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..

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..

Mapping IP Multicast Addresses to MAC Addresses

Assigning a Layer 3 multicast address to a multicast group (application) automatically generates a Layer 2 multicast address. Figure 19-6 shows how a multicast MAC address is calculated from a Layer 3 multicast address. The MAC address is formed using an IEEE-registered OUI of 01005E, then a binary 0, and then the last 23 bits of the multicast IP address. The method is identical for Ethernet and Fiber Distributed Data Interface (FDDI). Figure 19-6 Calculating a Multicast Des..

Weighted Random Early Detection

When a queue is full, IOS has no place to put newly arriving packets, so it discards them. This phenomenon is called tail drop. Oftentimes, when a queue fills, several packets are tail dropped at a time, given the bursty nature of data packets. Tail drop can have an overall negative effect on network traffic, particularly TCP traffic. When packets are lost, for whatever reason, TCP senders slow down their rate of sending data. When tail drops occur and multiple packets are l..

Automatically Configuring STP Timers

Modifying STP timers can be tricky, given the conservative nature of the default values and the calculations needed to derive proper STP operation. Timer values are basically dependent on the Hello Time and the switched network’s diameter, in terms of switch hops. Catalyst switches offer a single command that can change the timer values in a more controlled fashion. As described earlier, the spanning-tree vlan vlan-list root macro command is a better tool to use than setti..