Information Technology Reference
In-Depth Information
it sends partial updates and maintains neighbor state information just as link-state proto-
cols do. EIGRP does not send periodic updates as other distance-vector routing protocols
do. The important thing to consider for the test is that EIGRP could be presented as a hy-
brid protocol. EIGRP metrics and mechanisms are discussed later in this chapter.
Link-State Routing Protocols
Link-state routing protocols address some of the limitations of distance-vector protocols.
When running a link-state routing protocol, routers originate information about them-
selves (IP addresses), their connected links (the number and types of links), and the state
of those links (up or down). The information is flooded to all routers in the network as
changes in the link state occur. Each router makes a copy of the information received and
for wards it w ithout change. E ach router independently c alc ulate s the be st paths to each
destination network by using the Dijkstra shortest path algorithm, creating a shortest path
tree with itself as the root, and maintains a map of the network.
After the initial exchange of information, link-state updates are not sent unless a change in
the topology occurs. Routers do send small hello messages between neighbors to maintain
neighbor relationships. If no updates have been sent, the link-state route database is re-
freshed after 30 minutes.
The following is a list of link-state routing protocols:
OSPF
■
IS-IS
■
OSPFv3
■
OSPF, OSPFv3, and IS-IS are covered in Chapter 11, “OSPF, BGP, Route Manipulation,
and Multicast.”
Distance-Vector Routing Protocols Versus Link-State Protocols
When choosing a routing protocol, consider that distance-vector routing protocols use
more network bandwidth than link-state protocols. Distance-vector protocols generate
more bandwidth overhead because of the large periodic routing updates. Link-state rout-
ing protocols do not generate significant routing update overhead but do use more router
CPU and memory resources than distance-vector protocols. Generally, WAN bandwidth is
a more expensive resource than router CPU and memory in modern devices.
Ta ble 1 0 - 3 compares distance-vector to link-state routing protocols.
Ta b l e 1 0 - 3
Distance-Vector Versus Link-State Routing Protocols
Key
To p i c
Characteristic
Distance Vector
Link State
Scalability
Limited
Good
Convergence
Slow
Fast
Routing overhead
More traffic
Less traffic
Implementation
Easy
More complex
Protocols
RIPv1, RIPv2, EIGRP, RIPng
OSPF, IS-IS, OSPFv3
