1, IS-IS is using NASP address to build up adjacency between each node, not IP address.
2, IS-IS is used to solve L1 and L2 routing problem described in
https://yingsnotebook.wordpress.com/2017/05/27/osi-network-tcpip
Especially L1 is similar with OSPF Not So Stub Totally Stub area, L1 area has no information about l2 routing and any other l2 area’s routing, it has only a default router (L1L2 router) which will connect this L1 area to L2 area; L2 area is similar with OSPF backbone area, which has information of both L2 and L1 areas.
When OSI network model is used, L2 will carry only area id info because L2 only need to forward the traffic towards correct L1 area, it does not have to know all ES address in L1 area. When IP is used, L2 will have IP information for networks directly connected in L2 router, and also the networks routes learned from L1 route calculation.
L1L2 router will have two IS-IS link databases, one is for L1 area which it connects to, the other is for L2. L1 and L2 databases are separated and can not be shared with each other, but L1L2 router will advertise routes learned according to L1 calculation to L2 database.
3, In IS-IS there are two types of network link: point-to-point and broadcast link. While point-to-multipoint can be simulated as several point-to-point links.
All interfaces in broadcast link will participate in DIS selection, like DR and BDR selection in OSPF broadcast link, but unlike DR and BDR role in OSPF, DIS in IS-IS is not the only one router which can send out LSP update, instead, all routers in IS-IS broadcast link can send out LSP update. DIS is used to:
- Help routers on a broadcast segment to synchronize
- Representing the broadcast segment in the link-state database as a standalone object- The Pseudonode
4, Unlike OSPF which has several types of LSA, in IS-IS each router generate only one LSP, which includes all routes information of the router, LSP can be very long and need to be segmented to transfer in Layer two network, a sequence number will be used to identify the LSP packet sent by each router. Router selected as DIS will generate 2 LSP: One is the general LSP for routing information, the other is Pseudonode LSP to indicate broadcast segment information. LSPID will be system ID + Pseudonode ID(local circuit ID of the interface)
5, IS-IS has 3 adjacency states:
- Down: the initial state
- Initializing: IIHs have been received from the neighbout, but it is not certain that the neighbor is properly receiving this routers IIH
- Up: IIHs have been received and also it is certain that the neighbor is properly receiving this router’s IIHs
6, IS-IS is a true multiprotocol routing protocol in the sense that it does not require any particular Layer 3 routing to carry it packets, and in a single instance, it can carry informaton of destination described by different address families, for example IPv4 and Ipv6 can be carried at the same time a single LSP and maintained in the same link state datebase.
7, ISIS PDU type
IIH (IS-IS hello), ISIS adjacency is built when router received IIH from peer, this may cause problem in point-to-point link, so a 3way handshake is introduced to solve this problem.
Link state PDU (LSP), IS-IS has only one type of LSP, while there are several type of LSA in OSPF. similar to ospf LSA that are uniquely identified by their type and link-state ID. IS-IS LSPs are also identified by a number that consists of three parts: System ID, Pseudonode ID and LSP number
Complete Sequence Numbers PDUs CSNP: contain full sequence numbers of LSPs, the purpose is to advertise a complete list of LSPs in the senders link database.
Partial Sequence Numbers PDUS PSNP are used to request an LSP or ack its arrival.
8,The following are sample NETs-
1) 49.0001.1111.1111.1111.00
Area address = 49.0001
System ID = 1111.1111.1111
NSEL = 00
2) 49.0001.1234.AAAA.AAAA.AAAA.00
Area address = 49.0001.1234
System ID = AAAA.AAAA.AAAA
NSEL = 00
The NSEL (Network-Selector) is a field in the NSAP address that identifies the network layer service to which a packet should be sent. This part of the address for a router will always be 0x00. In the IS-IS routing protocol, the field is sometimes referred to as the SEL field
8, Ipv4 and ipv6 support in is-is
Single topology routing is where a single graph/topology is built with which different address-families (IPv4/IPv6) are imposed on top as “leafs” of the tree. Should a failure of a single node or link occur then both IPv4 and IPv6 routing can be influenced with them converging identically.
Multi-topology routing is the process of decoupling the address-family from a single SPF graph but rather generates multiple SPF graphs for each address-family.
A router running IS-IS for both IPv4 and IPv6 will not form anadjacency with a router running IS-IS for IPv4 or IPv6 only. In order to allowadjacency to be formed in mismatched address-families network, the adjacency–check command in IPv6 address family configuration mode must be disabled.
Yings notebook 