AS_PATH Prepending and Route Aggregation

The concept and motivation behind the AS_PATH prepend feature is simple—impact the AS_PATH length decision step by increasing the length of the AS_PATH. To do so, a router simply configures a route map, refers to it with a neighbor route-map command, with the route map using the set as-path prepend asn1 asn2… command. As a result, the route map prepends additional ASNs to the AS_SEQUENCE. Any ASN can be prepended, but in practice, it makes the most sense to prepend the lo..

Physical Versus Logical Connections

External BGP neighbors have a restriction in that they must be physically connected, adjacent to one another. BGP drops any UPDATE message from its external BGP peer if the peer is not physically connected, unless otherwise specified. However, some situations arise in which external neighbors cannot be on the same physical segment. Such neighbors are logically connected (multiple IP hops away) but not physically connected. An example would be running BGP between external neig..

Label Switching in Frame-Mode MPLS

After receiving the Layer 2 PPP frame from the San Jose router, the San Francisco router immediately identifies the received packet as a labeled packet based on its PPP Protocol field value and performs a label lookup in its Label Forwarding Information Base (LFIB). NOTE LFIB also is called Tag Forwarding Information Base (TFIB) in older Cisco documentation.The LFIB entry corresponding to inbound label 30 (and displayed in Example C-2) directs the San Francisco router to rep..

Local-Area Networks and Technologies

Local-area network ( LAN) technologies, as the name suggests, are extremely localized, covering a small geographic area up to only a few thousand meters. For example, they can connect computers within or between buildings, or within a particular department, such as Accounting or Marketing. Whenever there is a situation in which you are able to install your own physical media to connect peripherals, workstations, and terminals, you would employ LAN technologies. Because of the..

Changing the Default Log Facility

Use the logging facility configuration command to change the syslog facility that the router sends error messages to: Router#configure terminal Enter configuration commands, one per line.  End with CNTL/Z. Router(config)#logging host 172.25.1.1 Router(config)#logging facility local6 Router(config)#end Router# The default syslog facility setting is local7. By default, the router will forward all syslog messages to the server's local7 log facility. You can modify this ..