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Implementing Basic DDR 153
Bandwidth on Demand
Bandwidth on demand (BOD) is a Cisco proprietary implementation that allows the aggregation
of multiple B channels into a single logical connection. This implementation is widely used in
Cisco-centric networks.
The implementation of Cisco's BOD solution is accomplished through the dialer load-
threshold command. The variable parameter in the command is load. When the interface is
connected to the remote side, a measurement of load is kept and updated continually based on
utilization of the link. The load is measured on a scale of 1­255, with 255 representing link
saturation. This command is typically utilized in almost every DDR configuration to provide
load sharing over both B channels. Example 6-16 demonstrates this concept.
The dialer load-threshold 110 statement specifies that if the load of the first B channel reaches
110 (about 43 percent utilization), the second B channel should be initialized and, once
connected, the traffic should load balance across both channels.
The router recalculates the load of the link every 5 minutes to maintain an accurate picture
without unnecessarily using CPU cycles. In a multivendor environment, BOD may not be a
viable choice due to its proprietary nature. For such cases, Multilink PPP is more appropriate.
Multilink PPP
Multilink PPP is a specification that enables the bandwidth aggregation of multiple B channels
into one logical pipe. Its mission is comparable to that of Cisco's BOD. More specifically, the
Multilink PPP feature provides load-balancing functionality over multiple wide area network
(WAN) links, while providing multivendor interoperability, packet fragmentation and proper
sequencing, and load calculation on both inbound and outbound traffic.
Cisco's implementation of Multilink PPP supports the fragmentation and packet sequencing
specifications in RFC 1717. Multilink PPP enables packets to be fragmented and the fragments
to be sent (at the same time) over multiple point-to-point links to the same remote address.
Example 6-16
Implementing Cisco's BOD Feature
RouterA(config)#interface BRI0
RouterA(config-if)#ip address 10.12.1.1 255.255.255.0
RouterA(config-if)#encapsulation ppp
RouterA(config-if)#dialer idle-timeout 180
RouterA(config-if)#dialer load-threshold 110
RouterA(config-if)#dialer map ip 10.12.1.2 speed 56 2145552222
RouterA(config-if)#dialer map ip 10.12.1.2 speed 56 2145552223
RouterA(config-if)#dialer-group 1
!
RouterA(config)#dialer-list 1 protocol ip permit
!
RouterA(config)#ip route 10.13.1.0 255.255.255.0 10.12.1.2