Introduction

This document is about the link-state packet (LSP) pseudonode. A pseudonode is a logical representation of the LAN which is generated by a Designated Intermediate System (DIS) on a LAN segment. DIS is similar to OSPF's DR.

DIS

• DIS is similar to OSPF's DR, but there is no BDR.
• Pseudonodes is created for a broadcast segment to represent its all devices connected on that link. It reduces the number of adj that have to be maintained.
• Main functionality is to:
  • Create and update the pseudonode Link-State Packet (LSP) (it is a separate one from the router's LSP).
  • Flood LSPs over the LAN. Flooding complete sequence number protocol data units (CSNPs) every 10 seconds.

• Different DIS can exist for Level1 and Level2.
• How to detect which devices is the pseudonode? The router with metric 0 to all attached routers will be the pseudonode. All other devices will have a non-zero cost.
• The devices elected as DIS will generate a separate Link-State Packet (LSP) for the pseudonode, with metric 0 to all other routers. That same router will generate a LSP for itself with a cost to the pseudonode (see example below).

    router-8# show isis database router-6.00-00 detail
    IS-IS Level-1 LSP router-6.00-00
    LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
    router-6.00-00        0x00000071   0xD55C        456           0/0/0
      Area Address: 49.0001
      NLPID:       0xCC 
      Code:   137 Length: 8
      IP Address:  172.16.126.6
      Metric: 10 IP 172.16.126.0 255.255.255.0
      Metric: 10 IS router-6.01            <--- metric 10 to r6.
    router-8#

    Router-8# show isis database router-6.01-00 detail
    IS-IS Level-1 LSP router-6.01-00
    LSPID                 LSP Seq Num  LSP Checksum  LSP Holdtime  ATT/P/OL
    router-6.01-00        0x0000006D   0x65CF        759           0/0/0
      Metric: 0  IS router-6.00            <---metric 0 to r6, r2,r1.
      Metric: 0  IS router-2.00
      Metric: 0  IS router-1.00
    router-8#

DIS Election

• Election is done based on:
  • 1. Highest Priority
  • 2. Highest SNPA (MAC) address.
• Separate election is held for DIS Level 1 and DIS Level 2.
• DIS priority 0 is the lowest priority, but could still become the DIS, unlike OSPF.
• Default priority is 64.
• No Backup DIS, new one is elected upon primary failure.
• DIS election is preemptive.

Additional Resources

Understanding IS-IS Pseudonode LSP

Comments