Matching AS_PATHs Using AS_PATH Filters
NLRI filtering with AS_PATH filters uses two commands: ip as-path access-list access-list-number {permit | deny} as-regexp neighbor {ip-address | peer-group-name} filter-list access-list-number {in | out} Figure 13-4 shows a sample internetwork used in many of the upcoming examples, two of which show the use of AS_PATH filtering. Figure 13-4 Network Used for AS_PATH Filter Examples Example 13-4 shows an AS_PATH filter in which routes are filtered going from R4 to R3..
In a typical distance vector protocol, each router advertises its table of reachable destinations (vectors) and the distances to them (distance) on each of its interfaces on a regular basis (periodic updates). OSPF routers advertise the state of their directly connected links to all routers on the network (through flooding). Although OSPF uses periodic updates, long periods of time exist between them, reducing network traffic to a minimum. Each router receives these link-stat..
Using SNMP to Perform Mass Configuration Changes
The following Perl script will distribute configuration commands to a large number of routers. It works by using SNMP to trigger TFTP file transfers into the routers. In effect, this script lets you automatically distribute a series configuration commands to a list of routers. Automating routine changes like this saves time and effort, but more importantly, it virtually eliminates typographical mistakes. Here's some example output: Freebsd% ./snmpcfg.pl =====================..
The router provides a number of different methods to view active users. The output from all of these commands is nearly identical. For security purposes, for operational reasons, or just for curiosity, many administrators like to know which users are accessing the router. The format of the output is as follows: the absolute line number, the VTY line number, the username, listing of connected hosts, the inactivity timer, and the source address of the session. Also notice that..
Filtering BGP Routes Based on AS Paths
You can use AS Path filters, either inbound or outbound, to filter either the routes you send or the routes you receive, respectively. You must apply these filters to each peer separately: Router1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router1(config)#ip as-path access-list 15 permit ^65501$ Router1(config)#ip as-path access-list 25 permit _65530_ Router1(config)#ip as-path access-list 25 deny _65531$ Router1(config)#ip as-path ..
