Is-Is Area - Zte ZXR10 GER User Manual

ZXR10 GER (V2.6.03) General Excellent Router User Manual Volume-I
PDU
Link State PDU
Dijkstra SPF
Algorithm
Reduce Traffic
Three Types
212 Confidential and Proprietary Information of ZTE CORPORATION
Since the IS-IS protocol is based on CLNS (not IP), IS-IS uses
Protocol Data Unit (PDU) defined by ISO to implement
communications among routers. The types of PDUs used in the
IS-IS protocol are as follows:
Call PDU
Link state PDU (LSP)
Serial Number PDU (SNP)
Where, call PDU is similar to the HELLO packet in the OSPF
protocol, which is responsible for the formation of the adjacency
between routers, discovery of new neighbors and the detection
of exit of any neighbors.
IS-IS routers exchange routing information, set up and maintain
link state database by use of link state PDUs. An LSP indicates
important information about a router, covering area and
connected network. SNP is used to ensure reliable transmission
of LSPs. SNP contains summary information about each LSP on a
network.
When a router receives an SNP, it compares SNP with link state
database. If router loses an LSP in SNP, it originates a multicast
SNP and asks for necessary LSPs from other routers on the
network. LSPs are used in conjunction with SNPs so that IS-IS
protocol can complete reliable route interaction on a large
network.
Likewise, the IS-IS protocol also uses the Dijkstra SPF algorithm
to calculate routes. Based on the link state database, the IS-IS
protocol uses the SPF algorithm to calculate the best route and
then adds the route to the IP routing table.

IS-IS Area

For convenience of link-state database management, concept of
IS-IS area is introduced. Routers in an area are only responsible
for maintaining the link state database in the local area to
reduce the traffic of the routers themselves.
IS-IS areas are classified into backbone areas and non-backbone
areas:
Routers in the backbone area have the information about the
database of the entire network.
Routers in a non-backbone area only have information about
the area.
Based on the area division, IS-IS defines three types of routers:
L1 router: Exists in a non-backbone area and only exchanges
routing information with L1 router and L1/L2 router in the area.
L2 router: Exists in the backbone area and exchanges routing
information with other L2 routers and L1/L2 routers.