IS-IS: Link State PDUs

A Link State Protocol Data Unit (LSP) is used to advertise the routing information of the network.  In IS-IS the smallest element of a LSDB would be the LSP.  There are no different types of LSPs instead to describe specific network elements, with in an LSP distinct TLV records are used inside an LSPs variably sized payload.

LSPs are identified by a unique number that consists of three parts:

  • System ID – is this is System Id of the router that originated the LSP (6 octets taken from the NET address of the originator)
  • Pseudonode ID – this differentiates between the LSP describing the router itself and the LSPs for multiaccess networks in which the router is a DIS
  • LSP Number – this denotes the fragment number of the LSP.  the LSP number is also called the Fragment Number or Fragment for short.

these three parts together are referred to as the LSPID.

For LSPs that describe routers themselves, the Pseudonode ID is always 0.

to distinguish between various versions of the same LSP, a sequence number is used.  Sequence numbers are 32 bit unsigned integers starting at 0x00000001 and ending at 0xFFFFFFFF.  Each modification of an LSP causes the sequence number to increment by 1. This is how IS-IS informs neighbors of changes to a specific link, when the sequence number in the advertised LSPs increases, it causes neighbors to update their LSDBs with the most up to date LSP being received.

LSPs have a lifetime value associated with it.  This is set to 1200 seconds and is decreased.  If the lifetime decreases to 0, the router will delete the LSPs body from the LSDB, keep only the header, and advertise an empty LSP with the lifetime set to 0.  This is referred to as an LSP purge, this allows other routers to learn the purged route and Cisco devices will hold the empty LSP header for another 20 minutes before it’s removed entirely frio the LSDB.

Since IS-IS encapsulates its messages within a layer 2 frame, whose max payload size (MTU) is limited IS-IS must implement its own form of fragmentation for LSPs whose size exceeds that of the Frame MTU.  If an LSP were to have so many TLVs that it exceeded the MTU, it would simply break them up and send multiple LSPs.  This ‘fragmentation’ is only performed by the originating router.  This makes having MTU set to be the same across a network to ensure frames are not dropped.  If there is a varying difference in MTU within a network, IS-IS must be configured to account for the smallest MTU on the network.

The address information about all networks are contained in LSPs of each router connected to the network.  The topological information about the network and the connected routers are contained in the Pseudonode LSP generated by the DIS.  This means  that an IS-IS router on a specific level of routing will generate one LSP for itself and all topological information, and one LSP describing itself and one more Pseudonode LSP for each network it is a DIS in.


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