Foundation Summary


The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

Table 16-10 summarizes the commands you have seen throughout this chapter.

Table 16-10. Summary of Commands Used in This Chapter



neighbor ip-address route-reflector-client

Configures the route reflector to view a specified neighbor as a client. It reflects all routes received from this router to other clients and nonclients.

no neighbor ip-address route-reflector-client

Removes a neighbor as a client.

show ip bgp neighbor neighbor -address

Gives details about a specified neighbor.

ip prefix-list list- name [ seq seq-value ] deny permit network / len [ ge ge-value ] [ le le-value ]

Creates a prefix list that is used to filter routes from updates based on the prefix length of the route.

neighbor ip-address peer- group -name prefix-list prefix-list-name in out

Configures a router to use a prefix list as a filter in distributing routes.

show ip prefix-list detail name

Shows a prefix list by name. It shows what the list is filtering on and how many matches to the criteria have been logged. This is known as the number of hits.

neighbor { ip-address peer-group-name } weight weight

Sets the weight attribute to influence BGP path selection. Weight is a Cisco-proprietary attribute. It is a local attribute, which is not propagated to other routers. It selects the best path if there are multiple exits from the router into the Internet.

bgp default local-preference valu e

Sets the local preference attribute. Again, this is used to influence the router's selection of the best path based on its selection process. This attribute is passed in the routing updates to other BGP peers.

show ip bgp

Shows details about the BGP configuration on your network.

The benefits of route reflectors include the following:

  • The capability to scale the network, given the other characteristics

  • A strong hierarchical design

  • A reduction of traffic on the network

  • A reduction in the memory and CPU needed to maintain TCP sessions

  • Faster convergence and a simpler network because two routing protocols are implemented:

    - iBGP for external routing information traversing the autonomous system

    - IGP for routes internal to the autonomous system

Characteristics of route reflectors are as follows :

  • A route reflector is a router that forwards updates to its clients. When a client sends an update to the route reflector, it is forwarded or reflected to the other clients and nonclients.

  • The route reflector is the only router that is configured or that has the remotest idea that it is anything other than a peer.

  • A client is a router that receives updates from a route reflector that a route reflector has forwarded from another client or nonclient.

  • Both a route reflector and its clients, therefore, form a unit that shares information. This unit is called a cluster.

  • The autonomous system can be divided into clusters and be configured. There must be at least one route reflector per cluster; otherwise , the clients will not get the updates reflected to them.

  • The route reflector and the client no longer require a full mesh of peering relationships because the route reflector forwards updates from other clients.

  • In all probability, a route reflector is connected to peers for whom it is not forwarding routes. These are regular neighbors or peers, but from the route reflector's view, they are nonclients.

  • Nonclients must be fully meshed with the route reflector and each other.

  • The route reflector connects to other route reflectors. These route reflectors need to be fully meshed because the old rule of not propagating routes that are not defined in the network command is now operational. This is to ensure that the iBGP routing tables are complete.

  • When the route reflector forwards an update, the Originator-ID attribute is set. This is the BGP router ID of the router that originated the path. The purpose of this attribute is not to award honors to the originating router, but so that if this router receives the update, it will see its own ID and will ignore the packet. This prevents the possibility of routing loops .

  • If there are multiple route reflectors in the cluster to provide redundancy, then the originating router is identified by the Cluster-ID attribute. This serves the same purpose as the Originator-ID in preventing routing loops.

The rules by which route reflectors propagate updates are as follows:

  • If a route reflector receives multiple paths to the same destination, it chooses the best path.

  • If the route is received from a client, the route reflector reflects or forwards the update to clients and nonclients, except for the originator of the route.

  • If the route is received from a nonclient, the route reflector reflects the update only to clients.

  • If the route is received from eBGP, the route reflector or client reflects it to all nonclients, as well as clients.

Whether a prefix is permitted or denied is based upon the following rules:

  • If a route is permitted, the route is used.

  • If a route is denied, the route is not used.

  • At the bottom of every prefix list is an implicit deny any . Thus, if the given prefix does not match any entries of a prefix list, it is denied.

  • When multiple entries of a prefix list match a given prefix, the entry with the smallest sequence number (the first match in the list) is used.

  • The router begins the search at the top of the prefix list, with the sequence number 1. When a match is made, the search stops. Processing time will be reduced if the most common matches or denies are placed near the top of the list. This will prevent having to process criteria that are seldom met every time a route is examined.

  • Sequence numbers are generated automatically by default. To configure the sequence numbers manually, use the seq seq-value argument of the ip prefix-list command.

  • A sequence number does not need to be specified when removing a configuration entry.

Table 16-11 lists the various command options for prefix lists.

Table 16-11. Displaying Prefix List Command Options



show ip prefix-list [ detail summary ]

Displays information about all prefix lists, including the hit count, which is the number of times that a match has been found for the criteria in the prefix list. This is very important in troubleshooting for capacity planning and security.

show ip prefix-list [ detail summary ] name

Displays a table showing the entries in a prefix list identified by name.

show ip prefix-list name [ network/len ]

Displays the filtering associated with the node based on the absolute of the defined prefix.

show ip prefix-list name [ seq seq-num ]

Displays the prefix list entry with a given sequence number.

show ip prefix-list name [ network/len ] longer

Displays all entries of a prefix list that are more specific than the given network and length.

show ip prefix-list name [ network/len ] first-match

Displays the entry of a prefix list that matches the given prefix (network and length of prefix).

The show commands always include the sequence numbers in their output.

Table 16-12 summarizes the different approaches to obtaining routing information from the Internet.

Table 16-12. Receiving Routing Updates from Multiple ISPs

Which Routes Are Accepted from the Internet



IGP Chooses Best Metric to Default Network?

BGP Attribute Selects Best Path to External Network?

Can Exit Path Be Tuned via BGP Attributes?

Autonomous System Sends All Its Routes to ISPs?

ISP Chooses Entry Path to Autonomous System?

Default routes only from all ISPs




Go to nearest gateway that is advertising the route




Select routes and default routes from ISPs




Yes; normally the AS_Path is the attribute that selects the exit path to the Internet




Full routing tables from ISPs




Yes; normally the AS_Path is the attribute that selects the exit path to the Internet




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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