IS-IS

Oct 28 2008

ISIS adjacencies

IS-IS adjacencies

Point-to-point adjacencies are formed in a slightly differnet manner than those on broadcast links. Therefore different types of IS-IS hello packets are used. The three different types of IS-IS

hello packets are:-

Point-to-point hello
Level 1 LAN hello
Level 2 LAN hello

As specified in ISO 10589, the TLVs used in Point-to-point hellos are limited to the following:-

Area Addresses (Type 1 )
Padding (Type 8 )
Authentication Type (Type 10 )

Point-to-Point adjacencies

On a Point-to-point link IS-IS hellos are sent periodicaly every 10 seconds (hello interval) to maintain the adjacency.

Below is a list of header fields sent in Point-to-point hellos.

Circuit Type – Level 1, Level 1-2 or Level 2 only
Source ID – System ID of the router that generated the hello packet
Holding Time – Maximum interval between two consecutive hello packets before the router is considered no longer available
PDU Length – Length of the entire PDU, including header
Local Circuit ID – Unique link identifier
TLV Fields – Variable-length fields.

TLV 240 enhances the IS-IS Point-to-Point adjacency setup process by introduing the concept of a three way handshake by confirming the presence of its SysID in a TLV 240 attached to hellos received from its neighbor.

LAN adjacancies

Similar to OSPF, IS-IS over a multi-access network involved the use of a psuedonode. Nodes on the LAN send their hellos to well-known Level 1 and Level 2 broadcast MAC addresses.

Below is a list of header fields sent in LAN level 1 and 2 hellos.

Circuit Type – Tells whether this circuit is Level 1-only, Level 2-only, or both.
Source ID – The SysID of the originator
Holding Time – Maximum interval between two consecutive hello packets before the router is considered no longer available
PDU length – Length of the entire PDU, including header
Priority – This 7 bit value designates the priority to be the DIS on the LAN.
LAN ID – SysID of the DIS plus an octet-long unique ID for this router assigned by the DIS.

The broadcast addresses for the Level 1 and Level 2 hellos is as follows:-

01-80-C2-00-00-15 for Level 2 adjacencies (AllL2ISs)
01-80-C2-00-00-14 for Level 1 adjacencies (AllL1ISs)

By zismail • CCIE SP, IS-IS • 0 • Tags: ,

Oct 27 2008

Introduction into IS-IS

Ok, here are some introductory points on ISIS. ISIS is a manual cost link state protocol. ISIS was designed within the ConnectionLess Network Service framework(CLNS). CLNS is supported by the following ISO protocols.

  • CLNP – ISO 8473
  • ES-IS – ISO 9542
  • IS-IS – ISO 10589

 

      CLNP, ES-IS and IS-IS all operate at layer 3 of the 7 layer OSI model.  They are differentiated by the value of the initial protocol identifier field in the first octet.  As you can see from this ISIS packet capture*, the “Intra Domain Routing Protocol Discriminator” is set to 0×83. The list below shows the value of the Routing Protocol Discriminator in the first octet of their encoded protocl formats for CLNP, ES-IS and IS-IS.

    • CLNP:  0×81 (10000001)
    • ES-IS: 0×82 (10000010)
    • IS-IS: 0×83 (10000011)

     

      IS-IS packets

      There are three types of IS-IS packets. Hello packets, Link-state packets and sequence number packets.

      Hello packets are used to establish adjacencies between IS-IS neighbors.  

      Link-state packets are used to distribute routing information between IS-IS nodes.  

      Sequence number packets are used to control distribution of Link-state packets ie sequence number packets provide a method of synchronisation of the link state databases in an IS-IS routing area.

      There are 3 subcatagories of Hello packets: Level 1, level 2 and Point-to-point hello. There are 2 subcatagories of Link-state packets: Level 1 and Level 2. There are 4 subcatagories of sequence number packets: Level 1 and 2 complete sequence number packets and Level 1 and 2 partial sequence number packets

      From this packet capture* you can see a Point-to-point hello, layer 1 and 2 link state packet, complete and partial sequence number packet. 

       

      *packet capture taken from http://packetlife.net/captures/protocol/isis/

      By zismail • CCIE SP, IS-IS • 0 • Tags: ,

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