Router - Is-Is

Front 1

Routers sharing a common data link become IS-IS neighbors if the hello packets that they exchange meet the criteria for forming an adjacency. What must both interfaces agree on for an adjacency to be formed?

Back 1

The Maximum Packet size (MTU), common routing level (if level 1 routers must be in same area ), a unique system ID, Authentication (if configured must be on both routers), at least one is level 1-2, hello & hold timers must match

Front 2

What is the Type/Length/Variable field and its benefits?

Back 2

TLVs are one of the strengths of IS-IS. They provide flexibility and extensibility for the protocol. The protocol can adapt to changing needs and advances in technology by simply defining a new TLV.

Front 3

What is the system ID?

Back 3

Is the unique identifier for an area. The first part of the address is a very long area address, of which only the last 6 octets are available for addressing the router or host. The area address must be the same for all routers in the same area. Areas, recognized by the format of their NET, produce a summary into level 2 and the level 1-2 router and introduce a default route back into level 1.

Front 4

What are ISO Addresses?

Back 4

NET and NSAPs are ISO addresses, and are assigned to the system rather than the interface The Net address is specifically the NSAP address of the host, with the NSEL set to 0x00. Because the NET (unlike the NSAP) doesn't identify a process, the address is that of a transitional or intermediate system. Therefore is associated with the router or IS.

Front 5

What are the 3 parts of an ISO address?

Back 5

Area(AFI,IDI, High Order DSP)=Is like an IP subnet describing a group or location, ID (System ID) = Identifies a particular member at the location, like the host portion of an IP, SEL (NSEL)= Similar to a TCP port, since identifies a process on the host. When Interpreting an address, a Cisco Router knows that the first byte is AFI, last byte is SEL, and the proceeding 6 bytes are system ID

Front 6

What are the 3 Integrated IS-IS packet types?

Back 6

Hello=create and maintain neighbor relationships, LSP=holds info on the neighbors connected to the router, SNP=Describe the LSPs in the transmitting routers linkstate database.

Front 7

What are IS-IS advertisements called?

Back 7

They are called Sequence packet numbers (SNP) and list the LSPs in the transmitting routers link-state database in a condensed format. SNPs are never flooded, but only sent between neighbors. The ensure link-state database synchronization by: Distributing LSPs on a LAN without explicit individual acknowledgements, Acknowledging individual LSPs, requesting LSPs at startup.

Front 8

What is a Complete Sequence Number Packets (CSNP)?

Back 8

Is a list of all the links held in the link-state database, which triggers a synchronization of the link-state database on each machine. The DIS on a multicast network will send out CSNPs every 10 seconds.

Front 9

What is a Partial Sequence Number Packets (PSNP)?

Back 9

It contains a subset of LSPs, and is used to request LSPs and to acknowledge receipt.

Front 10

State a benefit and disadvantage of using the a level1-2 (Is default)

Back 10

It eliminates errors, but is a drain on network resources

Front 11

What is a DIS?

Back 11

Like a DR in OSPF, a DIS exists to simulate a point-to-point topology across a multipoint environment. The DIS exists at Level 1 and level 2. So the DIS has the responsibility of flooding LSPs to all connected systems running IS-IS

Front 12

How often does a DIS send out hellos?

Back 12

Every 3.3 secs, 3 times the speed of other routers on the multi-access link.

Front 13

How do you identify the DIS on a Multi-access link?

Back 13

Its identified in the output screen of show clns interface or any other show cmd that shows the pseudonode such as show isis database or show clns neighbors. If the medium is multi-access broadcast, the circuit ID field shows the ID of the psuedonode, which is the system ID of the DIS plus the nonzero octet. The psuedonode is identified by the non zero value in the octet following the system ID.

Front 14

What takes on much of the responsibility for synchronizing the databases on behalf of the pseudonode?

Back 14

The DIS, who sends out IS-IS LSPs on behalf of the pseudonode. The DIS has 3 tasks: Creating and maintaining adjacencies, creating and updating the pseudonode LSP and flooding the LSPs over the LAN.

