400-101: IS-IS

Understanding CLNS Addressing

• Apply to entire nodes and not interfaces.
• IS-IS requires CLNS node addresses to function properly.
• CLNS addresses used by routers are called network service access poins (NSAPs).
• One part of an NSAP address is the NSAP selector (NSEL) byte.
• When an NSAP is specified with an NSEL of 0, the NSAP is called the network entity title (NET).
• An NSEL of 0 identifies network services.

NSAP Addresses

• Maximum size of 20 bytes.
• The high-order bits describe inter-area structure, and the low-order bits identify unique systems within an area.
• IS-IS uses the OSI address in the LSPs to identify the router and build the topology table and the underlying IS-IS routing tree.
• OSI addresses are NSAPs. They contain the following:
• The device's OSI address.
• A link to the higher-layer process.
• Equivalent to the combination of the IP address and upper-layer protocol in an IP header.

IS-IS NSAP Address Structure

• Three fields: the area address, the system ID, and the NSEL.

+++++++++++++++++++++++++++++++++++++++++++++++++++++
|    IDP    |                 DSP                   |
+++++++++++++++++++++++++++++++++++++++++++++++++++++
| AFI | IDI |  High Order DSP  |  System ID  | NSEL |
+++++++++++++++++++++++++++++++++++++++++++++++++++++

• Initial Domain Identifier (IDI) identifies the domain of the NSAP address.
• Authority and Format Identifier (AFI) makes up the format of the address and the authority that assigned the address.
• Initial Domain Part (IDP) = AFI + IDI.

• Domain-Specific Part (DSP) contributes to routing within an IS-IS routing domain.
• High-Order Domain-Specific Part (HODSP) is more or less the OSI equivalent of a subnet in IP.
• System ID identifies an individual OSI device. (In OSI, a device has an address, where as in IP, each interface has an address.)
• NSEL identifies a process on the device. (It corresponds roughly to a port or socket in IP, not used in routing decisions.)
• DSP = HODSP + System ID + NSEL

Example:  49.0001.0000.0c12.3456.00

• AFI = 49 (private)
• Area ID = 0001
• System ID = 0000.0c12.3456
• NSEL = 0 (also called NET)

Identifying Sysems in IS-IS

• Unlike OSPF, an IS-IS router can be a member of only one area.
• Other restrictions:

1. All routers in an area must use the same area address.
2. ESs recognize only ISs and other ESs on the same subnetwork that share the same area address.
3. L1 intra-area routing is based on system IDs (must be unique).
4. L2 system IDs must also be unique.
5. It is customary to use a MAC address from the router as the system ID.
6. All system IDs in a domain must be of equal length.

NET Addresses

• NSAP addresses with an NSEL value of 0.
• Used to uniquely identify a host within an IS-IS routing domain.
• Because IS-IS originates from the OSI world, NET addresses are required even if the only routed protocol is IP.
• NET address (NSEL = 00) refers to the device itself.
• Routers use the NET to identify themselves in the LSPs.
• Addresses starting with the AFI value of 49 are private addresses, analogous to RFC 1918 for IP addresses.
• A circuit is the IS-IS term for an interface.

IS-IS PDUs

• Hello PDU (ESH, ISH, IIH): Used to establish and maintain adjacencies.
• Link-state PDU (LSP): Used to distribute link-state information.
• Partial Sequence Number PDU (PSNP): Used to acknowledge and request missing pieces of link-state information.
• Complete Sequence Number PDU (CSNP): Used to describe the complete list of LSPs in a router's LSDB.

