Information Technology Reference
In-Depth Information
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 and IGRP are no longer CCDA exam topics. Table 10-2 provides a quick
high-level summary of which protocol should be selected.
Ta b l e 1 0 - 2
IGP and EGP Protocol Selection
Description
Routing
Protocol
Used to connect to an ISP
BGP
IGP used in enterprise networks, supports large network and multi-vendor
OSPF
IGP used in large enterprise networks with Cisco routers
EIGRP
Distance-Vector Routing Protocols
The first IGP routing protocols introduced were distance-vector routing protocols. They
used the Bellman-Ford algorithm to build the 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 seri-
ous 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 (30 seconds for RIP). In the period be-
tween 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 com-
monly said that distance-vector protocols “route by rumor.”
Having to wait half a minute 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—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). Fur-
thermore, 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 is a list of IP distance-vector routing protocols:
RIPv1 and RIPv2
■
EIGRP (which could be considered a hybrid)
■
RIPng
■
EIGRP
link-state routing protocol characteristics. Although EIGRP uses distance-vector metrics,
