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Implementing Basic DDR 145
Step 3: Specifying Static Routes
In the classic DDR model, dynamic routing protocol updates are not moving across the link, so
it is important that static routes be used in place of dynamic updates. To provide bidirectional
reachability between the two sites in the absence of routing protocol traffic, static routes should
be configured at both the local and remote routers. As demonstrated in Example 6-6, any IP
traffic that needs to cross the link has been defined as interesting and will trigger a call setup.
Do not confuse the definition of interesting traffic with the implementation of security
measures. DDR defines only what types of traffic can initiate a call, not what can go across it.
Once a call has been established, any type of traffic that has been configured on the BRI
interface traverses the link freely. This includes routing updates. If the IP network on which the
BRI interface exists is included in the routing protocol configuration (and the BRI interface isn't
specified as passive), routing updates can flow across the link while it is active. Once static
routes have been specified, it is important to make the BRI interface(s) passive. Passive
interfaces are discussed later in this chapter.
Static routes are necessary in DDR because the ISDN link is not always active. In a dynamic
routing environment, the fact that the link is down could be construed as a network down
condition and reachability could be lost. To combat this, the link shows that it's spoofing while
it's down. Example 6-7 demonstrates this through the show interface bri 0 command.
The first line in the output shows that the interface is up and the line protocol is down, but the
line protocol is actually up because the router knows this is a DDR connection and keeps the
line protocol state at up (spoofing).
Example 6-7
show interface bri 0 Command Output
RouterA#show interface bri 0
BRI0 is up, line protocol is up (spoofing)
Hardware is BRI
MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation PPP, loopback not set
Last input 0:00:06, output 0:00:06, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0 (size/max/drops); Total output drops: 0
Output queue: 0/64/0 (size/threshold/drops)
Conversations 0/1 (active/max active)
Reserved Conversations 0/0 (allocated/max allocated)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
359 packets input, 5814 bytes, 1 no buffer
Received 0 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
70 packets output, 307 bytes, 0 underruns
0 output errors, 0 collisions, 6 interface resets, 0 restarts
0 output buffer failures, 0 output buffers swapped out
5 carrier transitions