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Perhaps the most technically accurate description is path vector. Common uses for BGP include the following: n Customer connected to one Internet service provider ISP not always required, however n Customer connected to several ISPs n Service provider networks transit AS n Network cores of very large enterprise networks Session Establishment BGP neighbors are not discovered; they must be conigured manually on both sides of the connection. TCP port number is used.

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Only one session remains if both connection attempts succeed. The show ip bgp summary command gives an overview of the session status. Keepalives are sent every 60 seconds. Peers can use an MD5 shared secret. Route Processing All routes received after the neighbor establishment are saved in memory. If more than one way to reach a destination exists, the best is selected. Use the show ip bgp command to view all the routing information received from all neighbors. The best BGP routes are copied into the IP routing table after the router checks administrative distance values.

The BGP process injects local routes in two different ways: n Using the network coniguration commands. This command lists networks that are candidates if they ap- pear in the routing table. When you disable automatic summarization, the routes introduced locally into the BGP table are not summarized. Although these are minor coniguration differences, they are handled in different ways: 1.

The next-hop ield is updated with the last eBGP peer. It is not updated when iBGP is used. The synchronization rule is a method that guarantees that a route is known to all routers within the AS even if they are not running BGP. The synchronization check can be turned off and is by default as of IOS version BGP Basic Coniguration To start BGP on your router, use the following global coniguration command: router bgp as-number A public AS number can be obtained from the appropriate agency, or a private AS number is possible in some situations 64, to 65, Only one BGP process is permitted per router.

To conigure your BGP neighbors, use the following router coniguration commands: neighbor ip-address remote-as as-number neighbor ip-address description neighbor description To temporarily disable a neighborship, use the following router coniguration command: neighbor ip-address shutdown To conigure MD5 authentication between neighbors, use the following router coniguration command. Keep in mind the password string must match on both routers. If auto-summarization is disabled, an exact match is required in the forwarding table.

You can use the mask keyword to specify a speciic subnet with the network command. If you would like to modify attributes before inserting preixes into the BGP table, you can use a route map in the network command in router coniguration mode: network network-number [mask network-mask] [route-map map-tag] This option might be used for one or more of the following: n Change the weight of a locally sourced route.

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This makes them seem inferior to other routes per the BGP route-selection process. Notice the optional use of the distribute list syntax to suppress certain networks from being advertised in updates. Redistribution can be conigured with a route map to reset the origin code or set other attributes. Aggregation in BGP Use the following router coniguration command to conigure route summarization to suppress the advertising of individual networks. Remember, at least one network of the summarized space must exist in the BGP table: aggregate-address address-preix mask summary-only Route Selection Using Policy Controls AS Path Filtering with Regular Expressions String matching: A string of characters in the regular expression matches any equivalent substring in the AS path; 29 has three matches in 29 , for example.

String matching grouping: Parentheses can group smaller expressions into larger expressions. Routes not selected are silently dropped. Routes selected if an outbound ilter is used are transmitted to the neighbor when the selection is applied. Routes not selected are used locally but are never sent to the neighbor.

The commands used to conigure an AS path list are relatively simple. To display routes matching the AS path regular expression, use the show ip bgp regexp command. To display routes that conform to a speciied ilter list, use the show ip bgp ilter-list command. To display a speciic access list or all AS path access lists in the router, use the show ip as-path- access-list command.

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Preix Lists Preix lists are a powerful method to control the updates coming from other BGP speaking routers. The exact match is assumed when neither ge nor le is speciied. The range is assumed to be from ge-value to 32 only if the ge attribute is speciied. The range is assumed to be from le to le-value only if the le attribute is speciied. The advertisement of the ORF capability indicates that a BGP-speaking router can accept a preix list from a neighbor and apply the preix list to locally conigured ORFs if any exist.

When this capability is enabled, the BGP speaker can install an inbound preix list ilter to the remote peer as an outbound ilter, which reduces unwanted routing updates. The neighbor uses the ORF preix list previously negotiated.

Filtering with Route Maps Route maps are also a power iltering tool. The routing information must be permitted by the route map to be accepted. If the route map has no statement explicitly permitting a route, the route is implicitly denied and dropped. The syntax required is as follows: Router conig-router neighbor ip-address route-map name in out The show ip bgp route-map command displays selected routes from a BGP routing table based on the contents of a route map.

When you conigure soft-reconiguration inbound for a neighbor, the router stores all routes received from that neighbor as an extra copy in memory. This copy is taken before any iltering is applied by the router to routes it receives. When you have completed the changes to ilters and route maps applied on incoming information, use clear ip bgp ip-address soft on the router in privileged EXEC mode.

When you have completed the changes to ilters and route maps applied on the outgoing information, execute clear ip bgp ip-address soft out on the router in privileged EXEC mode.

Routers use the route refresh feature to ask a neighbor to resend all the routing information when needed. Higher weights are preferred. You can also conigure the router so that all incoming routes that match an AS ilter receive the conigured weight. The default weight value is 32, for locally originating networks including those via redistribution and is 0 for all other networks. Using Local Preference You can use local preference to inluence route selection within the local AS; this attribute is stripped from outgoing updates via eBGP.

You should decide between the use of weight or local preference.

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The default local preference for iBGP and local routes is ; all others are 0 by default. You can apply local preference in the following ways: n Using a route map with the set local-preference command n Using the bgp default local-preference command to change the default local preference value applied to all updates coming from external neighbors or originating locally For veriication, you can use the command show ip bgp preix to display the locally applied value.

AS path prepending potentially enables the customer to inluence the route selection of its service providers. Because the AS paths sent over the unwanted link become longer than the AS path sent over the preferred path, the unwanted link is now less likely to be used as the return path. You can conigure manual manipulation of the AS path attribute prepending using a route map with the set as-path prepend command.

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The default value of the MED attribute is 0. A lower value of MED is more preferred. A router prefers a path with the smallest MED value but only if weight, local preference, AS path, and origin code are equal. The only exception is if the router is originating networks that have an exact match in the routing table through the network command or through redistribution. In that case, the router uses the metric in the routing table as the MED attribute value. Using the default-metric command in BGP coniguration mode causes all redistributed networks to have the speciied MED value.

You can use a route map to set MED on incoming or outgoing updates.

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Use the set metric command within route map coniguration mode to set the MED attribute. You must use the command bgp bestpath med confed when you use MED within a confederation to inluence the route selection process. A router can apply it to any BGP route by using a route map. Other routers can then perform any action based on the tag community that is attached to the route.

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Any BGP router can tag routes in incoming and outgoing routing updates or when doing redistribution.