IS-IS Standards History
- ISO 10589 specifies OSI IS-IS routing protocol for CLNS traffic
- Tag/Length/Value (TLV) options to enhance the protocol A Link State protocol with a 2 level
- RFC 1195 added IP support
- IS-IS runs on top of the Data Link Layer
- Requires CLNP to be configured
- RFC5308 adds IPv6 address family support to IS-IS
- RFC5I20 defines Multi-Topology concept for IS-IS
- Permits IPv4 and IPv6 topologies which are not identical
Terminology
- AIT: Authority and Format Identifier (the first octet of all OSI NSAP addresses—identifies format of the rest of the address)
- CLNP: Connection-Less Network Protocol (ISO 8473—the OSI connectionless network layer protocol—very similar to IP)
- ES: End System (the OSI term for a host)
- IS; Intermediate System(the OSI term for a router)
- ES-IS :End System to Intermediate System routing exchange protocol
- (ISO 9542—OSI protocol between routers and end systems)
- IS-IS : Intermediate System to Intermediate System routing exchange protocol (the ISO protocol for routing within a single routing domain)
- IS-IS Hello :A Hello packet (defined by the IS-IS protocol)
- LSP: Link State Packet (a type of packet used by the IS-IS protocol)
- TLV: Type Length Value
ISIS features
- Open standard (OSI)
- First developed by DEC and It was standardized by the ISO in 1992 communication between network devices which are termed Intermediate Systems by the ISO.
- Link-state routing protocol
- Classless
- VLSM and Manual summary
- Work on SPF or Dijkstra algonthm
- Incremental / trigger updates
- Updates are send as unicast /multicast (Layer 2)
- Metric= default (Cisco)
- Administrative distance = 115
- It is used by large ISP
- Protocol independent i.e. Support IP, IPX. hippie Talk
Integrated or Dual IS-IS
- Originally IS-IS was ECP for OSI layer, later developed to support TCP/IP called as DUAL IS-IS
- Dual IS-IS works with this change features:
- Function similar as any other IGP routing protocol
- Fast convergence ( default hello = 10 sec . dead timer = 3 times of hello)
- Stable
- Make efficient use of bandwidth, memory &CPU resources
METRIC in ISIS
- Cisco supports only default metric
- e. Metric= 10 on any type of interface ( ex : serial/Ethemet/fastethemet/gigethemet)
- Narrow metrics
- Max value on interface can be 64 (6bit) and 1024 (10bit) per path in routing table
- Other venders support different types of metrics (default, delay, expense, error)
IS IS Levels
- ISIS has a 2 layer hierarchy
- Level-2 (the backbone)
- Level- 1 (the areas)
- IS-IS support three type of router levels
- Level- 1 (LI) router (intra-area routing)
- Level-2 (L2) router (inter-area routing)
- Level-1-2 (L1L2) router (intra and inter-area routing)
- Manually Need to define IS-type
- Default is level 1 /Level 2
Level 1 Router
- Establishes neighbor ship with only LI and Ll/12 routers
- It acts like OSPF internal router (totally stub)
- Has the Level 1 LSDB with all routing information for the area
- It has a default route pointing towards nearest L1/L2 router
- LI should be continuous within a area.
Level 2 Router
- Establishes neighbor ship with only L2 and L1/L2 routers
- It act like OSPF Backbone router
- Has a Level 2 LSDB with all information about inter-area routing
- L2 should be continuous within a domain with other L2 orLl/L2 router
Level 1/Level 2 Routers
- Establishes neighbor ship with all LI , L2 and Ll/L2 routers
- It act like OSPF ABR
- Has two LSDBs:
- Level 1 for the intra-area routing
- Level 2 for file inter-area routing
- if the router has adjacencies to other areas, lt will inform the Level 1 routers (intra-area) it is a potential exit point for the area
- L1/L2 should be continuous within a domain with other L2 or Ll/L2 router
Backbone & Areas
- ISIS does not have a backbone area as such (like OSPF)
- Instead the backbone is the contiguous collection of Level-2/ or Level 1-2 capable routers
- ISIS area borders are on links, not routers
- Each router is identified with Network Entity Title (NET)
- NET is an NSAP where the n-selector is 0
Backbone must be L2 contiguous
EXAMPLE-1
EXAMPLE-2
An Addressing Example
Identifying Systems in IS-IS
- The area address uniquely identifies the routing area and the System ID identifies each node
- All routers within an area must use the same area address
- An ES maybe adjacent to a level-1 router only if they both share a common area address
- Area address is used in level-2 routing
- System ID maybe the MAC address (CLNS) or IP address of an interface (IP world)
- System ID used in level-1 routing and has to be unique within an area (and of same length)
- System ID has to be unique within level-2 routers that form routing domain
- General recommendation: domain-wide unique System ID
AREA COMPARISION OF OSPF with ISIS
ISIS PDU
ISIS PDU is directly encapsulated in to data-link header. There is no CLNP or IP header.
- Hello
- LSP
- CSNP
- PSNP
OSPF and ISIS Similarities
- Both use the same method of topology abstraction
- Both use [ practically}the same flooding algorithm
- Both use [practically] the same Dijkstra algorithm
- Both provide IP routing information
OSPF and ISIS Major differences
- History
- Transport protocol
- PDU format
- Details of hierarchical routing