Front 15

What is a pseudonode?

Back 15

It is a LAN identifier for a broadcast subnetwork, which has links to each of the ISs and each IS has a single link to the pseudonode. It’s the system ID of the DIS plus the circuit ID. The DIS generates link-state PDUs on behalf of the pseudonode, and these LSPs are sent to all the connected ISs.

Front 16

What is the major difference between IS-IS and OSPF in terms of Encapsulation?

Back 16

IS-IS is protocol independent because it runs directly on top of the data link layer, where as OSPF is encapsulated into IP. The difference means that IS-IS can be adapted to circumstances by simply drafting a new TLV.

Front 17

What triggers a new LSP to flooded throughout the network?

Back 17

An adjacency coming up or down, an interface changing state or assigned a new metric, an IP route changes

Front 18

What are the 3 fields in a LSP that help determine whether the LSP that has been received is more recent that held in the database?

Back 18

Remaining Lifetime (used to age out old LSPs), Sequence Number and Checksum

Front 19

What is the default deadtime?

Back 19

30 secs, 3 times that of the hello timer. Thus the path will wait 30secs before declaring the path dead and flushing the LSPs from the link-state database.

Front 20

What is used by IS-IS to make the best path decision?

Back 20

Link-state protocols employ the algorithm defined by Dikstra. They take all the LSPs from the link-state database, and use the algorithm to create a SPT tree that shows the shortest paths to all destinations. The SPT is used in turn to create the forwarding table, which is also known as the routing table.

Front 21

What happens if there is more than 1 path with the lowest hop count?

Back 21

Cisco IOS would place some or all of the equal cost paths into the routing table. The default number of load-sharing paths is 6, but newer versions of IOS can support more.

Front 22

What is used to determine the shortest path?

Back 22

The lowest metric is chosen, internal paths are chosen over external paths, and level 1 routes have precedence over level 2 routes

Front 23

When do suboptimal routing decisions occur?

Back 23

They occur when Level 1 areas have knowledge only of networks within their own areas. To reach another area, packets are sent to the nearest level 2 router. Without additional configuration, the level 1 router determines the nearest level 2 router to be the one with the lowest hop count. The metrics used are the default metric of 10 on each outbound interface (regardless of the b/w), therefore, the best route translates to that with the lowest hop count.

Front 24

What are the rules for summarizing IP routes in Integrated IS-IS?

Back 24

Summarization is configured at the level 1-2 router at the edge of the area, you must configure it on every level 1-2 router in the area injecting updates into level2, and level 1 routes cant be summarized within the area (only between areas) because this isn't permitted by the protocol

Front 25

What is the recommended solution for Using a NBMA link (eg Frame Relay or ATM) with IS-IS?

Back 25

IS-IS doesn’t have an NBMA link type, so non-broadcast multi-access links must be setup either as broadcasts or point-to-point networks. The recommended solution is to set them up as point-to-point using sub-interfaces.

Front 26

How many Link-state databases are there on a level 1-2 router?

Back 26

Level 1 and level 2 routers have their own Link-state databases. So L1 LSPs are flooded throughout the area, and Level 2 LSPs are sent across Level 2 adjacencies. A level 1-2 router has 2 SPF link-state databases, one for the level 1 routes and the other for the level 2 routes. A separate SPF algorithm is run for each database.

Front 27

How is the ATT bit used by level 1 and 2 routers?

Back 27

Level 1-2 routers send LSPs with an attached (ATT) bit in their level 1 LSPs. This indicates they are attached to another area and is interpreted by the receiver as a default route. Thus the level 2 router serves as a transit router between areas.

Front 28

Which cmds can be use to display the adjacencies on a router?

Back 28

Show clns interface and show clns neighbors. Show clns neighbors shows info on the state of the link, type of routing protocol, the data-link address (SNPA) and the hold time (how long since it received the last hello).

Front 29

To enable the routing process IS-IS

Back 29

router(config)# router isis

Front 30

To configure the Network Entity Title (NET) address, which assigns the area. Is similar to router ID in OSPF, but also implies area assignment.

