ISIS

1
IS-IS

• By Ted Cory

2
IS-IS Outline

• Brief History of IS-IS
• How IS-IS works
• Configuring IS-IS
• Troubleshooting IS-IS

3
About Me

• Senior WAN Tech, Raymond James Financial
• Certifications CCNP,MCSE,A

4

• A Brief History of IS-IS

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History of IS-IS

• IS-IS What is it?
• Stands for Intermediate System to Intermediate
  System
• Developed by DEC for DECnet Phase V
• Radia Perlman was chief developer
• Is the routing protocol for ISOs CLNS
• Is formally described in ISO 10589

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History of IS-IS

• IS-IS was passed over by IETF (OSPF was chosen).
• Was not chosen for political and technical
  reasons.
• Routes both IP and CLNS traffic.

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• How IS-IS works

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ISO terms

• Intermediate System- A router
• End System- A host
• LSP-Link State PDU
• Performs function of an LSA
• Is a packet itself, not encapsulated

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IS-IS similarities with OSPF

• Both maintain link state databases
• Both compute shortest path with Dijkstra based
  SPF algorithm. Both use hello packets
• Both can summarize address space
• Both elect designated router
• Both use areas
• Area borders are on links, not routers.

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IS-IS System Types

• Three types of Intermediate Systems
• Level 1(L1)-Routers that have no direct
  connectivity to another area
• Resides entirely within an area
• Similar to Internal Backbone OSPF router
• Has no knowledge of destinations outside its own
  area

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IS-IS System Types

• Level 2 (L2)-Routers that connect different areas
• Connect multiple areas
• Connect only to other L2 routers
• Similar to OSPF backbone router

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IS-IS System Types

• Level 1/Level 2 (L1/L2) Routers that are
  connected to both L1 and L2 routers.
• Maintain a separate link state for Level 1 and
  Level 2 connectivity
• Similar to OSPF ABRs
• Lets other routers know its connected to other
  networks via an attached bit in LSP.

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IS-IS System Types
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IS-IS Network Entity Titles

• Every Router has a unique address within an area.
  This address is called the routers NET.
• Network Entity Title (NET)- Composed of System ID
  and Area ID
• System ID-Similar to RID in OSPF
• Used by Level 1 routing
• Area ID-Up to three defined on a router at one
  time.
• Used by Level 2 routing

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IS-IS Network Entity Titles

• NET can be formatted in many ways.
• System ID of all nodes within an area must be the
  same length.
• Usually the MAC address

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IS-IS Network Entity Titles

• Three typical NET formats
• Eight octet format
• Area System ID SEL
• 08. 0000.4159.b9ab. 00
• (08.0000.4159.b9ab.00)

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IS-IS Network Entity Titles

• OSI NSAP address
• Domain Area System ID SEL
• 53.1256.23cd.0005. 0000.1256.b3df. 00
• (53.1256.23cd.005.0000.1256b3df.00)
• GOSIP NSAP address
• AFI ICD DFI AAI Reserved RDI Area System ID
  SEL
• 53.0005.73.0000c3. 0000. 12fd. 0007.
  0000.1256b3df .00

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IS-IS Network Entity Titles

• Three Rules for addresses
• NET must begin with single octet (ex 53.xxxx)
• NET must end with single octet, which should be
  00
• On Cisco routers NET must be six octets

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IS-IS Neighbors

• Routers exchange hellos
• Separate adjacencies for L1 and L2 routers.
• Default hello timer is 10 seconds.
• Default Hold timer is 3 times hello interval.
• L1 and L2 routers will NOT establish adjacencies

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IS-IS Neighbors

• Can view the neighbor table by using command
• show clns is-neighbors
• Valid states for neighbors are Init or Up.
• Init-Neighbor is known but not adjacent
• Up-Known and adjacent

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IS-IS Designated Routers

• Every router interface gets a priority for L1 and
  L2 routing
• Range is 0 to 127
• Default is 64
• Changed with isis priority interface command
• Priority 0 is ineligible
• Highest priority wins

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IS-IS Designated Routers

• Election process is different from OSPF
• No BDR elected
• Better priority forces reelection
• New LSPs must be flooded
• Not as stable

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IS-IS Useful commands

• Which-route
• Allows you to find IP address based on NLSP
  address.
• Show isis database
• Displays summary database info

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IS-IS

• R2show isis database
• ISIS Level-1 Link State Database
• LSPID LSP Seq Num LSP Checksum LSP Holdtime
  ATT/P/OL
• R1.00-00 0x0000008B 0x6843 55
  0/0/0
• R2.00-00 0x00000083 0x276E 77 0/0/0
• R2.01-00 0x00000004 0x34E1 57 0/0/0
• R3.00-00 0x00000086 0xF30E 84
  0/0/0
• ISIS Level-2 Link State Database
• LSPID LSP Seq Num LSP Checksum LSP Holdtime
  ATT/P/OL
• R1.00-00 0x00000092 0x34B 41 0/0/0
• R2.00-00 0x0000008A 0x7A59 115 0/0/0
• R2.01-00 0x00000004 0xC3DA 50 0/0/0
• R3.00-00 0x0000008F 0x0766 112 0/0/0
• High Hold time is good

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• Configuring IS-IS

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IS-IS Configuration

• Steps to configure basic IS-IS
• Determine area router will be located and
  interfaces that IS-IS will be enabled on.
• Enable IS-IS (router isis)
• Configure NET with the net command
• Enable IS-IS on interfaces (ip router isis)

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IS-IS Configuration

• Sample config
• Clns routing
• !
• Interface Ethernet0
• ip address 10.1.255.5 255.255.255.0
• ip router isis
• !
• Interface Serial0
• ip address 192.168.1.1 255.255.255.252
• ip router isis
• !
• Router isis
• Net 00.0001.0000.3090.6756.00

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IS-IS Other configuration options

• Setting summary addresses
• Configuring route authentication
• Changing router type
• Area migrations

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• Troubleshooting IS-IS

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IS-IS Troubleshooting

• Important to remember that CLNS is being used
• Similar to troubleshooting OSPF

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IS-IS Troubleshooting

• When inspecting routers config check the
  following
• Does the NET statement have the correct NET?
• Does the NET comply with NET format you are
  using?
• Is the command ip router isis on the correct
  interfaces?
• Are the IP addresses and subnet masks correct?

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IS-IS Troubleshooting

• Troubleshooting Adjacencies
• L1 routers can only form adjacencies with L1 or
  L1/L2 routers
• L2 routers can only form adjacencies with L2 or
  L1/L2 routers
• Are you receiving hellos from neighbor? If so
  are they at the correct level? Use the command
  debug isis adj-packets to view this info

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IS-IS Troubleshooting

• Troubleshooting Adjacencies
• Is the hello interval and hello multiplier the
  same between neighbors?
• Is authentication being used? If so are the
  password correct?
• Any ACLs?

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IS-IS Troubleshooting

• Troubleshooting the Link State Database
• Show isis spf-log lists all SPF calculations
• Debug isis spf-triggers displays messages about
  causes of SPF calculation
• Debug isis spf-statistics Shows SPF calculation
  itself. Also shows how long SPF calculation took.

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Sources

• Routing TCP/IP Volume 1
• By Jeff Doyle
• Interconnections, Second Edition
• By Radia Perlman
• Ciscos website
• http//www.cisco.com/warp/public/97/is-is-ip-confi
  g.pdf