Cisco Routing 4 - IS-IS - Intermediate System to Intermediate System

This entire series on routing was created when I was preparing for the Cisco 642-901 BSCI test, as my notes. I then made a simple edit and published the text. If you think something essential is missing, something is not described correctly or is not entirely clear, I will welcome your information in the comments.

IS-IS Properties

• standardized by ISO (open standard)
• uses Dijkstra's SPF algorithm, areas and hierarchical topology
• uses Hello packets to establish adjacency
• uses DR router election for broadcast multi-access
• it is a classless protocol / VLSM / CIDR
• uses summarization
• supports authentication
• runs on the OSI network layer (layer 3), it is an ISO CLNP protocol, and even though it only routes TCP/IP (thus also supporting IPv6), the packets through which IS-IS communicates are CLNS PDU (protocol data unit), so all routers must support the ISO CLNS protocol. Therefore, the IP addresses of the interface (they can be from different subnets) do not matter and adjacency will still be established (between neighboring routers)
• CLNS (Connectionless Network Service) is a network layer service that communicates through CLNP (Connectionless Network Protocol)
• uses Network Service Access Point (NSAP) addresses - an access point that lies between layers 3 and 4 of the OSI model - the OSI network services are available to the transport layer, there are several NSAP address formats, consisting of 3 parts - area address, system ID, NSAP selector, it has a length of 8 to 20 bytes, the beginning of 49 means a private address
  for Cisco, we start from the right (e.g. 49.0001.0002.0003.0004.00) - 1 byte (00) NSEL, 6 bytes (0002.0003.0004) system ID, the rest is the area ID (49.0001)
• NET address (Network Entity Titles) - area ID (first 1 or 2 octets), system ID (router identification, up to 8 octets - Cisco 6, usually MAC, but not necessarily), SEL (NSAP selector byte, 1 octet, represents a service on the router, Cisco 00)
• NET is NSAP, where the N-selector = 00
• does not use the IP protocol, but OSI networking
• LSP, Hello PDU, routing PDU are sent as OSI packets (not IP packets, but they can be encapsulated in them) = PDUs are encapsulated directly into the data-link frame
• more timers for tuning, fewer area types than OSPF, uses fewer link-state PDUs (LSP)
• more scalable (up to 1000 routers in a single area)
• lower CPU load, even faster convergence than OSPF
• uses Partial Route Calculations (PRC) to compute reachability
• hierarchy with Level 1 and 2 routers, as opposed to areas
• uses domains (similar to AS in TCP/IP) - network sections under the same administrative control
• area - a logical part of the network composed of interconnected routers and their data link
• L1 routers use LSP to build a topology table for the local area
• area boundaries are at network connections between routers - a router can be part of only one area (per routing process)
• creates adjacencies with all neighbors
• IS-IS backbone is a contiguous group of L2 and L1/L2 routers, each area is connected to it
• only one IS-IS routing process can run on a router
• uses only point-to-point (serial lines) and broadcast network (default for LAN), does not support point-to-multipoint and non-broadcast
• metric - cost max. 1024 for the path, max 64 for the link - the default is 10 for each link, manually set, for non-Cisco also delay (travel delay), expense (cost of using the link), error (probability of error)
• to enable IS-IS on a router, it is not enough to choose the protocol and set the networks using the network command (this command is not present here), but you must configure it on each interface where it is to be used
• Mesh Groups - a feature that optimizes LSP flooding in an NBMA network with point-to-point links

Hierarchy in IS-IS - Router Levels

IS - Intermediate System (router), ES - End System (station)

• Level 0 - End System to Intermediate System (ES-IS) in the same subnet
• Level 1 - L1 IS - routing between IS within the area, allows communication between ES in the area, manage the Link-state PDU database (LSPD), intra area
• Level 2 - L2 IS - routing between areas within the same domain, inter area
• L1L2 intermediate IS - connects L1 and L2, have separate link-state databases (L1 and L2)
• Level 3 - between domains (inter-domain) using the InterDomain Routing Protocol (IDRP), similar to BGP

IS-IS Router Types

• Designated Intermediate System (DIS) - (or also Designated Router - DR) elected based on priority (0-127), then higher SNPA (Subnetwork Point of Attachment - MAC in LAN or DLCI in Frame Relay), then system id in NSAP, a new election occurs immediately upon a change
• there is no BDR here

Protocol Data Unit - PDU in IS-IS

We can roughly say that PDU is a packet.

• Hello - ESH (from ES to IS), ISH (IS to ES), IIH (IS to IS), creates and maintains neighbor relationships
• Link State Packet - LSP - sends link-state info
• Complete Sequence Number - CSNP - distributes the router's complete link-state DB, sent by the DIS to ensure that all neighbors have the same table
• Partial Sequence Number - PSNP - announces, requests and acknowledges link-state info (LSP)

Cisco IOS Commands for Configuring IS-IS

ROUTER(config)#router isis 0   // if we have multiple processes, we specify them using a tag, default is 0

ROUTER(config-router)#net 49.1234.1111.1111.1111.00 // setting the router's address
ROUTER(config-router)#is-type level-1 // setting the instance to L1 router (level-1-2, level-2-only)
ROUTER(config-router)#redistribute eigrp 100 level-1 metric 50  // interconnecting from ISIS to EIGRP
ROUTER(config-router)#redistribute isis level-1 metric 512 10 255 1 1500 // interconnecting from EIGRP to ISIS

ROUTER(config-if)#ip router isis            // enables it for the interface, by default enables Level 1-2
ROUTER(config-if)#isis circuit-type level-1 // only L1 or level-2-only only L2, or level-1-2
ROUTER(config-if)#isis metric 35 level-2    // sets the metric for L2

ROUTER#show clns neighbors     // displays the System ID of all known (neighboring) ISIS routers (IS/ES)
ROUTER#show clns interface e0  // details about routing on the interface, if the Circuit ID is non-zero, the DIS is present
ROUTER#show isis database      // displays routing info about L1/L2 LSPs 
ROUTER#show isis route         // displays info about L1 routes
ROUTER#show ip route isis      // ISIS routes L1/L2, su - summary route (when we create a summary route, Cisco automatically creates a Null 0 interface and connects it to the summary