Back 30

router(config-router)# net [net address]

Front 31

To configure router as level 1 or level 2 Router, default is level 1-2. Once applied, all interfaces communicate only at that level. Routing at both levels means twice as many hellos and advertisements, twice as much memory consumed and twice as much processing. However routing at both levels makes configuration easier.

Back 31

router(config-router)# is-type [level-1 | level-2]

Front 32

To configure summarization, only do on the level 1-2 router. The level 1-2 router would summarize the routes and inject them into the level 2 network.

Back 32

router(config-router)# summary address [summary network add] [mask]

Front 33

To start IS-IS for IP on the specified interface

Back 33

router(config-if)# ip router isis

Front 34

To set the router level on a per-interface basis.

Back 34

router(config-if)# isis circuit-type [level-1 | level-2-only]

Front 35

To map the IP dst address to the outgoing Data-link Connection Identifier (DLCI) in a fully meshed Frame Relay cloud. Defines the interface as a broadcast interface

Back 35

router(config-if)# frame-relay map ip [ip add] [dlci] broadcast

Front 36

Maps a DLCI to the CLNS process of the dst router. IS-IS is encapsulated in a frame similar to CLNS and that frame must be received to build a routing table. So without this cmd, no routes appear in the IP routing table because IS-IS doesn’t receive IS-IS frames containing LSPs

Back 36

router(config-if)# frame-relay map clns [dlci] broadcast

Front 37

To configure a sub interface to be point-to-point

Back 37

router(config)# interface [type] [mod/num] point-to-point

Front 38

To define the DLCI to the dst on a sub interface in a P-t-P Frame Relay Cloud. No need to configure "frame-relay map", but must start the IS-IS process for each sub-interface. Is the recommended way to run IS-IS over NBMA and the only way to run it on anything less than full mesh.

Back 38

router(config-if)# frame-relay interface-dlci

Front 39

To see contents of the neighbor table and the state of the link, so is good for quickly checking connectivity. It shows neighbors system ID, interface, SNPA (L2 add), state, holdtime, protocol and type (what level).

Back 39

router# show clns neighbors [area]

Front 40

To see more detailed info about each neighbor including the area address of each neighbor, ip address of the transmitting interface and uptime. Specifying the "area" is optional.

Back 40

router# show clns neighbors [area] detail

Front 41

To check how the IS-IS interface is set up and check for any mismatch of parameters. Can see info like circuit ID, metric and DIS priority for each level. If circuit ID ends with a non zero value (eg 0x01), it is ID of pseudonode, so is the DIS.

Back 41

router# show clns interface

Front 42

To see the LSPs held in the local database. LSPID shows system ID of generating router, lsp seq num, checksum, holdtime and ATT(indicates L2 router with path out of area. L1 routers use the presence of this bit to identify closest L1-2 router)

Back 42

router# show isis database [level-1] [level-2]

Front 43

Shows the database in more detail, including the area addresses that may be reached from this router, ip address of transmitting interface, and the metric cost of the IP routes it knows. Stating level 1 or 2 is optional.

Back 43

router# show isis database [level-1] [level-2] detail

Front 44

To reveal the trigger for the last 20 occurrences for which the SPF was run. It displays When, duration, number of routers calculated in SPF, Last Triggered (LSPID that triggered SPF) and Trigger (list of events that triggered SPF calculation.

Back 44

router# show isis spf-log

Front 45

Displays info on all adjacency-related activity, including hello packets (sent and received), and any changes in the state of an adjacency in Integrated IS-IS.

Back 45

router# debug isis adjacencies-packets

Front 46

Displayed statistical information about building routes between routers. Using the statistical info provided, you can determine how long it takes to place a Level 1 IS or Level 2 IS on the shortest path tree (SPT) using the IS-IS protocol.

Back 46

router# debug isis spf-statistics

Front 47

Displays all the Integrated IS-IS packets both sent and received by the router. These packets are SNPs (CSNPs and PSNPs) and LSPs that are detected by the router.

Back 47

router# debug isis update-packets