244 CHAPTER 24: IP ROUTING OVERVIEW
Operational scope
■ Interior gateway protocols (IGPs): Work within an autonomous system,
including RIP, OSPF, and IS-IS.
■ Exterior gateway protocols (EGPs): Work between autonomous systems. The
most popular one is BGP.
n
An autonomous system refers to a group of routers that share the same routing
policy and work under the same administration.
Routing algorithm
■ Distance-vector protocols: RIP and BGP. BGP is also considered a path-vector
protocol.
■ Link-state protocols: OSPF and IS-IS.
The main differences between the above two types of routing algorithms lie in the
way routes are discovered and calculated.
Type of the destination address
■ Unicast routing protocols: RIP, OSPF, BGP, and IS-IS.
■ Multicast routing protocols: PIM-SM and PIM-DM.
This chapter focuses on unicast routing protocols. For information on multicast
routing protocols, refer to the “Multicast Routing and Forwarding Overview” on
page 701.
Version of IP protocol
IPv4 routing protocols: RIP, OSPFv2, BGP4 and IS-IS.
IPv6 routing protocols: RIPng, OSPFv3, IPv6 BGP, and IPv6 IS-IS.
Routing Protocols and
Routing Priority
Different routing protocols may find different routes to the same destination.
However, not all of those routes are optimal. In fact, at a particular moment, only
one protocol can uniquely determine the current optimal routing to the
destination. For the purpose of route selection, each routing protocol (including
static routes) is assigned a priority. The route found by the routing protocol with
the highest priority is preferred.
The following table lists some routing protocols and the default priorities for
routes found by them:
Routing approach Priority
DIRECT 0
OSPF 10
IS-IS 15
STATIC 60
RIP 100
OSPF ASE 150
OSPF NSSA 150
IBGP 255
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