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ISIS Protocol

IS-IS Rules

ISO Network Entity Title (NET)

Three of the different formats that an ISO NET can take:
(a) An 8-octet area IS/system ID format    07.000.3090.c7df.00
(b) An OSI NSAP format                             47.0004.30ac.0007.0000.3090.c7df.00
(c)  A GOSIP NSAP format                         47.0005.80.0000a7.0000.ffdd.0007.0000.3090.c7df.00

Rules for Creating a NET
• The NET must begin with a single octet.
• Addresses starting with 49 (AF I= 49) are considered private IP address, analogous to
RFC 1918.
— Routed by IS-IS
— Should not be advertised to other Connectionless Network Service (CLNS)
networks (outside this IS-IS domain)
• Additional 2 bytes added for the area ID.
• All routers in the same area must have the same area address.
• The system ID must be the same number of octets throughout the domain.
• Cisco has implemented a fixed length of 6 octets for the system ID of a NET.
• It is customary to use the MAC address of the router, or an IP address of a loopback
interface (192.168.111.3 = 192.168.111.003 = 1921.6811.1003).
• The practice of using a modified loopback IP address as the system ID may now be
considered outdated because of the dynamic host name feature. This feature uses a
new Type Length Value (TLV 137) to map the router’s host name to the system ID.
• Each device must have a unique system ID within the area.
• The NET must end with a single octet—the network service access point (NSAP)
selector byte (NSEL), usually set to 0x00.
— When the NSEL is set to 0, it identifies the device itself.
— The NSEL is like a TCP port number: It indicates the transport layer

Examples of NETs: Cisco Implementation
Example 1: NSAP 47.0001.aaaa.bbbb.cccc.00
Area ID is 47.0001
System ID is aaaa.bbbb.cccc
NSAP selector byte is 00
Example 2: NSAP 39.0f01.0002.0000.0c00.1111.00
Area ID is 39.0f01.0002
System ID is 0000.0c00.1111
NSAP selector byte is 00

Rules for IS-IS Adjacencies

• L1 routers form L1 adjacencies with L1 and L1-L2 routers in their area.
• L2 routers form L2 adjacencies with L2 and L1-L2 routers in their area or another area.
• L1/L2 routers form L1 and L2 adjacencies with each other in their area or another area.
• An L1 router does not form an adjacency with an L2 router, regardless of area.
• The system ID must be unique to each router.
• Hello intervals and hold times do not have to match.

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