ISIS: An IntraDomain Routing Protocol

1
IS-ISAn Intra-Domain Routing Protocol

• ECE 697A Fall 2002
• Presenter Jianhong XIA
• 09/26/2002

2
Outline

• Background
• IS-IS Overview
• Feature
• Terminology
• Level-1 / Level-2 Routing Systems
• How IS-IS works
• Design Consideration
• New Features
• Conclusion
• References

3
Background OSI Model

• ISO/OSI Network Model
• Complete Suite of Routing Protocols

• IS-IS (Intermediate System-to-Intermediate
  System)
• ES-IS (End System-to-Intermediate System)
• IDRP (Inter-Domain Routing Protocol)

Layer 7 Application
Layer 6 Presentation
Layer 5 Session
Layer 4 Transport
Layer 3 Network
Layer 2 Link
Layer 1 Physical
4
Background OSI systems

• Two type of systems
• End System (ES)
• Hosts or Workstations
• Intermediate System (IS)
• Routers
• Level 1 Routers (L1 Routers)
• Level 2 Routers (L2 Routers)
• Level 1 and Level 2 Routers (L1L2 Routers)

5
Background OSI Network

• Level 1 routing is routing within a Level 1 area
• Level 2 routing is routing between Level 1 areas.

6
Background TCP/IP Model

• TCP/IP Network Model
• A Little Different from OSI Network Model

Layer 5 Application
Layer 4 Transport
Layer 3 Network
Layer 2 Link
Layer 1 Physical
7
Background IS-IS Goal

• IS-IS
• A Protocol of Network Layer
• An Interior Gateway Protocol (IGP)
• To Route in ISO Connectionless Network Protocol
  (CLNP) Networks.
• Now to Support Both CLNP and Internet Protocol
  (IP) networks.

8
Background IS-IS ES-IS

• ES-IS
• Discovery protocol
• Define how ESs and ISs learn about each other
• IS Hello messages (ISHs) and ES Hello Messages(
  ESHs )
• IS-IS
• Link-State Hierarchical Routing Protocol
• Flood link-state information between intermediate
  systems (routes)
• Build a complete picture of network topology

9
Background - RIP

• Advantages of RIP
• Extremely Simple to configure and deploy
• Designed for small or medium-size network
• Disadvantages of RIP
• Inability to scale to large network
• High recovery time/convergence time
• Routing loops/infinity problem

10
Background -OSPF

• No OSPF at that time
• OSPF was derived from
• SPF algorithm
• Fault-tolerant Broadcasting of Routing
  Information
• BBNs work on area routing
• IS-IS routing protocol
• OSPF can only support IP

11
IS-IS Overview - Feature

• Support Both CLNP and IP network
• Link State Protocol
• Similar to OSPF
• But OSPF Supports Only IP
• Hierarchical Routing
• Simplify Backbone Design
• Embraced by Large Tier1 ISPs
• Stable and Scalable
• Very Fast Convergence
• Flexible in terms of tuning and easily extensible
  to new features

12
IS-IS Overview - Terminology

• Terminology and Acronyms
• Intermediate System (IS)
• Router
• End System (ES)
• Network Host or Workstation
• Packet Data Unit (PDU)
• Analogous to IP packet
• Link State PDU (LSP)
• Routing Information Packet

13
IS-IS Overview - Concepts

• Network Nodes
• Hosts (ES)
• Level-1 Routers (IS)
• Level-2 Routers (IS)
• Link Types
• Point-to-Point Links (WAN serial, ptp FR)
• Broadcast Links (Ethernet, etc.)
• Sub-networks
• Point-to-Point subnetworks
• Broadcast subnetworks
• General Topology subnetworks.
• Areas
• A group of contiguous networks and attached hosts

14
IS-IS Overview - Areas
Area 49.001
L1
Level-1 Area
L1L2
Level-2 Backbone
Area 49.003
Area 49.0002
Level-1 Area
L1L2
Level-1 Area
L1L2
L1
L1
15
IS-IS Overview - Routing
Backbone
Area 49.0002
Area 49.001
Level-1 Routing
Level-1 Routing
Level-2 Routing

• IS-IS supports 2-level routing hierarchy
• Routing domain is carved into areas. Routing in
  an area is level-1. Routing between areas is
  level-2

16
IS-IS Overview Benefit

• Hierarchical Routing
• Simplify Backbone Design
• Level 1 ISs need to know only how to get to the
  nearest Level 2 IS
• Backbone routing protocol can change without
  impacting the Intra-area routing protocol.

17
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
18
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
Listening ISH packet
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
19
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
ES
ES
IS
IS
Listening ESH packet
ES
L2 Routing
IS
IS
IS
IS
ES
ES
20
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
21
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
22
IS-IS Overview How it works

• Source ES and Destination ES are on the same
  subnetwork

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
23
IS-IS Overview How it works

• Source ES and Destination ES are on the same area

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
24
IS-IS Overview How it works

• Source ES and Destination ES are on the different
  areas

L1 Routing
L1 Routing
ES
ES
IS
IS
ES
L2 Routing
IS
IS
IS
IS
ES
ES
25
IS-IS Overview - Decomposition
26
IS-IS Overview PDU types

• Types of PDUs (Protocol Data Units)
• Level 1 LAN IS to IS Hello PDU
• Level 2 LAN IS to IS Hello PDU
• Point-to-Point IS to IS Hello PDU
• Level 1 Link State PDU
• Level 2 Link State PDU
• Level 1 Complete Sequence Numbers PDU
• Level 2 Complete Sequence Numbers PDU
• Level 1 Partial Sequence Numbers PDU
• Level 2 Partial Sequence Numbers PDU

27
IS-IS Overview - Messages

• Link-State Update Messages
• IS generates update messages specifying the ESs
  and ISs to which it is connected, also associated
  metrics
• All neighboring ISs forward it to their
  neighbors, and so on.
• Sequence numbers terminate the flood.
• Sequence numbers distinguish old updates from new
  ones.
• Using these information, each IS builds a
  complete topology of the network.

