every time I work with NTP I am reminded of when I took a certain exam. During one of the labs there was a need for configuring NTP on a router - simple enough, I though. I spent about 40% of the entire allotted exam time on trying to get NTP to synchronize - turns out, that everything was correctly configured and NTP was just slow… 🤦‍♂️

Quality of Service (QoS) is a technology that you hope you never need to use, but when you do, it’s really handy! The core principal of QoS is to allow us to affect the network resource allocation based on the given service and it’s requirements. QoS is a technology that only becomes relevant when you have a network that is strained in one or more of the following ways: Insufficient bandwidth Jitter or latency Packet loss In situations where you are not experiencing the aforementioned issues, QoS will not bring any benefit to your network.

I have a special appreciation for the multicast technology. Back when I used to work in first line support for a Swedish service provider of IPTV, I was always intrigued by the efficiency of using multicast to deliver the TV to peoples homes. Using multicast drastically reduced the bandwidth requirements for the central streaming sources that were used to send out the TV broadcasts onto the fiber networks. Now, in recent years service providers have moved away from this and into using unicast streaming in stead.

BGP Path selection One of the most essential parts of BGP is its path selection algorithm. It dictates how BGP is to select which paths to promote to the RIB. Using the different Path attributes you can affect how routing of your prefixes is done within your network, in your peers network and even on the internet. The path selection is done in a ascending order, where the first unequal attribute will be the determinator of which path is selected.

Big Gaming Protocol The big bad Border Gateway Protocol (BGP) is, unlike the OSPF, EIGRP and ISIS, an eBGP designed to connect together large scale networks. Because BGP is built for scale, it is tuned to be a bit more stable than the IGPs. For example it does not allow for dynamic neighbor discovery, but rather each neighbor must be explicitly specified. The BGP hold timers are also a lot more generous than the IGP timers, with the default being 3 minutes.

OSPF version 3 To support IPv6 routing, OSPF version 3 (OSPFv3) was created. It has support for both IPv4 and IPv6, but is not backwards compatible with the version 2 standard. In fact, the protocol is designed to be independent on the address family type. The communication between routers is based entirely on IPv6 though, using link-local addresses. There is also support for multiple instances within a network, which allows you to configure which routers are able to form adjacencies.

Path selection for good old OSPF Path selection is based on the Shortest Path Tree (SPT) found using Dijkstra’s Shortest Path First (SPF) algorithm. Generally OSPF prefers intra-area routes, inter-area routes and external routes, ordered from most to least preferred. If two paths exists for a given route, within the same area, then the one with the lost total metric is injected into the RIB. If two routes have equal total metrics, then both are generally injected into the RIB.

Open Shortest Path First (OSPF) 🗺️ Areas are a way to divide an OSPF domain into smaller, logical chunks, which allows for less resources to be used by the OSPF process. Each interface on an OSPF router can only be member of a single area, however, an OSPF router can have interfaces in multiple areas, thus becoming an Area Border Router (ABR). A unique LSDB will be created for each area that a router is a member of.