Troubleshooting ISDN BRI SPIDs
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Content 4.3
DDR Configuration 4.3.1 DDR operation
Dial-on-demand routing (DDR) is triggered when traffic that
matches a predefined set of criteria is queued to be sent out a
DDR-enabled interface. The traffic that causes a DDR call to be
placed is referred to as interesting traffic. Once the router
has transmitted the interesting traffic, the call is
terminated.The key to efficient DDR operation is in the
definition of interesting traffic. Interesting traffic is
defined with the dialer-list command. Dialer lists can
allow all traffic from a specific protocol to bring up a DDR
link, or they can query an access list to see what specific
types of traffic should bring up the link. Dialer lists do not
filter traffic on an interface. Even traffic that is not
interesting will be forwarded if the connection to the
destination is active. DDR is implemented in Cisco routers in
the following steps: - The router receives traffic,
performs a routing table lookup to determine if there is a
route to the destination, and identifies the outbound
interface.
- If the outbound interface is configured
for DDR, the router does a lookup to determine if the traffic
is interesting.
- The router identifies the dialing
information necessary to make the call using a dialer map to
access the next-hop router.
- The router then checks to
see if the dialer map is in use. If the interface is currently
connected to the desired remote destination, the traffic is
sent. If the interface is not currently connected to the remote
destination, the router sends call-setup information through
the BRI using the D channel.
- After the link is
enabled, the router transmits both interesting and
uninteresting traffic. Uninteresting traffic can include data
and routing updates.
- The idle timer starts and runs as
long as no interesting traffic is seen during the idle timeout
period and disconnects the call based on the idler timer
configuration.
The idle timer setting specifies the
length of time the router should remain connected if no
interesting traffic has been sent. Once a DDR connection is
established, any traffic to that destination will be permitted.
However, only interesting traffic resets the idle timer. Web
Links DDR http://www.cisco.com/en/US/tech/tk801/
tk133/tech_protocol_family_ home.html
Content
4.3 DDR Configuration 4.3.2
Configuring legacy DDR Legacy DDR is a term used to define a
very basic DDR configuration in which a single set of dialer
parameters is applied to an interface. If multiple unique
dialer configurations are needed on one interface, then dialer
profiles should be used. To configure legacy DDR perform the
following steps: - Define static routes
-
Specify interesting traffic
- Configure the dialer
information
Lab Activity Lab Exercise:
Configuring Legacy DDR In this lab, the student will configure
an ISDN router to make a Legacy dial-on-demand routing (DDR)
call to another ISDN capable router. Web Links
Configuring Legacy DDR Spokes http://www.cisco.com/en/US/products/sw/
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chapter09186a0080087272.html
Content 4.3
DDR Configuration 4.3.3 Defining static routes
for DDR To forward traffic, routers need to know what route to
use for a given destination. When a dynamic routing protocol is
used, the DDR interface will dial the remote site for every
routing update or hello message if these packets are defined as
interesting traffic. To prevent the frequent or constant
activation of the DDR link necessary to support dynamic routing
protocols, configure the necessary routes statically. To
configure a static route for IP use the following command:
Router(config)#ip route net-prefix mask
{address | interface} [distance]
[permanent] The Central router has a static route to
network 10.40.0.0 on the Home router. The Home router has two
static routes defined for the two subnets on the Central LAN.
If the network attached to the Home router is a stub network,
then all non-local traffic should be sent to Central. A default
route is a better choice for the Home router in this instance.
Home(config)#ip route 0.0.0.0 0.0.0.0 10.1.0.2 When
configuring static routes, consider the following: - By
default, a static route will take precedence over a dynamic
route because of its lower administrative distance. Without
additional configuration, a dynamic route to a network will be
ignored if a static route is present in the routing table for
the same network.
- To reduce the number of static route
entries, define a summarized or default static route.
Web Links Evaluating Backup Interfaces,
Floating Static Routes, and Dialer Watch for DDR Backup
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Content 4.3
DDR Configuration 4.3.4 Specifying interesting
traffic for DDR DDR calls are triggered by interesting traffic.
This traffic can be defined as any of the following:
- IP traffic of a particular protocol type
- Packets
with a particular source address or destination
- Other
criteria as defined by the network administrator
Use
the dialer-list command to identify interesting
traffic. The command syntax is as follows:
Router(config)#dialer-list dialer-group-num
protocol protocol-name {permit |
deny | list access-list-number} The
dialer-group-num is an integer between 1 and 10 that
identifies the dialer list to the router. The command
dialer-list 1 protocol ip permit will allow all IP traffic
to trigger a call. Instead of permitting all IP traffic, a
dialer list can point to an access list in order to specify
exactly what types of traffic should bring up the link. The
reference to access list 101 in dialer list 2 prevents FTP and
Telnet traffic from activating the DDR link. Any other IP
packet is considered interesting, and will therefore initiate
the DDR link. Web Links Configuration Examples
http://www.cisco.com/en/US/tech/tk801/
tk133/tech_configuration_ examples_list.html
Content
4.3 DDR Configuration 4.3.5
Configuring DDR dialer information There are several steps
involved in configuring the DDR interface. PPP is configured on
the dialer interface using the same commands that enable PPP on
a serial interface. HDLC is the default encapsulation for an
ISDN interface on a Cisco router, but most networks employ PPP
for circuit-switched connections. Because of its robustness,
interoperability, and additional features such as
authentication, PPP is the data link protocol in use on the B
channels of most routers. To configure PPP on the DDR interface
use the following commands: Home(config)#username Central
password cisco
Home(config)#interface
bri0/0
Home(config-if)#encapsulation ppp
Home(config-if)#ppp authentication chap
Home(config-if)#ip address 10.1.0.1 255.255.255.0 A
dialer list specifying the interesting traffic for this DDR
interface needs to be associated with the DDR interface. This
is done using the dialer-group group-number