What is Integrated IS-IS ?

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1.Introduction to Integrated IS-IS Routing Protocol

IS-IS is a dynamic routing protocol designed by the International Organization for Standardization (ISO) for its Connectionless Network Protocol (CLNP).

With the popularity of the TCP/IP protocol, in order to provide support for IP routing, the Internet Engineering Task Force (IETF) has extended and modified IS-IS in RFC1195 so that it can be applied to both TCP/IP and OSI (Open). In the System Interconnection environment, it is called Integrated IS-IS (Integrated IS-IS or Dual IS-IS).

In IS-IS, a router is described as an Intermediate System (IS), and a host is described as an End System (ES). Therefore, the protocol for providing communication between the host and the router is ES-IS; and the communication between the routers is IS-IS. Unlike using proxy ARP or IRDP in IP or configuring a default gateway on a host, CLNP forms an ES-IS adjacency between the ES and the IS.

With the construction of an IPv6 network, dynamic routing protocols are also required to provide accurate and effective routing information for IPv6 packet forwarding. The IS-IS routing protocol combines its own scalability with the support of the IPv6 network layer protocol to discover, generate, and forward IPv6 routes.

2.The basic principle of IS-IS

 IS-IS is a link-state routing protocol. Each router generates an LSP that contains the link state information of all IS-IS interfaces enabled on the router. By establishing IS-IS adjacencies with neighboring devices and updating the LSDBs of the local devices, you can synchronize the LSDBs with the LSDBs of other devices on the entire IS-IS network. Then, according to the LSDB, the SPF algorithm is used to calculate the IS-IS route. If the IS-IS route is the optimal route to the destination address, the route will be delivered in the IP routing table and the packet will be forwarded.

3.The classification of IS-IS routers

1, Level-1 router

A Level-1 router is responsible for routing in the area. It only forms a neighbor relationship with Level-1 and Level-1-2 routers in the same area. Level-1 routers in different areas cannot form neighbor relationships.

The Level-1 router is only responsible for maintaining the Level-1 Link State Database (LSDB). The LSDB contains the routing information of the local area. The packets that are forwarded to the nearest Level-1-2 router will sent to the local area

2, Level-2 router

The Level-2 router is responsible for inter-area routing. It can form neighbor relationships with Level-2 routers in the same or different areas or Level-1-2 routers in other areas. The Level-2 router maintains a Level-2 LSDB that contains routing information between areas.

All Level-2 (that is, Level-2 neighbor relationships) routers form the backbone of the routing domain and are responsible for communication between different areas. Level-2 routers in the routing domain must be physically contiguous to ensure continuity of the backbone network. Only Level-2 routers can exchange data packets or routing information directly with routers outside the area.

3, Level-1-2 router

A Level-1 and Level-2 routers are called Level-1-2 routers. They can form Level-1 neighbor relationships with Level-1 and Level-1-2 routers in the same area, or with other areas. Level-2 and Level-1-2 routers can form Level-2 neighbor relationships. Level-1 routers must be connected to other areas through Level-1-2 routers.

The Level-1-2 router maintains two LSDBs. The Level-1 LSDB is used for intra-area routes, and the Level-2 LSDB is used for inter-area routing.

4.IS-IS network type

IS-IS supports only two types of networks. According to different physical links, it can be divided into: broadcast links: Ethernet, Token-Ring, etc.; point-to-point links: PPP, HDLC, etc.

For a NBMA (Non-Broadcast Multi-Access) network, you need to configure a sub-interface. Note that the sub-interface type should be configured as P2P. IS-IS cannot run on point-to-multipoint link P2MP (Point to MultiPoint).

5.IS-IS network address

In IS-IS, you can define both the zone ID and the system ID through Network Entity Tiyle (NET). Even though integrated IS-IS is only used in the TCP/IP environment, since it is based on CLNP, the OSI CLNP address is still configured.

6. IS-IS packet type

There are several types of IS-IS packets: HELLO PDU (Protocol Data Unit), LSP, and SNP.

1、Hello PDU

Hello packets are used to establish and maintain neighbor relationships, also known as IS-to-IS Hello PDUs. Among them, Level-1 IS-IS in the broadcast network uses Level-1 LAN IIH; Level-2 IS-IS in the broadcast network uses Level-2 LAN IIH;P2P IIH is used in non-broadcast networks. Their message formats are different. In the P2P IIH, relative to the LAN IIH, there is an additional Local Circuit ID field indicating the local link ID, a Priority field indicating the priority of the DIS in the broadcast network, and a LAN ID field indicating the DIS and the pseudo node System ID are missing.

 2、LSP

Link State PDUs (LSPs) are used to exchange link state information. There are two types of LSPs: Level-1 LSPs and Level-2 LSPs. The Level-1 LSP is transmitted by the Level-1 IS-IS, the Level-2 LSP is transmitted by the Level-2 IS-IS, and the Level-1-2 IS-IS can transmit the above two LSPs.

The main fields in the LSP are explained as follows:

ATT field: When Level-1-2 IS-IS transmits a Level-1 LSP in the Level-1 area, if the ATT bit is set in the Level-1 LSP, the Level-1 IS-IS in the area can pass. This Level-1-2 IS-IS leads to the external area.

OL (LSDB Overload) field: Overload flag.

The LSP with the overload flag set will spread in the network, but will not be used when calculating the route through the overloaded router. That is, after setting the overload bit to the router, other routers will not use this router for forwarding when performing SPF calculation, and only calculate the direct route on the node. For more details, please refer to the -IS-IS overload bit described later.

IS Type field: Indicates whether the IS-IS type of the LSP is Level-1 or Level-2 IS-IS (01 means Level-1 and 11 means Level-2).

  3、SNP

The Sequence Number PDUs (SNPs) synchronize the LSDBs (Link-State DataBase) by describing the LSPs in all or part of the database, thus maintaining the integrity and synchronization of the LSDB.

The SNP includes a full sequence number message CSNP (Complete SNP) and a partial sequence number message PSNP (Partial SNP), and it can be further divided into Level-1 CSNP, Level-2 CSNP, Level-1 PSNP and Level-2 PSNP.

The CSNP includes summary information of all LSPs in the LSDB, so that the LSDB can be synchronized between adjacent routers. On the broadcast network, the CSNP is sent periodically by the DIS (the default transmission period is 10 seconds); on the point-to-point link, the CSNP is sent only when the adjacency is established for the first time.

The PSNP only lists the sequence numbers of one or more LSPs  which have been received recently. It can acknowledge multiple LSPs at a time. When the LSDB is found to be out of synchronization, the PSNP is also used to request neighbors to send new LSPs.