BSCI – IS-IS Pt.1

What does the OSI use to provide connectionless delivery of data?
The OSI protocol suite uses Connectionless Network Service (CLNS) to provide connectionless delivery of data
What is the actual layer 3 protocol that allows OSI to provide connectionless delivery of data?
The actual layer 3 protocol is Connectionless Network Protocol (CLNP), similar to what IP does for TCP/IP
What type of addresses are used by the IS-IS routing protocol to identify routers and build the LSDB?
The IS-IS routing protocol used CLNS addresses to identify the routers and build the LSDB.
How do IS-IS routing exchange link-state information?
IS-IS routers use link-state packets (LSPs) to exchange link-state information.
How do routers running IS-IS pick the best paths to a particular network?
Each router runs Dijkstra’s shortest path first (SPF) algorithm against its LSDB to pick the best paths.
Describe the L1 routing level used by IS-IS.
L1 builds a common topology of system IDs in the local area and routes traffic within the area using the lowest-cost path. All devices in an L1 area have the same area address.
Describe the L2 routing level used by IS-IS routers.
The L2 routing level exchanges prefix information (area addresses) between areas and routes traffic to an area using the lowest-cost path, using an interarea routing table.
What are the 3 different types of IS-IS routers?

– L1 routers use LSPs to learn about paths within the areas they connect to (intra-area)

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– L2 routers use LSPs to learn about paths among areas (interarea)

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– L1/L2 routers learn about paths both within and between areas. ( Much like an ABR in OSPF);

How are area boundaries defined with IS-IS?
IS-IS area boundaries fall on the links, not within the routers. Each router belongs to exactly 1 area. Neighboring routers learn that they are in the same or different areas and negotiate appropriate adjacencies- L1, L2, or both.
What protocol type of routing can be accomplished using IS-IS?
IS-IS is capable of combining ISO CLNS and IP routing in one protocol. It can be used for IP routing, CLNS routing, or for a combination of the two.
How does Integraded IS-IS transport information between routers?
Integraded IS-IS uses its own protocol data units (PDUs) to transport information between routers. IS-IS information is not carried within a network layer protocol, but is instead carried directly within data-link layer frames.
What’s the difference between narrow metrics and wide metrics when used on IS-IS routers?
The default (and older) metric type is narrow, which limits the maximum interface metric to 63 (6 bits) and the max path metric to 1023 (10bits). Wide metrics allows a 24-bit interface and a 32-bit path metric.
What is the default interface metric on Cisco routers running IS-IS?
All IS-IS interfaces have a default metric of 10, which can be a problem if not manually changed to reflect different bandwidths.
What is the basic function of the ES-IS protocol?
The ES-IS protocol permits ESs (hosts) and ISs (routers) to discover one another. ES-IS also allows ESs to learn their network layer addresses. It finally handles topology information discover and exchange between ESs and ISs
How do ESs and ISs discover each other when using the ES-IS protocol?
ESs send End System Hellos (ESH) to well-known addresses that annouce their presence to routers (ISs). Routers listen to ESHs to find the ESs on a segment. Routers transmit IS Hellos (ISHs) to well-known addresses, announcing their presence to ESs. ESs listen for these ISHs and randomly pick an IS to which they forward all the packets.
How do routers establish and maintain adjacencies between ISs?
Routers use IS-IS Hellos (IIHs) to establish and maintain adjacencies between ISs.
How does ES-IS handle IP end-systems?
ES-IS is not used by IP end-systems
Describe the level 0 routing used by IS-IS.
Level 0 routing is done between ESs and ISs on the same subnet. When an ES needs to send a packet to another ES, it sends the packet to an IS on an attached network.
Describe the level 1 routing done in IS-IS.
Level 1 routing is done between ISs within the same area. L1 routing is also called intra-area routing.
How does an IS notify an ES that a direct route is available?
ISs can provide a redirect message back to the source that tells it that a more direct route is available
Describe how level 2 routing is done in IS-IS.
Level 2 routing is done between different areas within the same domain. If a destination address is in another area, the L1 IS sends the packet to the nearest L1/L2 IS, which sends it through the backbone and to its final destination.
Describe how level 3 routing is used in IS-IS.
Level 3 routing is done between separate domains. It does this through the Interdomain Routing Protocol (IDRP). L3 OSI routing is not implemented on Cisco routers
How many updates can be sent in one IS-IS LSP?
IS-IS updates are sent in just one LSP.
What type of addresses are required to run IS-IS?
IS-IS requires CLNS node addresses (also called Network Service Access Points, or NSAPs) even if the router is used for routing only IP
What are the 2 main components of NSAP addresses?

– The device’s OSI address

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– A link to the higher-layer process

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The NSAP address is equivalent to the combination of the IP address and upper-layer protocol in an IP header.;

How is the NSAP address divided in Cisco’s implementation of Integraded IS-IS?
Cisco’s implementation of Integraded IS-IS divides the NSAP into 3 fields: the area address the system ID, and the NSAP selector (NSEL).
The simplest NSAP is comprised of what parts?

1. Authority and format identifier (AFI)

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2. Area ID (aka prefix)

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3.; System ID (The MAC address of a LAN interface)

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4. NSAP selector (NSEL) of 0;

What is the NSAP selector (NSEL) field used for in an NSAP address?
The NSEL field is used to identify a process on the device, corresponding roughly to a port number in IP. NET addresses are NSAP addresses with an NSEL of 0
Describe the prupose of a Subnetwork Point of Attachment.

The Subnetwork Point of Attachment (SNPA) is the point that provides subnetwork services. It is equivalent to a Layer 2 address. An SNPA can be one of the following:

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– The MAC address of a LAN interface

– The VC ID from ATM of DLCI for Frame Relay;

In terms for IS-IS, what is a circuit?
A circuit is the IS-IS term for an interface. Because the NSAP and NET refer to the entire device, a circuit ID is used to distinguish a particular interface.