network
Table of Content

ISIS and OSPF

  • IGP
  • link state protocol

Link State database

describes it’s neighbors and links connected to it

PN

pseudo node, extra router, convert Ehernet segment to star network with 0 cost

DIS

Designated IS (DIS), which is responsible for creating and distributing details of router PN

LSP – Link State PDU

Router’s details, its links and neighbors

Flooding

An information distribution mechanism

Building network topology summary

  • IS-IS routers base their forwarding decision on the network topology view they know from Link State database
  • Link State PDUs (LSPs) are used to describe routers, each router generates one LSP about itself
  • Every router must have the same set of LSPs
  • Fast LSP propagation is achieved via mechanism called flooding
  • Each IS-IS router computes optimal path to all links present in link state database
  • Optimal path is selected via Shortest Path First algorithm. It is based on lowest cost of complete path

Neighbor discovery

  • R1 send hello to possible neighbor R2
  • R1 receives hello from R2 and modifies its hello with info from R2
  • R1 sets R2 state to up
    after they became neighbors they both send own LSPs, syncing LSD

Credentials exchanged in hello packets
System ID , Area ID, Priority, Authentication, Holding time.

Neighbors vs adjacent

  • neighbor stage routers exchange very limited amount of information
  • adjacent routers need to learn everything about each other. They exchange LSPs until their link state databases are identical.

Neighbor vs adjacency summary

  • IS-IS router automatically discovers directly connected neighbors
  • Neighbor relation is formed by exchanging special PDUs called hello
  • Different hello types are used depending on medium type: point-to-point or broadcast. Routers connected to broadcast network elect DIS during hello exchange
  • Neighbors exchange topology information using Link State PDUs (LSP’s)
  • IS-IS routers are called adjacent when their link state databases are identical
  • Routers can establish adjacency only on the same level
    Level 1 router with Level 1 only
    Level 2 router with Level 2 only

Area

A way to limit the size of LSPs is to group routers into areas
Link state databases need to be synchronized only within an area

backbone area or Level 2 area

an extra area needs to be introduced to connect areas 1 and 2 together
L2 router has both L1 LSP and L2 LSP

Areas summary

  • To scale IS-IS in larger networks it is possible to create network segments referred to as area’s, Routers need to synchronize databases only within an area
  • Local areas are called Level-1
  • Backbone linking Level-1 areas is called Level-2. It must be contiguous
  • IS-IS router can belong to one area only
  • Border routers (L1/L2) treat L1 and L2 areas as separate islands on IS-IS level. They merge IP information from both levels in their routing table
  • Default IS-IS behavior introduces suboptimal routing between areas

IP Prefix Redistribution

a process of inserting IP routing information into protocol from outside

OSI addressing

  • IS-IS does not run over IP but directly over layer 2 network
  • IS systems have to identify themselves somehow
  • Special address called Network Service Access Point (NSAP) is used by IS systems
  • The same address is used for entire node (unlike IP which assigns IP address to each interface)
  • IS-IS only carries information about IP networks inside non IP packets

OSI Terminology

  • IS – Intermediate System – Router
  • ES – End System – Host
  • CLNS – Connectionless Network Service – IP/UDP stack + applications running on top of UDP
  • CLNP – Connectionless Network Protocol – IP
  • NSAP – Network Service Access Point – IP address + TCP/UDP port
  • NET – NSAP Address where NSEL=00 – IP address
  • PDU – Protocol Data Unit – Packet
  • LSP – Link State PDU – LSA in OSPF