• L1 and L2 establish separate adjacencies for L1 and L2, one for each level.
• Unlike OSPF, IS-IS routers in a LAN establish adjacencies with all other routers on the LAN.
• IIH PDUs announce the area ID.
• Separate IIH packets announce the L1 and L2 neighbors.
• Adjacencies form based on the area address communicated in the incoming IIH and the type of router (L1 or L2).
• L1 routers accept L1 IIH PDUs from their own area and establish adjacencies with other routers in their own area.
• L2 routers (or the L2 process within any L1/L2 router) accept only L2 IIH PDUs and establish only Level 2 adjacencies.
• Dijkstra's algorithm requires a virtual router (pseudonode) called the DIS to build a directed graph for broadcast media.
• DIS selection criteria:

1. highest priority (default 64, within 0-127)
2. highest MAC address

• The L1 DIS and L2 DIS on a LAN might or might not be the same router.
• Preemptive. Any adjacent IS with a higher priority automatically takes over the DIS role.
• No backup DIS.
• Routers on a LAN establish adjacencies with all other routers and with the DIS.
• Each IS is required to advertise a single adjacency to the pseudonode.
• A pseudonode LSP details only the adjacent ISs.
• The pseudonode LSP is used to build the map of the network and to calculate the shortest path first (SPF) tree (equivalent to the network LSA in OSPF).

• L1 LSPs are flooded within the local area.
• L2 LSPs area flooded throughout the backbone.
• Each IS originates its own LSP (one for L1 and one for L2).
• Identified by the originator's system ID and an LSP number starting at 0.
• If an LSP exceeds the MTU, it is fragmented into several LSPs, numbered 1, 2, 3, and so on.
• IS-IS maintains the L1 and L2 LSPs in separate LSDBs.
• IS examines the checksum of received LSPs and discards any invalid LSPs, flooding them with an expired lifetime age.
• If the LSP is valid and newer than what is currently in the LSDB, it is retained, acknowledged and given a lifetime of 1200 seconds.
• When the LSP expires, it is kept for an additional 60 seconds before it is flooded as an expired LSP.

• Sequence Number PDUs (SNPs) are used to acknowledge the receipt of LSPs and to maintain LSDB synchronization.
• Two kinds: Complete SNPs (CSNPs) and Partial SNPs (PSNPs).
• Adjacent IS-IS routers exchange CSNPs to compare their LSDBs.
• CSNPs are multicast every 10 seconds.
• Adjacent IS-IS routers use PSNPs to acknowledge the receipt of LSPs and to request transmission of missing or newer LSPs.CSNPs are not periodically sent on point-to-point links.
• A CSNP is sent only once, when the point-to-point link first comes up.
• After that, LSPs are sent to describe topology changes, and they are acknowledged with a PSNP.

Configuring IS-IS for IP

• OSI CLNS protocol must also be configured.
• Each IS-IS router requires a NET, and IS-IS packets are directly encapsulated onto the data link layer instead of traveling inside IP packets.
• A NET address identifies a device (an IS or ES), not an interface.

Basic Configuration for Integrated IS-IS

1. Define areas, prepare the addressing plan (NETs) for routers, and determine which interfaces to enable IS-IS.
2. Enable IS-IS on the router.
3. Configure the router's NET.
4. Enable IS_IS on the proper interfaces.

Three commands to enable Integrated IS-IS on a router for IP routing. Additional commands to fine-tune the IS-IS process.

• router isis: enables IS-IS as an IP routing protocol.
• net [number]: assigns a NET to a router to identify it.
• ip router isis [tag]: enables IS-IS on the interface (no network statement).

A router's CLNS address (NET) consists of three main parts:

1. The prefix, which identifies the area that the router is a part of.
2. The system ID, which uniquely identifies each device.
3. The NSEL, which must be 0.

• The IS-IS neighbor relationships are established over CLNS, not over IP.
• Two ends of a CLNS adjacency can have IP addresses on different subnets, with no impact on the operation of IS-IS.
• Multiple IS-IS processes is possible (marked with a tag). 
• The process name is locally significant. If omitted, Cisco IOS assumes a null tag.
• By default, the Cisco IOS makes the router an L1/L2 router.

• NET = area ID + system ID + NSEL (00).
• Area ID = 1-13 bytes.
• System ID = 6 bytes.

• If no tag is listed, the Cisco IOS assumes a tag value of 0.
• If only one IS-IS process is active on the router, not tag value is needed.

Example:

router isis
 net 49.0001.0000.0000.0001.00
!
interface ethernet0
 ip address 10.1.1.1 255.255.255.0
 ip router isis
!
interface ethernet1
 ip address 10.1.2.1 255.255.255.0
 router isis