- Links-State, use SPF algorithm,VLSM
- Uses helo to establish adjacencies
- Open Standard , comparable to OSPF
- was originally designed as IGP for CLNS Connectionless Network Service
- the OSI Layer 3 protocol is the CLNP Connectionless Network Protocol(unreliable like IP)
- uses CLNS address to identify routers and build the LSDB
Supports 2 Routing Levels
Level 1:
- occurs w/in an IS-IS area, routing is accomplished by looking at the lowest significant address portion (systemID) and choose the lowest-cost path
- Level 1 routing all devices having the same address
Level 2:
- learn the locations of L1 routing areas and build an interarea routing table
- all IS in L2 routing used the destination address to route traffic using the lowest cost path
Note:
- in
a. L1 Routers: (intra-area)
- learn about paths w/in the areas they connect
- maintain topology database on their area
b. L2 Routers: (interarea)
- learn about paths between areas
- backbone routers
c. L1/L2 Routers:
- acts as borders between L1 and L2
- similar to OSPF ABR, learn about paths both within and between areas
Note:
- the path of connected L2 and L1-2 routers is called backbone
Integrated or (Dual) IS-IS Routing
- for multiple protocols, IP and CLNS
- combines ISO CLNS and IP routing in one protocol
- uses its own PDUs to transport IP routing info and
IS-IS Design
- metric is not based on bandwidth
- value of 0-63 and 10 is the default
- narrow metrics are limited to max of interface metric of 63(6 bits) and max total path metric of 1,023(10 bit)
ES-IS Protocol
- End System to Intermediate System
- permits ESs(host) and ISs(routers) to discover one another
ES-IS performs the ff task:
- ESs send End System Hellos (ESHs) to well known address to announce presense to IS
- Routers (IS) listen to ESH to find the ES on a segment
- Routers (IS) transmits Intermediate System Hellos (ISHs) to well-known address announcing their presense to ES
- ESs listens for these ISH and randomly pick an IS to which they will forward all their packet
- When an ES needs to send a packet to another ES, it sends packet to one the IS on its directly attached network
Note:
- Routers uses IS-IS Hellos (IIHs) for establishing and maintaining adjacencies between ISs (Routers)
- IP systems do not use ES-IS
OSI Routing Levels
Level 0 Routing:
- ES to IS, within the same domain
- ES discover the nearest IS by listening to ISH packets
- when ES needs to send packets to another ES, it sends the packet to the IS on an attached network
Level 1 Routing:
- IS to IS within the same domain
Level 2 Routing
- IS to IS between different area within the same domain
- Also called interarea routing
- if a destination is in another area, the L1 IS sends the packet to the nearest L1-L2 IS, then forwards to L2 IS until reaches a L1-L2 or Level2 IS in the destination area
Level 3 Routing
- routing between separate domain
- comparable to BGP
- passes traffic to different AS
- not specified on Cisco routers but being accomplished through IDRP (Interdomain Routing Protocol)
Comparing IS-IS to OSPF
Similarities:
- both open standard link-state protocols
- both support VLSM
- similar mechanisms, LSAs, aging timers, LSDB synchronization to maintain LSDB
- uses SPF algorithm, similar update, decision and flooding process
- converge quickly after network changes
Differences
| OSPF | Integrated |
| Area border inside routers | Area border on links |
| Each link in only one area | Each router in only one area |
| More complex to extend backbone | Simple extension of backbone |
| Many small LSAs sent | Fewer LSPs sent |
| Runs on top of IP | Runs on top of Data Link Layer |
| Requires IP address | Requires IP and CLNS addresses |
| Default metric is scaled on interface bandwidth | Default metric is always 10 for all interfaces |
| Not easy to extend | Easy to support new protocols with new TLS tuples |
| Equipment personnel and information more available | Not as available |
II. IS-IS Routing Operations
- unlike IP addresses, CLNS addresses apply to entire nodes and not to interface
- requires CLNS addresses, are used by routers care called NSAP (Network Service Access Points)
- one part of NSAP is the NSAP selector (NSEL) byte
- when NSAP selector is 0, then the NSAP is called NET (Network Entity Title)
NSAP Address
- OSI network layer addressing is implemented with NSAP address
- An NSAP address identifies a system in the OSI network, an address represents an entire node, not an interface
- NSAP address are maximum of 20 bytes
