Equipment Needed
The following equipment is needed to perform this lab exercise:
Two Cisco routers, each of which must have a single ISDN BRI interface. One of the routers also needs an Ethernet interface
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Cisco IOS 11.2 or higher
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Two ISDN BRI circuits
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A PC running a terminal emulation program for console port connection on the routers
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Configuration Overview
This configuration will demonstrate an OSPF demand circuit. With OSPF demand circuits, periodic hellos are suppressed and periodic refreshes of LSAs are not flooded over the ISDN link. The ISDN link will be brought up initially to exchange routing information. After initial route exchanges, the link will only be activated when there is a change in the routing topology. Without demand circuits, the OSPF periodic hello and LSA updates will keep the ISDN circuit active even though there are no changes to the routing table.
RouterA and RouterB are connected as shown in Figure 3−19.
Figure 3−19: OSPF demand circuits
A PC running a terminal emulation program should be connected to the console port of one of the routers using a Cisco rolled cable.
ISDN Switch Setup
If you do not have access to actual ISDN circuits, you can use an ISDN desktop switch. Information on configuring an ISDN desktop switch can be found in the ISDN switch configuration section earlier in this chapter.
Router Configuration
The configurations for the two routers in this example are as follows. OSPF demand commands are highlighted in bold.
RouterA
Current configuration:
!
version 12.1
service timestamps debug uptime service timestamps log uptime no service password−encryption
!
hostname RouterA
!
!
username RouterB password 0 cisco
!
ip subnet−zero
!
lane client flush
isdn switch−type basic−ni cns event−service server
!
interface Loopback0
ip address 1.1.1.1 255.255.255.255
!
interface Ethernet0/0
ip address 135.25.2.1 255.255.252.0
ip ospf demand−circuit ← RouterA is configured with the ip ospf demand−circuit command
dialer map ip 135.2.4.2 name RouterB broadcast 8995201 dialer load−threshold 255 either
dialer map ip 135.2.4.1 name RouterA broadcast dialer load−threshold 255 either
!
router ospf 64
network 2.0.0.0 0.255.255.255 area 0 network 135.0.0.0 0.255.255.255 area 0
!
ip classless no ip http server
!
dialer−list 1 protocol ip permit
!
line con 0
transport input none line aux 0
line vty 0 4 login
! end
Monitoring and Testing the Configuration
Let's start by connecting to RouterA. Verify that the ISDN circuit is up and active with the show isdn status command. We see that both SPIDs have been sent to the switch and are valid. Also notice that there are no active calls on RouterA. This is important to note since we are running OSPF over the ISDN interface. We will see shortly that our routing table is maintaining active OSPF routes without keeping the ISDN circuit active at all times.
RouterA# show isdn status
Global ISDN Switchtype = basic−ni ISDN BRI1/0 interface
dsl 8, interface ISDN Switchtype = basic−ni Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 64, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED TEI = 65, Ces = 2, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED TEI 64, ces = 1, state = 5(init)
spid1 configured, spid1 sent, spid1 valid Endpoint ID Info: epsf = 0, usid = 70, tid = 1 TEI 65, ces = 2, state = 5(init)
spid2 configured, spid2 sent, spid2 valid Endpoint ID Info: epsf = 0, usid = 70, tid = 2 Layer 3 Status:
0 Active Layer 3 Call(s) Activated dsl 8 CCBs = 0
The Free Channel Mask: 0x80000003 Total Allocated ISDN CCBs = 0
Let's view the routing table on RouterA with the show ip route command. We see that RouterA has learned about the 2.2.2.2 network via the BRI interface. Recall that the BRI interface is not currently active (no calls exist on the router). When RouterA initially powers on, the ISDN circuit will activate so that OSPF routes can be exchanged. After the initial exchange of routes, OSPF demand will bring down the ISDN call. The before OSPF demand circuit keeps the routing table entries active even though the ISDN circuit is not active. OSPF keepalive messages are suppressed.
