Monday, April 19, 2010

BSCI - IS-IS - Differences between IS-IS and OSPF

  • OSPF supports 50 routers per area
  • IS-IS can support up to 1000 routers per area
  • OSPF includes more features, including route tags, stub/nssa configuration, on demand circuits
  • OSPF requires many LSA's to advertise and withdraw routes.
  • IS-IS only requires one LSP per IS-IS router in a given area.
  • IS-IS doesn't support a Backup designated router (BDR).i.e. you only have the primary Designated Intermediate System (DIS).
  • IS-IS repeats the DIS election process every time a new router is added to an area.
  • If the new router has the same of higher priority it will become the new DIS.
  • IS-IS is designed to be deterministic so that the same set of IS-IS routers always produce the same DIS.
  • Every time a DIS changes a new set of LSP's are flooded to the neighbours.
  • IS-IS form adjacencies with all routers in the area not just the DIS.
  • OSPF uses a scaled metric by default where as IS-IS uses a default metric of 10 for all links.

BSCI - IS-IS - High level operations

Routers using IS-IS issue Hello packets out of all interfaces enabled for IS-IS to discover neighbours and establish adjacencies.

Routers will become IS-IS neighbours  if their Hello packets contain certin information for the creation of an adjacency.

Routers then build Link-State Packets (LSP's) based on the adjacencies present and then flodd the LSP's to their neighbours.

All routers then build a Link-State Database (LSDB) using the information in the received LSP.

The router then runs the SPF algorithm, the shortest path to each dsestination is then calcultated and added to the Routing Information Base (RIB or OSI routing table).

Next, the Partial Route Calcultion is performed using the IP information contained within one of the Time, Length, Value (TLV) fields in the LSP. This generates shortest paths for IP traffic and in turn is entered in to the IP routing table.