Is-Is - 3Com Switch 7700 Configuration Manual

128 C 5: IP R
HAPTER OUTING

IS-IS

P O
ROTOCOL PERATION
As shown in Figure 8, RTA and RTD are each configured to belong to only
one area, whereas RTB and RTC are both configured to belong to two
areas. RTB belongs to area0, which complies with the backbone area
membership requirement. However, RTC does not belong to area0.
Therefore, a virtual link must be set up between RTC and RTB to insure that
area2 and area0 (the backbone area) are connected.
Figure 8 OSPF Areas
area0
RTA
The backbone area (area0) cannot be configured as the STUB area and the
■
virtual link cannot pass through the STUB area. So, if a virtual link has been
set up between RTB and RTC, neither area1 nor area0 can be configured as
a STUB area. Only area2 can be configured as a STUB area.
Routers in the STUB area cannot redistribute the external routes.
■
Intermediate System-to-Intermediate System (IS-IS) intra-domain routing
information exchange protocol is the dynamic routing protocol used in the AS
issued by the International Organization for Standardization (ISO). An intermediate
system (IS) in the OSI reference model is basically equivalent to a router in the
TCP/IP reference model. The IS-IS protocol, based on the link state algorithm, uses
the Shortest Path First (SPF) algorithm. It is similar to the Open Shortest Path First
(OSPF) protocol.
Integrated IS-IS is an implementation of IS-IS for IP regulated by the IETF.
This section introduces IS-IS routing protocol terms.
Intermediate System (IS). An IS equals a router of TCP/IP. It is the basic unit in
■
the IS-IS protocol used for propagating routing information and generating
routes. In the following text, IS is equal to router.
End System (ES). An ES equals the host system of TCP/IP. An ES does not
■
process the IS-IS routing protocol, and therefore it can be ignored in the IS-IS
protocol.
Routing Domain (RD). A group of ISs exchange routing information with the
■
same routing protocol in a routing domain.
Area. Area is the division unit in the routing domain.
■
Link State DataBase (LSDB). All the link states in the network form the LSDB. In
■
an IS, at least one LSDB is available. The IS uses the SPF algorithm and the LSDB
to generate its own routes.
Link State Protocol Data Unit (LSP). In the IS-IS, each IS will generate an LSP
■
which contains all the link state information of the IS. Each IS collects all the
LSPs in the local area to generate its own LSDB.
Network Protocol Data Unit (NPDU). NPDUs are the network layer packets of
■
ISO and are basically equivalent to the IP packet of TCP/IP.
Designated Intermediate System (DIS), is the elected router on the broadcast
■
network, equivalent to the DR in OSPF.
area1
RTC
RTB
area2
RTD