RouterA#show ip route
Codes: C − connected, S − static, I − IGRP, R − RIP, M − mobile, B − BGP D − EIGRP, EX − EIGRP external, O − OSPF, IA − OSPF inter area N1 − OSPF NSSA external type 1, N2 − OSPF NSSA external type 2 E1 − OSPF external type 1, E2 − OSPF external type 2, E − EGP
i − IS−IS, L1 − IS−IS level−1, L2 − IS−IS level−2, ia − IS−IS inter area * − candidate default, U − per−user static route, o − ODR
P − periodic downloaded static route Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1is directly connected, Loopback0 2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/1563] via 135.2.4.2, 00:06:07, BRI1/0 135.25.0.0/22 is subnetted, 1 subnets
C 135.25.0.0 is directly connected, Ethernet0/0 135.2.0.0/22 is subnetted, 1 subnets
C 135.2.4.0 is directly connected, BRI1/0
Let's get some information on the OSPF configuration of RouterA with the show ip ospf interface bri 1/0 command. We see that the interface is configured as a demand circuit. We also see that the OSPF hello messages are being suppressed. RouterA is keeping its OSPF adjacencies even though the ISDN circuit is not active.
RouterA#sh ip ospf int bri 1/0
BRI1/0 is up, line protocol is up (spoofing) Internet Address 135.2.4.1/22, Area 0
Process ID 64, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 1562 Configured as demand circuit.
Run as demand circuit.
DoNotAge LSA allowed.
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:00
Index 3/3 , flood queue length 0 Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 2.2.2.2 (Hello suppressed)
Suppress hello for 1 neighbor(s)
Now let's connect to RouterB. Verify that the ISDN circuit is active on RouterB with the show isdn status command. Also notice that there are no active calls on RouterB.
RouterB#show isdn status
Global ISDN Switchtype = basic−ni ISDN BRI1/0 interface
dsl 8, interface ISDN Switchtype = basic−ni Layer 1 Status:
ACTIVE
Layer 2 Status:
TEI = 64, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED TEI 64, ces = 1, state = 5(init)
spid1 configured, spid1 sent, spid1 valid Endpoint ID Info: epsf = 0, usid = 70, tid = 1
TEI Not Assigned, ces = 2, state = 3(await establishment) spid2 configured, spid2 NOT sent, spid2 NOT valid Layer 3 Status:
0 Active Layer 3 Call(s) Activated dsl 8 CCBs = 0
The Free Channel Mask: 0x80000003 Total Allocated ISDN CCBs = 0
Let's view the routing table on RouterB with the show ip route command. Notice that RouterB is learning about the 1.1.1.1 and the 135.25.0.0 network via the ISDN interface. Notice that these routes are still being maintained in the routing table even though the ISDN interface is not active.
RouterB#show ip route
Codes: C − connected, S − static, I − IGRP, R − RIP, M − mobile, B − BGP D − EIGRP, EX − EIGRP external, O − OSPF, IA − OSPF inter area N1 − OSPF NSSA external type 1, N2 − OSPF NSSA external type 2 E1 − OSPF external type 1, E2 − OSPF external type 2, E − EGP
i − IS−IS, L1 − IS−IS level−1, L2 − IS−IS level−2, ia − IS−IS inter area
* − candidate default, U − per−user static route, o − ODR P − periodic downloaded static route
Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/1563] via 135.2.4.1, 00:06:42, BRI1/0 2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0 135.25.0.0/22 is subnetted, 1 subnets
O 135.25.0.0 [110/1572] via 135.2.4.1, 00:06:42, BRI1/0 135.2.0.0/22 is subnetted, 1 subnets
C 135.2.4.0 is directly connected, BRI1/0
The show ip ospf neighbor command shows us that that RouterB and RouterA are still neighbored even though the ISDN circuit is not active. Also notice that there is no OSPF dead time associated with this neighbor.
RouterB#sh ip ospf neigh
Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/ − − 135.2.4.1 BRI1/0
We also see that RouterB is configured to run as a demand circuit. Recall that the ip ospf demand−circuit command was only entered into RouterA's configuration, not RouterB's. This is because the ip ospf demand−circuit command only needs to be entered on one side of the link.
RouterB#sh ip ospf int bri 1/0
BRI1/0 is up, line protocol is up (spoofing) Internet Address 135.2.4.2/22, Area 0
Process ID 64, Router ID 2.2.2.2, Network Type POINT_TO_POINT, Cost: 1562 Run as demand circuit.
DoNotAge LSA allowed.
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:04
Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec Neighbor Count is 1, Adjacent neighbor count is 1 Adjacent with neighbor 1.1.1.1 (Hello suppressed)
Suppress hello for 1 neighbor(s)
Now let's reconnect to RouterA. Let's see how OSPF demand circuit handles a change to the network
topology. First, enable PPP authentication debugging with the debug ppp authentication command. Next, go into global configuration mode and shut down the e0/0 interface.
