When designing enterprise routing, network architects should first figure out whether it is best to use static or dynamic routing. Static routing involves manually defining routes on devices, while dynamic routing involves the use of a dedicated routing protocol that will build the routing table.
The most common dynamic routing protocols used in modern networks are as follows:
- RIP
- EIGRP
- OSPF
- BGP
Large networks, including the Internet, are based on the Autonomous System concept. An AS defines a group of network devices under a common administration, and most often, this defines a large company or a service provider. Routing protocols can be classified based on different criteria. Depending on the zone in which they operate, they can be considered interior (inter-AS) routing protocols or exterior (intra-AS) routing protocols.
Interior routing protocols can be classified further as distance vector protocols or link-state protocols, based on their behavior regarding the router update exchange process.
Distance vector routing protocols include the following:
- RIPv1
- RIPv2
- IGRP
- RIPng
Link-state routing protocols include the following:
- OSPF
- IS-IS
- OSPFv3
The main difference between distance vector routing protocols and link-state routing protocols is the way they exchange routing updates. Distance vector protocols function using the “routing by rumor” technique, as every router relies on its neighbors to maintain correct routing information. This means the entire routing table is sent periodically to all the neighbors.
Link-state routing protocols do not “route by rumor”. Instead, the routing devices exchange information between them about their link-states. Devices build a map of the network, independently and loop-free, based on the link-state information each router generates and propagates to the other routers.
Link-state protocols offer a series of important advantages when compared to distance vector protocols. The most important advantage relates to the convergence factor, which occurs much faster because as soon as a network topology changes, only that specific information is sent to the routers in a given area.
Exterior routing protocols run between ASs (inter-AS) and the most common example is BGPv4. The main reason for using different types of routing protocols to carry routes outside the AS boundaries is the need to exchange a large amount of route entries.
Routers use Administrative Distance in order to select the best route when multiple routing protocols advertise the same prefix. The AD value represents how trustworthy a particular routing protocol is.
RIPv2 has a classless behavior, meaning subnet mask information is sent in updates so VLSM can be achieved. RIPv2 also supports authentication to ensure that the person you are sending the information to is authorized to receive that information.
EIGRP is a unique protocol because it uses a hybrid approach, combining distance vector and link-state characteristics. Combining these features makes EIGRP very robust and allows for fast convergence, even in large topologies. EIGRP functions by using DUAL, and it is the only IGP that can perform unequal cost load balancing.
The OSPF protocol is one of the most complex routing protocols that can be deployed in modern networks. Unlike EIGRP, OSPF is an open-standard protocol. OSPF functions by using the Dijkstra SPF algorithm.
OSPF defines the following types of areas:
- The backbone area (Area 0)
- Non-transit areas (which must touch the backbone area)
OSPF defines the following router roles:
- Backbone router
- Internal router
- Area Border Router (ABR)
- Autonomous System Boundary Router (ASBR)
OSPF functions by exchanging Link-State Advertisements between neighbors. LSA types are as follows:
- Type 1 – Router LSA
- Type 2 – Network LSA
- Type 3 – Network Summary LSA
- Type 4 – ASBR Summary LSA
- Type 5 – External LSA
- Type 6 – Multicast LSA
- Type 7 – NSSA External LSA
- Type 8, 9, 10 – Opaque LSA
OSPF offers the capability to create different area types, which relates to the various LSA types presented below and the way they flow inside a specific area. The different area types are as follows:
- Regular Area
- Stub Area
- Totally Stubby Area
- Not-So-Stubby Area (NSSA)
- NSSA Totally Stubby Area
BGP is a highly scalable path vector routing protocol. Its metric is based on multiple attributes that you can tune and control in order to affect which path through ASs data is taken.
BGP can be used in transit networks (i.e., ISPs that want to provide transit to other destinations on the public Internet) or multihomed networks (i.e., big enterprises that connect to multiple ISPs).
BGP can use multiple attributes in order to define a routing policy, the most important of which are as follows:
- Next hop
- AS path
- Origin
- Local preference
- Multi-Exist Discriminator (MED)
- Atomic aggregate
- Aggregator
IPv6 allows the use of static routing and supports specific dynamic routing protocols (i.e., variations of the IPv4 routing protocols modified or redesigned to support IPv6), such as the following:
- RIPng (RIP new generation)
- OSPFv3
- EIGRPv6
- IS-IS
- BGP