Chapter 11


"Do I Know This Already?" Quiz


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Which systems would you configure as Level 1-2 systems?


It is necessary to configure routers that straddle more than one area as Level 1-2 routers so that they can receive updates from both Level 1 and Level 2 routers and thus forward datagrams from Level 1 routers out of their area. Some designs allow for every router to be configured as a Level 1-2 router; this is the default configuration on Cisco routers. This eliminates errors but is heavy on network resources.


Which IS-IS configuration uses a full mesh and simulates a broadcast technology?


The multipoint configuration option is used with a full mesh and is seen by IS-IS as a broadcast network. It will therefore elect a DIS for the network.


What are the four stages of the routing process?


The four stages of the routing process are update, decision, forwarding, and receive.


What does an LSP contain?


An LSP contains the list of neighbors connected to the originating router.


When are LSPs generated?


LSPs are generated whenever there is a change in the network, often because of a configuration change. However, any of the following instances trigger a new LSP to be flooded throughout the network:

- An adjacency comes either up or down (for example, a new router comes online).

- An interface on the router changes state or is assigned a new metric.

- An IP route changes (for example, because of redistribution).


State at least one of the main steps of the flooding process on a point-to-point link.


The following list describes the flooding process on a point-to-point link:

  1. When an adjacency is established, both sides send a CSNP packet with a compressed version of their link-state databases.

  2. If there are any LSPs in the receiving router's database that were not in the CSNP it received, it sends a copy of the missing LSPs to the other router.

  3. Likewise, if the database is missing any LSPs received in the CSNP, the receiving router requests the detailed LSP to be sent.

  4. The individual LSPs are requested , sent, and acknowledged via PSNPs.

  5. When an LSP is sent, the router sets a timer. If no explicit acknowledgement has been received before the timer expires , the LSP is resent . This timer is the minimumLSPTransmission-interval and can be configured; the default on a Cisco router is 5 seconds.


Which three fields determine whether the LSP is valid?


The LSP contains three fields that help determine whether the LSP that has been received is more recent than that held in the database and whether it is intact or has been corrupted. These three fields are as follows :

- Remaining Lifetime : This is used to age-out old LSPs. If an LSP has been in the database for 20 minutes, it is assumed that the originating router has died. The refresh timer is set to 15 minutes.

- If the lifetime expires, the LSP has the content removed, leaving only the header. The lifetime is set to show that it is a new LSP, and then it is flooded through the network. All receiving routers accept the mutilated LSP, recognize that this means the route is bad, and purge the existing LSP from their databases.

- Sequence Number : This is an unsigned 32-bit linear number. The first LSP is allocated the sequence number 1, and the following LSPs are incremented by 1.

- Checksum : If a router receives an LSP and the checksum does not compute correctly, the LSP is flushed and the lifetime is set to 0. The router floods the LSP, all routers purge the LSP, and the originating router retransmits a new LSP.


Once the link-state databases are synchronized, the Dijkstra algorithm is run. Describe where the router places itself in the tree.


Each router builds a shortest path tree (SPT) with itself as the root. This is achieved by taking all the LSPs from the link-state database and using the Dijkstra algorithm to create the SPT. The SPT is used in turn to create the forwarding table, which is also known as the routing table.


State two criteria in determining which paths are to be placed in the forwarding database.


If there is more than one path to a remote destination, the criteria by which the lowest cost paths are selected and placed in the forwarding database are as follows:

- If there is more than one path with the lowest value metric, Cisco equipment places up to six equal-cost paths into the table. The default number of equal-cost paths is four.

- Optional metrics are chosen before the default metric, but because Cisco supports only the default metric, this is a moot point.

- Internal paths are chosen before external paths, because going outside the autonomous system is likely to be a suboptimal route and might be the result of a routing loop.

- Level 1 paths within the area are more attractive. If the path is within the area, not only is it more efficient to route directly to it, but also going outside the area and returning can be the cause of a routing loop, demanding greater resources and time.

- The address with the longest match or most specific address in IP is the address with the longest IP subnet mask. This ensures that the closest router is chosen, because prefix routing is configured by summarization that can occur only on area boundaries.


What are the ISO metrics?


The metrics defined in ISO 10589 are as follows:

- Default : Sometimes referred to as cost . Every Integrated IS-IS router must support this metric. Cisco set the default for all interfaces to be 10.

- Delay : This optional metric reflects the transit delay.

- Expense : This optional metric reflects the monetary expense of the network.

- Error : The reliability of the path is determined as the metric.


How many equal-cost paths is it possible to have in the IS-IS routing table of a Cisco router?


The default number of equal-cost paths allowed in the routing table is four, although Cisco allows six to be placed in the table.


What is a narrow metric?


A narrow metric is the default metric, which has a 6-bit field. Cisco increased the size of the field to 24 bits.


Where is the IS-IS metric applied?


The IS-IS metric is applied to the outgoing interface.


What action will the routing process take if it sees an incomplete LSP fragment?


If an LSP fragment is incomplete, the routing process ignores it, safe in the knowledge that it will be retransmitted if the sending router does not receive an ACK within a specified time frame.


Why is the IS-IS default of cost the only metric supported by Cisco?


Each metric that is configured for use in IS-IS requires its own database. If the router is a Level 1-2 router, it will need a database for each metric and each level of routing. This could result in eight databases and the use of many resources from both the router and the network.


When designing a network for fast convergence, what is the compromise that you need to consider?


Typically, the trade-off is between reliability and speed. To increase the speed of convergence for a routing protocol, it might be sufficient to tune the update process, although this results in the compromise of resources and reliability. If you reduce the update timers, the databases converge more quickly, but the network could be depleted of necessary resources to route data.


What is a suboptimal routing decision?


Suboptimal routing decisions occur when Level 1 areas have knowledge of only 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; therefore, the best route translates to that with the lowest hop count. As you know, the router two hops away might include a 16 Mbps Token Ring and a 56 kbps link as opposed to the three hops of Fast Ethernet and ATM.


Where does route summarization take place?


Route summarization can be configured on a Level 1-2 router at the area boundary.


When is a DIS elected in a WAN environment?


A DIS is elected on a WAN when the NBMA cloud is configured as multipoint.


Explain briefly how the IS-IS NBMA cloud is different than the configuration of the OSPF cloud.


Frame Relay and ATM are examples of NBMA networks, which are not accommodated in Integrated IS-IS. OSPF has a point-to-multipoint configuration option, but Integrated IS-IS does not. The solution in Integrated IS-IS is to configure the link as multipoint, allowing the election of a DIS. The alternative is to configure the interfaces with subinterfaces that are point-to-point.

CCNP BSCI Exam Certification Guide
CCNP BSCI Exam Certification Guide (CCNP Self-Study, 642-801) (3rd Edition)
ISBN: 1587200856
EAN: 2147483647
Year: 2002
Pages: 194
Authors: Clare Gough © 2008-2017.
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