- Higher-order bits identify the interarea structure
- Lower-order bits identify the systems within area
- ISIS Link-State Packets (LSPs) use NSAP addresses to identify the router and build the topology table
- NSAP address is equivalent to the combination of the IP address and upper-layer protocol in an IP header
Three Fields of NSAP Address (Cisco)
- Area Address
- System ID
- NSEL
IDP (Initial Domain Part)
- (AFI) Authority and Format Identifier and ( IDI) Initial Domain Identifier
- correspond to major network
AFI(Authority and Format Identifier) Value
- specifies the format of the address and the authority that is assigned
39 – ISO Data Country Code (DCC\
45 – E.164
47 – ISO 6523 International Code Designator (ICD)
49 – Private Locally Administered
IDI (Initial Domain Identifier)
- corresponds to subdomain under AFI, example: 47.0005 assigned to US govt., 47.0006 assigned to US DoD
DSP Domain Specific Part
- contributes to routing within an IS-IS routing domain
- comprises of HO-DSP (High-Order DSP), the system ID and the NSEL
- HO-DSP subdivides the domain into areas, equal to subnet in IP
- System ID identifies an individual OSI device
- NSEL identifies a process on the device and corresponds to a port or socket in IP, not used in routing decision
Typical NSAP Address Strucure
Example: HEX
49.1234.AA15.B322.1B41.00
49.1234 – area ID with the AFI of 49 means private
AA15.B322.1B41 – system ID, MAC address
00 – NSEL, zero means router
Rules for IS-IS as IGP:
1. Area address must be at least 1 Byte, separated into 2 parts
1st. the AFI set to 49 (private)
2nd. Area Identifies I, the octets of the area address after the AFI
2. System Id requires 6-byte , compliant with Govt. OSI Profile GOSIP version 2.0
3. NSEL always set to 0 for a router, 1 byte
Note:
- NSAP is called the NET when it has a NSEL of 0, router use NET to identify themselves in the ISIS PDUs
49.0001.0000.0c12.3456.00
AFI of 49
Area ID of 0001
Systemd ID of 0000.0c12.3456, the MAC address of a LAN interface
NSEL of 00
Note:
- the area address is also referred to as the prefix
- some documents uses the terms area ID and area address as synonyms
Area Address
- area address uniquely identifies the routing area and is associated with routing process
- system id identifies each node and used only in Level 1 routing and must be unique w/in an area
- all routers within the area must use the same area address
- an ES may be adjacent to a router only if they share a common area address
- area address is used in Level 2 routing
- ES recognize only IS and other ES on the same subnetworks that share the same area address
NET Address
- includes NSEL field(process of port number)
- NET is called when NSEL field of 0
- NET address refers to the device itself (equivalent to Layer3 OSI), used in routers to identify themselves in the LSP and to form the basis of routing calculations (SPF)
SNPA, Circuit and Link
- SNPA (Subnetwork Point of Attachment) is the point subnetwork services, equivalent of the Layer 2 address
- SNPA is assigned using the MAC address, virtual circuit ID from X.25 or ATM, DLCI form FR, HDLC
- Circuit is the
- Link is the path between two neighbor IS
Intra-Area and InterArea Addressing and Routing
- area address is used to route between areas, system ID is no considered
- system Id is used to route within area, area address is not considered
How to Forward packet
1. IS checks a packet destination
- different area, routed based on the area addres
- same area, routed based on system id
2. If you are a L1 router
- interarea packet sent closest to L1/L2 router
- intra-area packet are routed based on L1 database
3. IF you are a L1/L2 router
- inter-area packets are routed based on L2 database
- intra-area packets are routerd based on L1 database
Route Leaking
- helps reduce suboptimal routing by allowing Level 2 information to be leaked into Level 1
- use up/down bit in TLV (Type,Length,Value) field
if set to 0 the route originated w/in that Level 1 area
if set to 1 the route is redistributed into the area from Level 2
PDU (OSI) ----- Packet (TCPIP )
OSI PDU
- Network PDU = datagram, packet
- Data-Link PDU = frame
4 Types of PDUs
- ISIS PDU are encapsulated directly into a data-link fram anre no CLNP or IP Header on a PDU
- Hello (ESH,ISH,IIH)
- used to maintain and establish adjacencies
- ESH is ES to IS
- ISH is IS to ES
- IIH is IS to IS, between IS sent every 10 secs
- LSP (Link-State Packet)
- used to distribute link-state information
- PSNP (Partial Sequence Number PDU)
