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IS-IS notes

General:
- routing protocol for ISO CLNP (Connectionless Network Protocol)
- NET (Network Entity Title) required by configuration (L3 address) Has different formats. One practical is below:
   Area (1-13 bytes)| System ID (6 bytes) | Selector (1 byte) (eg. 47.000|1921.6810.0001|00)
   Selector is 00 in a NET. The NET must begin with one octet (eg 47) and end with one octed (00)
   Selector is non 00, the address is NSAP (Network Service Access Point)
   NSAP describes a service attachment at the network layer (similar to IP protocol at the IP layer)
- operates over Ethernet 802.2 LLC  (not over the common Ethernet II)
- dual ISIS (RFC1195) supports CLNS and IP
- hierarchical with 2 level hierarchy (L2 - core)
- ignores TLVs it does not understand


Adjacencies:
- L1 area ID must be the same

- L2 area ID can be different
- L1 never forms adjacency with L2

Broadcast segment:
- DIS election is deterministic (highest priority)
- separate for L1/L2
- no backup DIS
- all routers establish adjacenies with each other on a broadcast segment
- an ISIS network running on broadcast is considered 1 Router (pseudo-node) in regards to SPF calculations
- all routers advertise 1 adjacency to others (the adjacency is with the pseudo-node)
- the DIS as the pseudo-node repradvertise advertise a link to all attached routers

LSP flooding scope:
- L1 LSP stay within the L1 area
- The L1/L2 at the edge places the routing information in a L2 LSP and forwards this accross the area boundry
- all L2 LSPs are flooded across L2 areas
- The closest L1/L2 sets the ATT bit in its L1 LSPs. L1 routers install a default route through the closest router that set the ATT bit
- L1 LSPs are advertised into L2 (as L2)
- External routes can be advertised as L1 in L1
- L2 LSPs are not advertised into L1
- Using wide metrics (TLV 135) eliminates external/internal route distinction
- L1 routes advertised as external are not advertised further by the L2 routers in L2 areas
- Routes can be imported from L2 -> L1. Such routes have the UP/DOWN bit set to DOWN. (To prevent advertisement back to L2)

ISIS Packet format:
Protocol ID | Header len | Version | ID len | PDU type | Version | Reserved | Max area address | PDU headers & TLVs

PDU type identifies the type of packet (eg. type 16 Hello L2 IS PDU, type 20 L2LSP PDU)

Packets:
LSPs (Link State Packets):
- are flooded periodically or when a change occurrs
- separate LSPs for L1 / L2
- TLVs
- Few fields:
   - LSP ID (router system ID, circuit ID, LSP number)
      LSP number begins with 0 and increments for every fragment of the same LSP
      Circuit ID - 0x0 or 0xX for LSPs originated by DIS
   - Protocols supported
   - IS Type determines L1 or L2 PDU
   - ATT (attached bit) - set by the L1/L2 routers in their L1 LSPs
   - OL (overload bit) - indicates the node should not be used for transit. Other routers ignore the LSPs from the OL node. Turns off ATT bit

Partial sequence number PDU:
- used for database synchronization
- acknowledges LSPs from a neighbor on P2P networks
- requests a copy of a missing LSP on a broadcast segment
- contains specific LSP header for the requested/acked LSP
- separate for L1/L2

Complete sequence number PDU:
- sent periodically on P2P
- sent by DIS on broadcast
- contains header info for all LSPs in the database
- separate for L1/L2

Hello PDUs:
- neighbors can have different intervals for hold timers
- used for discovery and to identify devices
- describes capabilities and parameters on interfaces
- separate (L1 and L2) on LAN (coded to mcast MAC)and on P2P

TLV:

Some ISO 10589 (CLNS) TLVs:
1 Area address - Lists area on wihch the router is configured. Can be multiple (TLVs)
2 IS Neighbors (LSPs) - lists the originating router's neighbors and cost to these
6 IS Neighbors (Hello) - list System ID for neighbors for which the local router has seen Hellos (only LANs, no P2P)
8 Padding - used to pad the Hellow PDU to its min size (1492 bytes) or to the interface MTU size
10 Authentication info

Some RFC1195 (Dual ISIS):
128 IP internal reachability information - advertises local router's IPv4 interface addresses/masks
Protocols supported - L3 protocols supported for routing (eg IPv4, IPv6)
130 IP external reachability information - network external to the routing domain reachable via one of the local router interfaces

Extension TLVs from RFC3784:
135 Extended IP reachability - adds traffic engineering and wide metrics. Replaces 128,130
211 Restart - graceful restart

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