Term
| What does the OSI use to provide connectionless delivery of data? |
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Definition
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The OSI protocol suite uses Connectionless Network Service (CLNS) to provide connectionless delivery of data
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Term
| What is the actual layer 3 protocol that allows OSI to provide connectionless delivery of data? |
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Definition
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The actual layer 3 protocol is Connectionless Network Protocol (CLNP), similar to what IP does for TCP/IP
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Term
| What type of addresses are used by the IS-IS routing protocol to identify routers and build the LSDB? |
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Definition
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The IS-IS routing protocol used CLNS addresses to identify the routers and build the LSDB.
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Term
| How do IS-IS routing exchange link-state information? |
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Definition
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IS-IS routers use link-state packets (LSPs) to exchange link-state information.
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Term
| How do routers running IS-IS pick the best paths to a particular network? |
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Definition
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Each router runs Dijkstra's shortest path first (SPF) algorithm against its LSDB to pick the best paths.
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Term
| Describe the L1 routing level used by IS-IS. |
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Definition
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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.
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Term
| Describe the L2 routing level used by IS-IS routers. |
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Definition
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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.
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Term
| What are the 3 different types of IS-IS routers? |
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Definition
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- L1 routers use LSPs to learn about paths within the areas they connect to (intra-area) - L2 routers use LSPs to learn about paths among areas (interarea) - L1/L2 routers learn about paths both within and between areas. ( Much like an ABR in OSPF)
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Term
| How are area boundaries defined with IS-IS? |
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Definition
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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.
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Term
| What protocol type of routing can be accomplished using IS-IS? |
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Definition
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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.
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Term
| How does Integraded IS-IS transport information between routers? |
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Definition
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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.
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Term
| What's the difference between narrow metrics and wide metrics when used on IS-IS routers? |
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Definition
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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.
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Term
| What is the default interface metric on Cisco routers running IS-IS? |
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Definition
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All IS-IS interfaces have a default metric of 10, which can be a problem if not manually changed to reflect different bandwidths.
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Term
| What is the basic function of the ES-IS protocol? |
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Definition
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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
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Term
| How do ESs and ISs discover each other when using the ES-IS protocol? |
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Definition
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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.
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Term
| How do routers establish and maintain adjacencies between ISs? |
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Definition
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Routers use IS-IS Hellos (IIHs) to establish and maintain adjacencies between ISs.
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Term
| How does ES-IS handle IP end-systems? |
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Definition
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ES-IS is not used by IP end-systems
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Term
| Describe the level 0 routing used by IS-IS. |
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Definition
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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.
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Term
| Describe the level 1 routing done in IS-IS. |
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Definition
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Level 1 routing is done between ISs within the same area. L1 routing is also called intra-area routing.
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Term
| How does an IS notify an ES that a direct route is available? |
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Definition
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ISs can provide a redirect message back to the source that tells it that a more direct route is available
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Term
| Describe how level 2 routing is done in IS-IS. |
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Definition
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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.
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Term
| Describe how level 3 routing is used in IS-IS. |
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Definition
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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
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Term
| How many updates can be sent in one IS-IS LSP? |
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Definition
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IS-IS updates are sent in just one LSP.
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Term
| What type of addresses are required to run IS-IS? |
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Definition
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IS-IS requires CLNS node addresses (also called Network Service Access Points, or NSAPs) even if the router is used for routing only IP
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Term
| What are the 2 main components of NSAP addresses? |
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Definition
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- The device's OSI address - A link to the higher-layer process The NSAP address is equivalent to the combination of the IP address and upper-layer protocol in an IP header.
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Term
| How is the NSAP address divided in Cisco's implementation of Integraded IS-IS? |
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Definition
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Cisco's implementation of Integraded IS-IS divides the NSAP into 3 fields: the area address the system ID, and the NSAP selector (NSEL).
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Term
| The simplest NSAP is comprised of what parts? |
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Definition
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1. Authority and format identifier (AFI) 2. Area ID (aka prefix) 3. System ID (The MAC address of a LAN interface) 4. NSAP selector (NSEL) of 0
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Term
| What is the NSAP selector (NSEL) field used for in an NSAP address? |
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Definition
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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
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Term
| Describe the prupose of a Subnetwork Point of Attachment. |
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Definition
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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: - The MAC address of a LAN interface - The VC ID from ATM of DLCI for Frame Relay
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Term
| In terms for IS-IS, what is a circuit? |
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Definition
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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.
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