Integrated IS - IS

1: What is an intermediate system?

** An Intermediate System is the ISO term for a router.


2: What is a network protocol data unit?

** A Network Protocol Data Unit is the ISO term for a packet.


3: What is the difference between an L1, an L2, and an L1/L2 router?

** An L1 router has no direct connections to another area. An L2 router only routes inter-area traffic. An L1/L2 router routes both inter-area and intra-area traffic and acts as an inter-area gateway for L1 routers.


4: Explain the basic difference between an IS-IS area and an OSPF area.

** The borders of IS-IS areas are between routers, on links. The borders of OSPF areas are defined by the routers themselves.


5: What is a network entity title (NET)?

** The Network Entity Title is an address by which a router identifies both itself and the area in which it resides.


6: To what value must the NSAP Selector be set in a NET?

** The NSAP Selector should be set to 0x00 in a NET.


7: What is the purpose of a System ID?

** The System ID uniquely identifies a router within an IS-IS domain.


8: How does a router determine what area it is in?

** The portion of the NET preceding the last seven octets is the area address.


9: Does IS-IS elect a Backup Designated Router on a broadcast subnetwork?


** IS-IS does not elect a BDR.


10: What is the purpose of the Pseudonode ID?

** The Pseudonode ID is the last octet of a LAN ID. Its purpose is to distinguish LAN IDs which are originated by a single router which is the DR on multiple LANs.



11: What is the maximum age (MaxAge) of an IS-IS LSP?

** The MaxAge of an IS-IS LSP is 1200 seconds (20 minutes).


12: What is the basic difference between the way OSPF ages its LSAs and the way IS-IS ages its LSPs?


** OSPF increments the age up to MaxAge; IS-IS decrements the age down to 0. A new OSPF LSA has an age of 0, whereas a new IS-IS LSP has an age of MaxAge.


13: How often does an IS-IS router refresh its LSPs?

** The refresh rate of an IS-IS router is 900 seconds (15 minutes).


14: What is a Complete Sequence Number Packet (CSNP)? How is it used?

** A Complete Sequence Number Packet contains a full listing of all LSPs in a database. A CSNP is periodically sent by the Designated Router on a broadcast network to maintain database synchronization.



15: What is a Partial Sequence Number Packet (PSNP)? How is it used?

** A Partial Sequence Number Packet contains a listing of one or more LSPs. It has two uses: On point-to-point networks, it is used to acknowledge the receipt of LSPs. On broadcast networks, it is used to request LSPs.