3Com Switch 4800G 24-Port Configuration Manual page 244

244 C 24: IP R
HAPTER OUTING
n
Routing Protocols and
Routing Priority
O
VERVIEW
Operational scope
Interior gateway protocols (IGPs): Work within an autonomous system,
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including RIP, OSPF, and IS-IS.
Exterior gateway protocols (EGPs): Work between autonomous systems. The
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most popular one is BGP.
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
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protocol.
Link-state protocols: OSPF and IS-IS.
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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.
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Multicast routing protocols: PIM-SM and PIM-DM.
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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.
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
DIRECT
OSPF
IS-IS
STATIC
RIP
OSPF ASE
OSPF NSSA
IBGP
Priority
0
10
15
60
100
150
150
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