backbone area

In OSPF, a network that has more than one area must have a backbone area, configured as All areas in an autonomous system (AS) must connect to the backbone area.

backup port

In RSTP, the backup port supports the designated port on the same attached LAN segment. Backup ports exist only when the bridge is connected as a self-loop or to a shared media segment.

backup router

In VRRP, the backup router is any VRRP router in the VRRP virtual router that is not elected as the master. The backup router is available to assume forwarding responsibility if the master becomes unavailable.


Backup designated router. In OSPF, the system elects a designated router (DR) and a BDR. The BDR smooths the transition to the DR, and each multi-access network has a BDR. The BDR is adjacent to all routers on the network and becomes the DR when the previous DR fails. The period of disruption in transit traffic lasts only as long as it takes to flood the new LSAs (which announce the new DR). The BDR is elected by the protocol; each hello packet has a field that specifies the BDR for the network.


Border Gateway Protocol. BGP is a router protocol in the IP suite designed to exchange network reachability information with BGP systems in other autonomous systems. You use a fully meshed configuration with BGP.

BGP provides routing updates that include a network number, a list of ASs that the routing information passed through, and a list of other path attributes. BGP works with cost metrics to choose the best available path; it sends updated router information only when one host has detected a change, and only the affected part of the routing table is sent.

BGP communicates within one AS using Interior BGP (IBGP) because BGP does not work well with IGP. Thus the routers inside the AS maintain two routing tables: one for the IGP and one for IBGP. BGP uses exterior BGP (EBGP) between different autonomous systems.

bi-directional rate shaping

A hardware-based technology that allows you to manage bandwidth on Layer 2 and Layer 3 traffic flowing to each port on the switch and to the backplane, per physical port on the I/O module. The parameters differ across platforms and modules.


In the Extreme Networks implementation, you can configure the switch so that traffic is silently dropped. Although this traffic appears as received, it does not appear as transmitted (because it is dropped).


Bootstrap Protocol. BOOTP is an Internet protocol used by a diskless workstation to discover its own IP address, the IP address of a BOOTP server on the network, and a file that can be loaded into memory to boot the machine. Using BOOTP, a workstation can boot without a hard or floppy disk drive.


Bridge protocol data unit. In STP, a BPDU is a packet that initiates communication between devices. BPDU packets contain information on ports, addresses, priorities, and costs and they ensure that the data ends up where it was intended to go. BPDU messages are exchanged across bridges to detect loops in a network topology. The loops are then removed by shutting down selected bridge interfaces and placing redundant switch ports in a backup, or blocked, state.


In conventional networking terms, bridging is a Layer 2 function that passes frames between two network segments; these segments have a common network layer address. The bridged frames pass only to those segments connected at a Layer 2 level, which is called a broadcast domain (or VLAN). You must use Layer 3 routing to pass frames between broadcast domains (VLANs).

In wireless technology, bridging refers to forwarding and receiving data between radio interfaces on APs or between clients on the same radio. So, bridged traffic can be forwarded from one AP to another AP without having to pass through the switch on the wired network.


A broadcast message is forwarded to all devices within a VLAN, which is also known as a broadcast domain. The broadcast domain, or VLAN, exists at a Layer 2 level; you must use Layer 3 routing to communicate between broadcast domains, or VLANs. Thus, broadcast messages do not leave the VLAN. Broadcast messages are identified by a broadcast address.


Basic Service Set. A wireless topology consisting of one access point connected to a wired network and a set of wireless devices. Also called an infrastructure network. See also IBSS.