RouterA#config term
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#int e 0/0 RouterA(config−if)#shut RouterA(config−if)#^Z
After exiting out of configuration mode, you will see RouterA dial RouterB. The debug trace is shown in the following.
00:47:12: ISDN BR1/0: TX −> SETUP pd = 8 callref = 0x4E ← RouterA calls RouterB
00:47:12: Bearer Capability i = 0x8890 00:47:12: Channel ID i = 0x83
00:47:12: Keypad Facility i = '8995201'
00:47:12: ISDN BR1/0: RX <− CALL_PROC pd = 8 callref = 0xCE 00:47:12: Channel ID i = 0x89
00:47:13: ISDN BR1/0: RX <− CONNECT pd = 8 callref = 0xCE 00:47:13: Channel ID i = 0x89
00:47:13: ISDN BR1/0: TX −> CONNECT_ACK pd = 8 callref = 0x4E 00:47:13: %LINK−3−UPDOWN: Interface BRI1/0:1, changed state to up 00:47:13: BR1/0:1 PPP: Treating connection as a callout
00:47:13: BR1/0:1 CHAP: O CHALLENGE id 10 len 23 from "RouterA"
00:47:13: BR1/0:1 CHAP: I CHALLENGE id 10 len 23 from "RouterB"
00:47:13: BR1/0:1 CHAP: O RESPONSE id 10 len 23 from "RouterA"
00:47:13: BR1/0:1 CHAP: I SUCCESS id 10 len 4
00:47:13: BR1/0:1 CHAP: I RESPONSE id 10 len 23 from "RouterB"
00:47:13: BR1/0:1 CHAP: O SUCCESS id 10 len 4
00:47:14: %LINK−5−CHANGED: Interface Ethernet0/0, changed state to administratively down
00:47:14: %LINEPROTO−5−UPDOWN: Line protocol on Interface BRI1/0:1, changed state to up
00:47:15: %LINEPROTO−5−UPDOWN: Line protocol on Interface Ethernet0/0, changed state to down
00:47:19: %ISDN−6−CONNECT: Interface BRI1/0:1 is now connected to 8995201 RouterB ← The ISDN call is now connected and the
updated routing table can be exchanged
The show dialer command on RouterA reveals that there is an active call. Notice that the dial reason is destination traffic to 224.0.0.5. This is OSPF traffic. Since we shut down our Ethernet interface, OSPF demand activated the ISDN circuit in order to update the routing table on RouterB.
RouterA#show dialer
BRI1/0 − dialer type = ISDN
Dial String Successes Failures Last DNIS Last status 8995201 18 74 00:00:15 successful 0 incoming call(s) have been screened.
0 incoming call(s) rejected for callback.
BRI1/0:1 − dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs) Wait for carrier (30 secs), Re−enable (15 secs) Dialer state is data link layer up
Dial reason: ip (s=135.2.4.1, d=224.0.0.5) ← OSPF demand caused the ISDN circuit to dial Time until disconnect 108 secs
Connected to 8995201 (RouterB)
BRI1/0:2 − dialer type = ISDN
Idle timer (120 secs), Fast idle timer (20 secs) Wait for carrier (30 secs), Re−enable (15 secs) Dialer state is idle
Now reconnect to RouterB. The show ip route command will reveal that the 135.25.0.0 network (E0/0 on RouterA) is no longer in the routing table.
RouterB#sh ip route
Codes: C − connected, S − static, I − IGRP, R − RIP, M − mobile, B − BGP D − EIGRP, EX − EIGRP external, O − OSPF, IA − OSPF inter area N1 − OSPF NSSA external type 1, N2 − OSPF NSSA external type 2 E1 − OSPF external type 1, E2 − OSPF external type 2, E − EGP
i − IS−IS, L1 − IS−IS level−1, L2 − IS−IS level−2, ia − IS−IS inter area * − candidate default, U − per−user static route, o − ODR
P − periodic downloaded static route Gateway of last resort is not set
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/1563] via 135.2.4.1, 00:00:14, BRI1/0 2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
135.2.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 135.2.4.1/32 is directly connected, BRI1/0 C 135.2.4.0/22 is directly connected, BRI1/0