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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter1 Product Overview
Layer 3 Software Features
interfaces and their metrics is used in OSPF LSAs. As routers accumulate link-state information, they
use the shortest path first (SPF) algorithm to calculate the shortest path to each node. Additional OSPF
features include equal-cost multipath routing and routing based on the upper-layer type of service (ToS)
requests.
OSPF employs the concept of an area, which is a group of contiguous OSPF networks and hosts. OSPF
areas are logical subdivisions of OSPF autonomous systems in which the internal topology is hidden
from routers outside the area. Areas allow an additional level of hierarchy different from that provided
by IP network classes, and they can be used to aggregate routing information and mask the details of a
network. These features make OSPF particularly scalable for large networks.
IS-IS
The Intermediate System-to-Intermediate System Protocol (IS-IS Protocol) uses a link-state routing
algorithm. It closely follows the Open Shortest Path First (OSPF) routing protocol used within the
TCP/IP environment. The operation of ISO IS-IS Protocol requires each router to maintain a full
topology map of the network (that is, which intermediate systems and end systems are connected to
which other intermediate systems and end systems). Periodi cally, the router runs an algorithm over its
map to calculate the shortest path to all possible destinations.
The IS-IS Protocol uses a two-level hierarchy. Intermediate Systems (or routers) are classified as Level
1 and Level 2. Level 1 intermediate systems deal with a single routing area. Traffic is relayed only within
that area. Any other internetwork traffic is sent to the nearest Level 2 intermediate systems, which also
acts as a Level 1 intermediate systems. Level 2 intermediate systems move traffic between different
routing areas within the same domain.
An IS-IS with multi-area support allows multiple Level 1 areas within in a single intermediate system,
thus allowing an intermediate system to be in multiple areas. A single Level 2 area is used as backbone
for inter-area traffic.
Only Ethernet frames are supported. The IS-IS Protocol does not support IPX.
IGRP
The Interior Gateway Routing Protocol (IGRP) is a robust distance-vector Inte rior Gateway Protocol
(IGP) developed by Cisco to provide for routing within an autonomous system (AS). Distance vector
routing protocols request that a switch send all or a portion of its rou ting table data in a routing update
message at regular intervals to each of its neighboring routers. As routing information proliferates
through the network, routers can calculate distances to all nodes within the internetwork. IGRP uses a
combination of metrics: internetwork delay, bandwidth, reliability, and load are all factored into the
routing decision.
EIGRP
The Enhanced Interior Gateway Routing Protocol (EIGRP) is a version of IGRP that combines the
advantages of link-state protocols with distance-vector protocols. EIG RP incorporates the Diffusing
Update Algorithm (DUAL). EIGRP includes fast convergence, variable-length subnet masks, partially
bounded updates, and multiple network-layer support. When a network topology change occurs, EIGR P
checks its topology table for a suitable new route to the desti nation. If such a route exists in the table,
EIGRP updates the routing table instantly. You can use the fast convergence and partial updates that
EIGRP provides to route Internetwork Packet Exchange (IPX) packets.