- used to acknowledge and request missing pieces of link-state information
- sent evry 3 secs
- CSNP (Complete Sequence Number PDU)
- used to describe/distribute the complete list of LSPs in the LSDB of a router, summary of LSDB
- periodically sent every 10 secs (broadcast) and only once for point-to-point
LSP
- router LSPs contain an LSP header and TLV fields
LSP Header – PDU type, length, LSP ID, LSP sequence number to identify duplicate, remaining lifetime of LSP
- area sequenced to avoid duplication of LSPs, begin at 1
- assist with synchronization,
- sequenced numbers are increased to indicate the newest LSP
- remaining lifetime is used for removal of outdated and invalid LSP from the topology table, known as count to zero operation, 1200 seconds is the default start value
TLV Fields – IS neighbor, ES neighbors, authentication info, attached IP subnet
- TLV examples. Area address type code is 1, IS neighbor type code is 2
Implementing
-
- Point-to-Point for all other media
- Has no concept of NMBA networks, it is recommended to use p2p over NBMA networks such as ATM,FR or X.25
- In broadcast mode, you must use enable CLNS mapping and include broadcast keyword
Implementing
- used for LAN and multipoint WAN interface
- DIS (Designated IS) creates psuedonode and represents LAN
- DIS is electected based on 1. only routers with adjacencies, 2. highest interface priority, 3. highes SNPA (MAC) break ties, there is no backup DIS
Note:
- interface have a default Level and Level 2 priority of 64, you can configure from 0 to 127 using the <
LSP and IIH Levels
- two levels nature of
- DIS representative of LAN
- LSPs are sent as unicast on p2p
- LSPS are sent multicast on broadcast networks
- LAN uses separate Level 1 and Level 2 IIHs, sent as multicast
- P2p uses a common IIH format, sent as unicast
Comparing Broadcast and Point-to-point Topologies
LSDB Synchronization
- single procedure for flooding, aging and updating LSP
- Level 1 LSPs are flooded within an area
- Level 2 LSPs are flooded throughout the Level2 backbone
- Large PDUs are divided into fragments that are independently flooded
- Each PDU is assigned an LSP fragment number, starting at 0 and incrementing by 1
- Separate LSDBs are maintained for Level 1 and Level 2 LSPs
- An LSP is typically flooded to all adjacent neighbors except the neighbor from which it was received
- LSP are identified by the system ID of the originator and an LSP fragment number starting at 0
- If an LSP exceeds the max transmission unit MTU, it is fragmented into several LSPs numbered 1,2,3 and so on
-
- When an IS receives an LSP, it examines the checksum and discards any invalid LSPs
- If the LSP is invalid and newer than what is currently in the LSDB, it is retained, ack, and given lifetime 1200 secs
- The age is decremented every second until it reaches 0, at which point the LSP is considered to have expired
- When the LSP has expired, it is kept for an additional 60 secs before it is flooded as an expired LSP
Sequence Number PDUs
SNP packets are used to acknowledge the receipt of LSPs and ensure synchronization and reliability
- separate CSNPs and PSNPs are used for Level1 and Level2 adjacencies
Two Types of SNP
1.PSNP are used for acknowledgement of LSP on p2p links and to request missing pieces of LSDB(subset only)
2.CSNP periodically multicast every 10 secs. by DIS on LAN to ensure LSDB accuracy and on p2p link when the link comes up
Neighbors and Adjacencies
- discover neighbors and from adjacencies by exchanging
- transmitted every 10 secs , command <
- hello identify itself and describe the parameters of the interface
- neighbors can advertise different hello intervals
L1/L2 Adjacencies
-
- L1-only routers form L1 adjacency with L1 and L1/L2
- L2-only routers from L2 adjacency with L2 and L1/L2
- L1/L2 can form both L1 adj and L2 adj
- L1-only and L2-only router will not form adjacency
- Cisco routers default is L1/L2
Rules for Adjacency
- L1 to L1 adjacency – area id must match
- L2 to L2 adjacency – even area id is different
- L1-only forms L1 adj with L1/L2 only if area id match
- L2-only forms L2 adj with L1/L2 even area id is different
- L1/L2 form both L1 and L2 adj if their area id is match
- L1/L2 form only L2 adj if their area id don’t match
Configuring Basic Integrated
- NET address identifies a device, an IS or ES and not an interface
- The OSI hello PDUs are sued to form the neighbor relationship between routers and SPF rely on configured NET address to identify the routers
-
Building the OSI Forwarding Database
1.
