Routing protocols can be categorized as interior gateway protocols (IGPs) or exterior gateway protocols (EGPs). IGPs are meant for routing within an organization’s administrative domain (in other words, the organization’s internal network). EGPs are routing protocols used to communicate with exterior domains, where routing information is exchanged between administrative domains. Figure 3-2 shows where an internetwork uses IGPs and EGPs with multiple autonomous administrative domains. BGPs exchange routing information between the internal network and an ISP. IGPs appear in the internal private network.
Figure 3-2 Interior and Exterior Routing Protocols
One of the first EGPs was called exactly that: Exterior Gateway Protocol. Today, BGP is the de facto (and the only available) EGP.
Potential IGPs for an IPv4 network are
- RIPv2
- OSPFv2
- IS-IS
- EIGRP
Potential IGPs for an IPv6 network are
- RIPng
- OSPFv3
- EIGRP for IPv6
- IS-IS
RIPv1 is no longer recommended because of its limitations. RIPv2 addresses many of the limitations of RIPv1 and is the most recent version of RIP. IGRP is an earlier version of EIGRP. RIPv1, RIPv2, and IGRP are not included on the ENSLD 300-420 exam blueprint. Table 3-2 provides a quick high-level summary of routing protocol uses.
Table 3-2 Routing Protocol Uses
| Routing Protocol | Description |
| BGP | Used to connect to an ISP. |
| OSPF | IGP used in enterprise networks. Supports large networks and is multivendor. |
| EIGRP | IGP used in large enterprise networks with Cisco routers. |
| IS-IS | Used in large enterprise networks and by service providers. |
Distance-Vector Routing Protocols
The first IGP routing protocols introduced were distance-vector routing protocols. They used the Bellman-Ford algorithm to build routing tables. With distance-vector routing protocols, routes are advertised as vectors of distance and direction. The distance metric is usually router hop count. The direction is the next-hop router (IP address) toward which to forward the packet. For RIP, the maximum number of hops is 15, which can be a serious limitation, especially in large nonhierarchical internetworks.
Distance-vector algorithms call for each router to send its entire routing table to only its immediate neighbors. The table is sent periodically (every 30 seconds for RIP). In the period between advertisements, each router builds a new table to send to its neighbors at the end of the period. Because each router relies on its neighbors for route information, it is commonly said that distance-vector protocols “route by rumor.” A 30-second wait for a new routing table with new routes is too long for today’s networks. This is why distance-vector routing protocols have slow convergence.
RIPv2 and RIPng can send triggered updates—that is, full routing table updates sent before the update timer has expired. A router can receive a routing table with 500 routes with only one route change, which creates serious overhead on the network (another drawback). Furthermore, RFC 2091 updates RIP with triggered extensions to allow triggered updates with only route changes. Cisco routers support this on fixed point-to-point interfaces.
The following are IP distance-vector routing protocols:
- RIPv1 and RIPv2
- EIGRP (which could be considered a hybrid)
- RIPng
EIGRP
EIGRP is a hybrid routing protocol. It is a distance-vector protocol that implements some link-state routing protocol characteristics. Although EIGRP uses distance-vector metrics, it sends partial updates and maintains neighbor state information just as link-state protocols do. EIGRP does not send periodic updates as other distance-vector routing protocols do. The important point to consider for the ENSLD 300-420 exam is that EIGRP could be presented as a hybrid protocol. EIGRP metrics and mechanisms are discussed in detail later in this chapter.