Section 3: ISDN (4 Points)

• Ensure that the ISDN is used should the Frame Relay network fail; use a Layer 3 method of invoking the line. Make R1 initiate the call. Use CHAP for authentication.

You have two methods of providing Layer 3 ISDN backup, Dialer Watch and an OSPF Demand circuit. As you have not been provided with which networks in particular require backing up, you have to configure the Demand circuit. When you configure an OSPF demand circuit, you should be aware that any topology change will cause your ISDN circuit to become active so you must ensure you have a stable network; otherwise, your line will constantly be connected. You configure only one end of the link with the ip ospf demand-circuit command and this should be placed on R1 as the question states that R1 should initiate the call. You can, therefore, configure R4 without a number to dial R1 so R4 never dials R1. Example 2-38 shows R1 with the ISDN line disconnected and still maintaining a full neighbor adjacency with R4 over the ISDN; Example 2-39 shows R4 in the same state.

Example 2-38. R1 OSPF Demand Status

 R1#sh isdn act --------------------------------------------------------------------------------                                 ISDN ACTIVE CALLS -------------------------------------------------------------------------------- Call    Calling      Called       Remote  Seconds Seconds Seconds Charges Type    Number       Number       Name    Used    Left    Idle    Units/Currency -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- R1#sh ip ospf neigh Neighbor ID     Pri   State           Dead Time   Address         Interface 10.4.4.4          1   FULL/  -        00:00:34    10.100.100.3    Virtual-Access1 10.4.4.4          1   FULL/  -           -        10.10.10.2      BRI0/0 

Example 2-39. R4 OSPF Demand Status

 R4#sh isdn act --------------------------------------------------------------------------------                                 ISDN ACTIVE CALLS -------------------------------------------------------------------------------- Call    Calling      Called       Remote  Seconds Seconds Seconds Charges Type    Number       Number       Name    Used    Left    Idle    Units/Currency -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- R4#sh ip ospf neigh Neighbor ID     Pri   State           Dead Time   Address         Interface 10.1.1.1          1   FULL/  -        00:00:31    10.100.100.1    Virtual-Access1 10.200.200.5      1   FULL/  -        00:00:37    10.100.101.2    Virtual-Access2 10.1.1.1          1   FULL/  -           -        10.10.10.1      BRI0/0 

It is good practice to configure PPP circuits with the command no peer neighbor-route to remove the /32 host route that is added to the routing table when a PPP link becomes active. You might find that when your ISDN line disconnects, this route will be removed from the routing table and cause an OPSF topology change; this will cause your ISDN line to re-establish. You should have already configured the peer route option on your virtual templates on R1 and R4 earlier within the OSPF section to remove the host routes. As requested in the question, you must use CHAP authentication; you should already have username and passwords configured for the PPP over Frame Relay section so simply enable ppp authentication chap on the ISDN interfaces of R1 and R4. Do not forget to add authentication to the ISDN network as itis part of OSPF area 0. If you have configured this correctly as shown in Example 2-40 and Example 2-41, you have scored 4 points.

Example 2-40. R1 ISDN Configuration

 username R4 password 0 cisco ! interface BRI0/0  ip address 10.10.10.1 255.255.255.240  encapsulation ppp  ip ospf message-digest-key 1 md5 james  ip ospf demand-circuit  dialer map ip 10.10.10.2 name R4 broadcast 2222  dialer-group 1  isdn switch-type basic-net3  no peer neighbor-route  ppp authentication chap ! dialer-list 1 protocol ip permit 

Example 2-41. R4 ISDN Configuration

 username R1 password 0 cisco ! interface BRI0/0  ip address 10.10.10.2 255.255.255.240  encapsulation ppp  ip ospf message-digest-key 1 md5 james  dialer map ip 10.10.10.1 name R1 broadcast  dialer-group 1  isdn switch-type basic-net3  no peer neighbor-route  ppp authentication chap ! dialer-list 1 protocol ip permit