2. When the database are synchronized, routers use the LSDB to calculate the SPF tree to OSI destinations, the NETs
3. The total of the link metrics along each path determines the shortest path to any given destination
4. Level 1 and Level 2 routers have separate LSDBs and therefore may run the SPF algorithm twice
5. Routers insert the best paths in the CLSN routing table (OSI Forwarding database)
Building an IP Routing Table
1. Routers find the best way to reach each other based on the lowest cost (SPF algorithm)
2. L1 Router and L2 router performs completely separate route calculations
3. All the best paths are added to the routing table
4. A second algorithm is run (PRC) Partial Route Calculation for the ip routing table
5. Best path from the PRC algorithm end up in the IP Routing table
Integrated
1. Define areas, prepare the addressing plan (NET) for routers and determine interface
CLNS addreess of a router is called the NET, consist of 3 main parts
- prefix – defines the area that the router is a part of
- system ID – which uniquely identifies each device
- NSAP selector NSEL which must be 0
2. Enable ISIS on the router
Router(config)# router
- this enables
- area-tag – name for process
- when CLNS packets is also needed, CLSN routing is disabled by default clns routing command
3. Configure the NET
Router(config-router)#net
- configures an ISIS NET address for the routing process
- the NET is a combination of area number, system id and NSEL of 00 at the end
- the area number must be at least 1 byte in length and can be as long as 13 bytes
- system id has a fixed length of 6 bytes
4. Enable Integrated
Router(config-if)#ip router
- includes an interface in an
- use the clsn router issi [area-tag] command to enable
Example:
1. Simple Integrated
The configured router acts as an IP-only Level1-2 router. CLNS routing is not enabled. This configuration specifies only one
interface FastEthernet0/0
ip address 10.1.1.2 255.255.255.0
ip router
!
interface Serial 0/0
ip address 10.2.2.2 255.255.255.0
ip router
!
router
net 49.0001.0000.0000.0002.00
Optimizing
Changing
Router(config-router)#is-type [level-1 | level1-2 | level 2-only]
Changing
Router(config-if)#
Changing
Router(config-if)#
-configures metric on interface, form 1 to 63, default is 10
Router(config-router)#metric
Example:
R3 – it is appropriate to change the IS type to Level 2 only since it only needs Level 2 routing
R1 – is internal router, does not connect to in any other area and appropriate to configure it to IS type Level 1
R2 – must do both Level 1 and Level 2 routing, left by default. It is appropriate to set the
Remember that the metric for all interface is 10 regardless of the speed of the serial or FastEthernet.
Configuring Route Summarization in
Router(config-router)#summary-address
- creates summary, default is Level 2
Example:
Router3(config-router)# summary-address 10.3.2.0 255.255.254.0 level-1-2
- summarizes 10.3.20/23 into Level 1-2
Verifying
show ip protocols
Verifying CLNS Structures
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