IS-IS LSP packets are not IP packets (unlike OSPF LSAs). Therefore on the BRI, ATM and Frame Relay interface, we need to define layer 2 mapping for CLNS protocol.

IS-IS LSP’s MaxAge is 20 minutes, and Refresh interval is 15 minutes.

Level-1 and Level-2 Domain:

· IS-IS level-1 and level-2 domains are different from OSPF areas. A level-2 domain can have more than one areas in it.
· A network can have multiple level-1 domains but only one level-2 domain.
· A level-1 domain can only have one area. It is similar to OSPF Totally NSSA area. It will see intra-area routes, a default route from the L1/L2 router for inter-area destinations, and external routes.
· A level-2 domain can have multiple different areas. It is similar to OSPF backbone area. And level-2 domain must be contiguous.

Note: Level-1 can learn routes in a different area through IS-IS route leaking.

Areas and Level-1 and Level-2 Routers:

· IS-IS area is not defined on the interface but on the router itself using a CLNS NET address.
· A level-1/level-2 or level-2 router is similar to OSPF ABR.
· A level-1 router can be a ASBR but not ABR.
· A level-2 router can have adjacency with other level-2 routers in different areas
· A level-1 router can only establish adjacency with level-1 routers in the same area.
· Two types of IS-IS adjacency: L1-L1 and L2-L2. Adjacency cannot be established between a level-1 router and a level-2 router.

Note: When redistributing other routing protocols into IS-IS, you can choose whether to redistribute into level-1 or level-2.

CLNS NET Address:

· A CLNS NET address consists of three parts: area_id.system_id.nsel, in which system_id is 6 Bytes and N-Selector (nsel) is 1 Byte and its value is always 0x00.
· A CLNS NET address always starts with a single Byte (e.g. 000A.0000.0000.0001.00 is not valid, but 00.000A.0000.0000.0001.00 is valid).
· A CLNS NET address is in hex-decimal format. (so area 10 should be 0x0A).
· A CLNS NET address length has to be consistent throughout the domain.
· The first Byte of area_id is AFI, an AFI value of 0x49 means it’s private IS-IS address.
· A router can only have one CLNS NET (Network Entity Title) address.

Network Types:

· Broadcast network (default type for any multipoint interface)
· Point-to-Point network

Note: Only interfaces with matching network types can establish adjacency. IS-IS does NOT support NBMA network, tunnel has to be created for this scenario.

Hello Packet Types:

· Level-1 LAN Hello
· Level-2 LAN Hello
· Serial Hello (level-1 and level-2 Hello on a point-to-point segment use are same)

Note: IS-IS Hello packets will use padding to increase the packet size to MTU, and this may cause problem when using tunneling. This behavior can be disabled.

Designated Intermediate System:

· Unlike OSPF, IS-IS router on a broadcast/multi-access network will establish adjacency with not only the DIS, but all of its neighbors.
· IS-IS doesn’t have BDR.
· IS-IS DIS election is preemptive.
· DIS is responsible for advertising the network out to the whole domain.

LSP Option Bits:

· ATT bit – attach bit, if set to 1 means the originating router is attached to multiple areas.
· OL bit – overload bit, setting OL bit to 1 turns the originating router into a stub router, it will not be used as a transit router.

Three ways to include an interface into ISIS:

1. Enable isis on the interface
2. Redistribute connected interface
3. Use passive-interface in IS-IS for the interface

IS-IS Metric:

The original metric is 1-63, and new metric is 1-16777214. This can be controlled by using the metric-style command under IS-IS configuration.

IS-IS sample configuration:

router isis
net 49.000A.0000.0000.0001.00
interface Serial 0/0
encapsulation frame-relay
frame-relay map clns 301 broadcast
ip router isis

Note: IS-IS has to be enabled under global config as well as under interface config.

Can a Level 1 area connect to more than one Level 2 area?

I have a problem where I might need to split my Level 2 area into two and redistribute into MP-BGP (we are replacing point to point links with a VPN)
Can a Level 1 area connect to more than one Level 2 area?

I have a problem where I might need to split my Level 2 area into two and redistribute into MP-BGP (we are replacing point to point links with a VPN)
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