Why is ISIS protocol preferred in the ISP side?
In this lesson I am going to explain about ISIS briefly.
Introduction to ISIS
IS-IS is protocol agnostic as it runs at OSI layer 2. This means it can handle absolutely any IPv4, IPv6 or non-IP address scheme. ISIS is scalable.
ISIS levels
L1 is an intra-area. L1 is the same as a stub-network of OSPF network
L2 or L1/L2 for both intra-area & inter-areas. L2 is the backbone area in ISIS routing protocol
ISIS network types
ISIS supports P2P & broadcast networks. This is the limitation of ISIS.
ISIS LSP
The ISIS routing uses LSP for exchanging link state information. LSP is the same as LSA in OSPF.
LSP PDU-----------used to exchange link state information
SNP----------------to maintain and synchronize LSDB information
CSNP --------for the Pseudo node (for the DIS router) containing summary of all LSP
PSNP---------for the real node LSP, containing summary of partial LSP of the router
Figure. ISIS topology
ISIS network-entity
ISIS network entity, have AFI field, system-ID similar to OSPF router-ID. NSEL value which is 00.
49.0001 | 0000.0000.0001 | 00 |
49.0001 is the AREA-ID. The AREA-ID is 1-13 bytes
0000.0000.0001 is the SYSTEM-ID. The SYSTEM-ID is 6 bytes
00 is the SEL value. The NSEL value is 1byte
ISIS cost style and SPF calculation
Default cost is 10
For the SPF calculation ISIS will use incremental SPF (ISPF) when there is any change in the topology. PRC (Partial route calculation) is used if there is any change in the routing information. This will make ISIS more preferable in the case of route convergence than ISIS.
Narrow cost 1-63, which is used for simple network
Wide cost 1-16,777,215 which is used for a complex network
DIS concept
In broadcast network to reduce the adjacencies Pseudo-node is used. Pseudo-node is generated by the DIS router. Then every router will connect to the Pseudo-node, making it look like a P2P connection. And all routers will make adjacencies unlike OSPF. Preemption is present in ISIS unlike OSPF. That is why CSNP (complete sequence network protocol) is created. Because of the adjacencies between all routers. Pseudo-node will have CSNP (it will have complete list of LSP), whereas the other routers will have PSNP (partial sequence network protocol).
How to select DIS
DIS is the large cost router will become DIS on the network
What makes ISIS scalable
TLV (type, length & value)
TLV area expandable values
And also ISIS supports both layer 2 & layer 3 protocols making it the preferable routing protocol than OSPF.
Summary
The above concepts of ISIS make ISIS the number 1 choice in the eyes of service providers. To generalize
The scalability of ISIS is very high due to TLV
Fast convergence than OSPF due to ISPF and PRC
Reduced number of adjacencies
ISIS support both layer 2 & layer 3 protocols.
I hope this is been explanatory. If you have any questions you can comment me.
