When designing enterprise routing, network architects should first figure out whether they should use static or dynamic routing. Static routing implies manually defining routes on devices and dynamic routing implies using a dedicated routing protocol that will build the routing table. The dynamic routing protocols most often used in modern networks are EIGRP, OSPF, and BGP.
Large networks, including the Internet, are based on the Autonomous System (AS) concept. An AS defines a group of network devices under a common administration and most often this defines a large organization or an ISP. Routing protocols can be classified based on different criteria. Depending on the zone in which they operate, they can be interior (inter-AS) routing protocols or exterior (intra-AS) routing protocols.
Interior routing protocols can be further classified as distance vector routing protocols or link-state routing protocols, based on their behavior regarding the router update exchange process. Distance vector routing protocols include RIP, IGRP, and RIPng. Link-state routing protocols include OSPF, IS-IS, and OSPFv3.
The main difference between distance vector routing protocols and link-state routing protocols is the way they exchange routing updates. Distance vector routing 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 the neighbors.
Link-state routing protocols do not generally “route by rumor” like distance vector routing protocols do. 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 routing protocols offer a series of important advantages compared to distance vector routing protocols. The most important advantage relates to the convergence factor. Link-state routing protocols converge must 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 different types of routing protocols are used to carry routes outside of the AS boundaries is the need to exchange a large amount of route entries.
Routers use Administrative Distance (AD) to select the best route when multiple routing protocols advertise the same prefix. The AD value represents how trustworthy a particular routing protocol is.
Route summarization helps make the routing design more scalable, regardless of the routing protocol used (e.g., RIPv2, EIGRP, or OSPF). It also helps place network addresses into usable blocks (listed below), which can be used for multiple purposes:
- NAT blocks
- Blocks used in redistribution
- Blocks for management VLANs
- Blocks for content services
A special form of route summarization can be implemented in the form of default routing. Every modern network usually uses some type of default routing. The best practice is to dynamically advertise the default route 0.0.0.0/0 out of the Enterprise Network to the ISPs, as opposed to performing a static route configuration on every router in the organization.
Route filtering allows the control of network traffic flow and prevents unwanted transit traffic, especially in situations that feature redundancy or multiple paths. Route filtering protects against erroneous routing updates and there are several techniques that can be used in this regard. For example, with OSPF, if Core Layer connectivity is lost, traffic should not be rerouted through a remote site.
There are two ways to migrate between routing protocols:
- Using AD
- Performing redistribution by moving the boundary routers in small steps
Enhanced Interior Gateway Routing Protocol (EIGRP) is a unique protocol in that 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 IGRP that can perform unequal cost load balancing.
When analyzing scalability on a router, there are three resource areas that should be taken into consideration: memory, CPU, and interface bandwidth.
The most efficient methods for limiting EIGRP queries and achieving a scalable EIGRP design are as follows:
- Using a solid summarization design
- Using distribute lists
- Using stub areas
- Ensuring there are feasible successors
- Implementing equal cost routing
- Avoiding scaling by multiple EIGRP ASs, if possible
Open Shortest Path First (OSPF) protocol is one of the most complex routing protocols that can be deployed in modern networks. OSPF is an open-standard protocol, while EIGRP is not. OSPF functions by using the Dijkstra SPF algorithm and by exchanging Link-State Advertisements (LSAs) 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
- Types 8, 9, and 10 – Opaque LSA
OSPF offers the capability to create different area types that relate to the various LSA types presented above 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)
- Totally not so stubby area (totally NSSA)
The major points that should be taken into consideration when designing a scalable OSPF design are:
- The number of adjacent neighbors the router has (OSPF neighbors in this case)
- The number of adjacent routers in an area
- The number of areas supported by each router
- Choosing the designated router
BGP is a highly scalable path vector routing protocol. Its metric is based on multiple attributes that can be tuned and controlled to decide which path the AS’s data will take.
BGP can be used in transit networks (ISPs that want to provide transit to other destinations on the public Internet) or in multi-homed networks (big organizations that connect to multiple ISPs).
When using BGP as an interior routing protocol, a full mesh of iBGP routers is necessary because they do not re-advertise routes that were learned from other iBGP peers, according to BGP protocol behavior.
Two methods for scaling iBGP and avoiding the need for a full-mesh topology are as follows:
- Route Reflectors
- Confederations
IPv6 allows the use of static routing and also supports specific dynamic routing protocols that are variations of the IPv4 routing protocols modified or redesigned to support IPv6:
- RIPng
- OSPFv3
- EIGRPv6
- IS-IS
- BGP