ISIS part 3

ISIS knows 2 types of addresses which are NSAP (Network Service Access Point) address and NET (Network Entity Title) address. The NSAP address is the layer 2 (network layer) address for CLNS packets. The majority of routers have only 1 NSAP address. The NET address is the address of the host router and has the same fields as NSAP address. The difference is that NET address has the network selector bit set to 0.

The NSAP/NET address has 3 major segments which are: areaID.systemID.NSEL.

The area ID is used to route between the areas (L2 routing), and contains 2 or more digits. The area ID must be the same for every router in the same area. This address contains of AFI (Authentication and Format ID) to identify the assigning authority, IDI (Inter-Domain ID) to identify the domain, and High Order DSP to identify sub domain/area. Routers will check the area ID upon arriving packets and determines whether the area ID is the same. If it is the same, then it does an L1 routing. If the area ID is different, it will send the packet to an L1/L2 router where it will send the packet to another area.

The system ID is used to route routers or hosts in an area (L1 routing), and contains exactly 12 digits. Every L2 router must have a unique system ID in the entire domain, while every L1 router must have a unique system ID in the entire area.

The NSEL (Network Selector) field is used to do the routing within the ES (on a router it is marked by 0x00), and contains 2 digits. If the NSEL is set to 00, then it is the NET address instead of NSAP address.

A basic router setup using ISIS is as below:

(config)#int

(config-if)#ip router isis

(config)# router isis

(config-router)#net AA.BBBB.BBBB.BBBB.BBBB.CC (A=Area ID, B=System ID, C=NSEL).

IP router isis has to be set on every interface in the ISIS process. If a new router comes to the area, the new router interface to the existing neighbor router has to configure the router isis and net A.B.C command. In the meanwhile, on the neighbor router the router isis command needs to be applied to the interface connected to the new router.

-- 1 March 2011 --

ISIS part 2

ISIS router has 2 network types which are broadcasts (LAN and Multipoint WAN) and point-to-point (everything else). If the router configuration is point-to-point, the adjacency will be formed and the routers will send a complete sequence number package, which contain the routers link-state database and can therefore synchronize with the other. If the routers are in a broadcast network, the network will elect a DIS (Designated Intermediate System) to send the latest configuration to the other routers. The DIS is elected to the router with the highest ISIS interface priority (default = 64). If the interface priorities are the same, the highest SNPA (Sub network Point of Attachment) will determine the DIS. The SNPA is the highest MAC address if it is a LAN configuration, and the highest DLCI number in a Frame Relay configuration. A DIS does not sync with all other routers. Therefore, a pseudonode (a virtual router) is created by the DIS. In this way, all routers forms an adjacency with the pseudonode, including the DIS. The DIS can detect problems quickly because it sends hello packets every 3.3 seconds, whereas CSNP sends hello packets every 10 seconds.

ISIS has 4 types of PDU which are Hello, LSP, CSNP and PSNP packets.

Hello packets consists of 3 different packets:

1. - ESH (End System Hello) which is sent by an ES (host) to an IS (router)

2. - ISH (Intermediate System Hello) which is sent by IS and listened by ES.

3. - IIH (IS-IS Hello) which is send from an IS to another IS to form and adjeceny.

The router is able to send multiple hello packets at the same time (ISH and IIH).

LSP (Link State Packets) is used to build a link-state database sent from adjacent routers, which are necessary to build a routing table together with the SPF algorithm. There are 2 types of LSP which are the L1 LSP and L2 LSP.

CSNP (Complete Sequence Number PDU) contains a complete list of LSP, sent to adjacent routers to update and sync the adjacent router. This list is sent by the DIS every 10 seconds.

PSNP (Partial Sequence Number PDU) is used to request LSP and to acknowledge if an LSP has been received.

-- 1 March 2011 --

ISIS part 1

IS-IS (Intermediate System to Intermediate System) is one of the routing protocols using the SPF/Dijkstra algorithm. ISIS has an AD of 115 and has a default metric of 10. ISIS itself is originally designed to work at layer 2 (network layer) using CLNS (Connectionless Network Service), while its layer 3 routing protocol uses CLNP (Connectionless Network Protocol). There are some key terminologies in ISIS protocol. The protocol is divided into logical entities (domain and area) and physical bodies (end system –ES = host, and intermediate system-IS = router).

There are 3 types of ISIS routers:

1. L1 (Level 1) router which is placed in a single area. This router has only a database of the neighboring L1 routers. An L1 router can only connect to another L1 router in the same area. If this router wants to connect to an L1 router in another area, is has to through a L1/L2 router as its default gateway.

2. L2 (Level 2) router which is placed into border areas. This router can be used to route between different areas. L2 routers has only a database of the neighboring L2 routers and therefore can connect to only L2 routers in different border areas. If it wants to go to a L1 router, it has to connect to a L1/L2 router.

3. L1/L2 router is a router that can act as a L1 router (intra area) and L2 router (inter area). Therefore, this router has 2 databases, each for L1 routes and L2 routes. The L1/L2 router together with the L2 router makes sending data possible to other areas. The L1/L2 router can connect to another L1/L2 area in any area, to L2 router in any area, and to L1 router in the same area.

The L1 router acts as an access router, where the backbone routers consist of L1 /L2 routers, L2 routers or a combination of both. L1 has different hello packets than that of L2, and maintains adjacency with its neighboring L1 router only. The only possible way for an L1 router to communicate with an L2 router is through the L1/L2 routers (gateway). Changing the levels is possible, and can be done on the interface (locally) or on the router (globally).

To configure this on the interface use the command:

(config)#interface

(config-if)#isis circuit-type

To configure this on the router use the command:

(config)#router isis

(config-router)#is-type

To see the levels on the neighboring routers use the (config)#show clns neighbor command.

ISIS can do route summarization. The routes can be shown using the #show ip route isis command. This route summarization needs to be configured on the border router (L1/L2 or L2 router), else the summarization will not work. If there are multiple routes from an isis area to another isis area (load balancing), both of the border routers needs to have the same summary routes, else all the traffic goes to the non-summarized router.

-- 1 March 2011 --