28
IS-IS Overview - SPF

• Shortest Path First (SPF) Algorithm
• Any change of the topology in one area, SPF
  computation will be done in all the routers in
  this area
• L1L2 Routers
• Have 2 instances of SPFs
• Two databases separately
• Computation in L2 takes up to 150ms
• For about 1200 routers

29
IS-IS Overview - Metrics

• Arbitrary and typically assigned by network
  administrator
• Metrics
• Single link lt 64
• Path links sum of its link values
• But lt 1024
• Three optional metrics (costs)
• Delay
• Expense
• Error

30
IS-IS Overview - packets

• Three logical headers
• Common Header (8 Bytes)

Common Header
Packet Type-specific, fixed header
Packet type-specific, variable length header
Protocol Identifier
Header Length
Version
ID Length
Packet Type
Version
Reserved
Maximum Area Addresses
31
IS-IS Overview - IDRP

• Inter-Domain Routing Protocol
• Specifies how routers communicate with routers in
  different domains.
• Designed to operate seamlessly with CLNP, ES-IS,
  IS-IS
• Based on the Border Gateway Protocol (BGP).

32
IS-IS Overview Over Load

• Design Consideration (1)
• A special bit in each LSP
• called the LSPDB Over Load bit
• While having some problems
• A router could set the OL bit

33
IS-IS Overview Over Load
When R1 computes SPT, it will find That R5 LSP
has Overload-bit-set. Therefore R5 cannot be used
as Transit node and shortest path to R4 is
R1-gtR2-gtR3-gtR4
Rtr-2
Rtr-1
Rtr-3
Rtr-5

• Why/When use Overload-Bit ?
• When the router is not ready to forward traffic
  for ALL destinations
• Typically when IS-IS is up but BGP (or even
  MPLS) not up yet.
• When the router has other functions (Network
  Management)

Rtr-4
34
IS-IS Overview LSP flooding

• Design Consideration (2)
• LSP flooding on a LAN
• limited to 30 LSPs per second
• But on slow speed links,
• 30 LSP/second may be too much.
• Default time between consecutive LSP
• Minimum of 33 milliseconds
• Configurable

35
IS-IS Overview LSP triggering

• Design Consideration (3)
• What triggers a new LSP
• When something changes
• adjacency came up or went down
• interface up/down (connected IP prefix !)
• redistributed IP routes change
• inter-area IP routes change
• an interface is assigned a new metric
• most other configuration changes
• periodic refresh

36
IS-IS Overview LSP triggering

• New LSP
• Create new LSP
• install in your own LSPDB and mark it for
  flooding
• Send the new LSP to all neighbors
• Neighbors flood the ISP further
• IS-IS throttles it main events SPF/PRC
  computation, LSP generation
• Partial Route Calculation (PRC)
• Throttling slows down convergence
• Find a compromise

37
New Features Route Leaking

• RFC 1195
• No information is leaked from routers in L2 into
  routers in L1
• May result in sub-optimal routing
• New Features
• Allows redistribution of L2-IP routes into L1
  areas
• Enables Level 1-only routers to pick the best
  path to exit the area
• Enable shortest-exit and MED for BGP

38
New Features - Others

• Extension for MPLS-TE
• Support MPLS-Traffic Engineering
• Fast Hellos
• Hold-time can be set to 1 second
• By default, Hello packets sent every 333
  milliseconds
• Reduced link failure detection time
• But increased BW/buffer/CPU usage, potentially
  can cause instability
• Others

39
Conclusion - Advantage

• IS-IS
• Suitable to deploy in large networks
• Open, described in detail in RFC 1142
• Supports both CLNP and IP networks
• Embraced by the large Tier1 ISPs
• Proven to be a very stable and scalable, with
  very fast convergence
• Link-state hierarchical routing protocol
• Level 1 routing
• Level 2 routing

40
Conclusion - Disadvantage

• IS-IS
• Extra LSP flooding
• Constant LSP regeneration
• Processor intensive SPF runs, etc
• Configuration is not as simple as RIP
• Others ???

41
References

• http//www.cisco.com/univercd/cc/td/doc/cisintwk/i
  to_doc/osi_rout.htm
• http//nanog.org/mtg-0202/shankar.html
• http//www.nanog.org/mtg-0010/isis.html
• http//www.faqs.org/rfcs/rfc1195.html
• http//www.faqs.org/rfcs/rfc1142.html
• http//www.geocities.com/Heartland/4394/work/ospf.
  html
• http//www.cisco.com/univercd/cc/td/doc/cisintwk/i
  to_doc/ospf.htm