19-13 ip ospf network broadcast

Syntax Description:

This command has no arguments.

Purpose: Used to configure an NBMA network as a broadcast network.

Initial Cisco IOS Software Release: 10.0

Configuration Example 1: Fully-meshed OSPF Neighbors on an NBMA Network

OSPF views networks as being of one of three types:

• A broadcast multiaccess network ” On a broadcast multiaccess network, all routers attached to the network have a direct communication link with all other routers on the network. Examples of a broadcast multiaccess network are Ethernet, Token Ring, and FDDI. When OSPF sends a protocol packet onto a broadcast multiaccess network, all OSPF routers on the network will receive the packet.

• A point-to-point network ” On a point-to-point network, only two routers exist on the network, one at each end of the point-to-point link. Examples of a point-to-point network are High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP).

• An NBMA network ” Examples of an NBMA network are Frame Relay and X.25. On an NBMA network, all OSPF routers could possibly have connections to all other OSPF routers on the network, but all the connections are logical point-to-point links so an OSPF protocol packet sent on one link will not reach all OSPF neighbors. NBMA networks can be configured, from an OSPF point of view, as broadcast, point-to-point, or multipoint.

This example will investigate configuring Frame Relay as an OSPF broadcast network.

In Figure 19-10, three OSPF routers are fully meshed over a Frame Relay network. Every OSPF router has a connection to every other OSPF router. For this case, the Frame Relay network can be made to behave like a multiaccess network by configuring the network type as broadcast. As with all OSPF broadcast networks, a Designated Router (DR) and Backup Designated Router (BDR) will be elected for the network. Initially, the routers in Figure 19-10 are configured without specifying a network type in order to observe the OSPF behavior on an NBMA network.

Figure 19-10. An NBMA Network Is Typically Configured as a Broadcast Network when the OSPF Routers Are Fully Meshed and the PVCs Are on the Same IP Subnet

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/0   bandwidth 64   ip address 10.1.1.1 255.255.255.248   encapsulation frame-relay   frame-relay map ip 10.1.1.2 101 broadcast   frame-relay map ip 10.1.1.3 102 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi   !   router ospf 1   network 1.1.1.1 0.0.0.0 area 1   network 10.1.1.0 0.0.0.7 area 0  _________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.248   encapsulation frame-relay   frame-relay map ip 10.1.1.1 110 broadcast   frame-relay map ip 10.1.1.3 112 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi   !   router ospf 1   network 2.2.2.2 0.0.0.0 area 2   network 10.1.1.0 0.0.0.7 area 0  _________________________________________________________________ Router C  interface Loopback0   ip address 3.3.3.3 255.255.255.255   !   interface Serial0   ip address 10.1.1.3 255.255.255.248   encapsulation frame-relay   frame-relay map ip 10.1.1.1 120 broadcast   frame-relay map ip 10.1.1.2 121 broadcast   no frame-relay inverse-arp   !   router ospf 1   network 3.3.3.3 0.0.0.0 area 3   network 10.1.1.0 0.0.0.7 area 0  

Frame Relay inverse ARP is disabled on Routers A, B, and C and static frame-relay map statements are used to map remote IP addresses to the proper Frame Relay DLCI. This is not necessary for the operation of OSPF over Frame Relay but only prevents the routers from leaning DLCIs that are not used. Chapter 11 shows how the OSPF neighbor command is used to configure OSPF over an NBMA network. For this example, the OSPF interface command ip ospf network broadcast is used to have OSPF treat the Frame Relay network as a broadcast network. Without the neighbor command or the ip ospf network broadcast command, OSPF does not know how to treat the NBMA network so OSPF neighbor relationships will not be formed , as shown in the output of the show ip ospf neighbor command.

 rtrA#  show ip ospf neighbor  (no output) 

OSPF is not sending any protocol packets to the NBMA network. Modify the configurations on Routers A, B, and C to configure the NBMA network as a broadcast network.

 Router A  interface Serial0/0   bandwidth 64   ip address 10.1.1.1 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.2 101 broadcast   frame-relay map ip 10.1.1.3 102 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi  _________________________________________________________________ Router B  interface Serial0   ip address 10.1.1.2 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.1 110 broadcast   frame-relay map ip 10.1.1.3 112 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi  _________________________________________________________________ Router C  interface Serial0   ip address 10.1.1.3 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.1 120 broadcast   frame-relay map ip 10.1.1.2 121 broadcast   no frame-relay inverse-arp  

Verification

Verify that OSPF is treating the Frame Relay network as a broadcast network.

 rtrA#  show ip ospf interface serial 0/0  Serial0/0 is up, line protocol is up   Internet Address 10.1.1.1/29, Area 0   Process ID 1, Router ID 1.1.1.1,  Network Type BROADCAST  , Cost: 1562   Transmit Delay is 1 sec, State DROTHER, Priority 1   Designated Router (ID) 3.3.3.3, Interface address 10.1.1.3   Backup Designated router (ID) 2.2.2.2, Interface address 10.1.1.2   Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5     Hello due in 00:00:01   Index 1/2, flood queue length 0   Next 0x0(0)/0x0(0)   Last flood scan length is 0, maximum is 2   Last flood scan time is 0 msec, maximum is 0 msec   Neighbor Count is 2, Adjacent neighbor count is 2     Adjacent with neighbor 3.3.3.3  (Designated Router)     Adjacent with neighbor 2.2.2.2  (Backup Designated Router)   Suppress hello for 0 neighbor(s) 

Verify that Routers A, B, and C have formed a FULL OSPF neighbor relationship.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 3.3.3.3           1  FULL/DR  00:00:35    10.1.1.3        Serial0/0 2.2.2.2           1  FULL/BDR  00:00:36    10.1.1.2        Serial0/0 

Router C has been elected the DR and Router B has been elected the BDR. For a fully meshed configuration, the router selected as the DR is not important as long as all PVCs remain active and the full mesh is maintained . In the next example, the selection of the DR becomes an important issue if the routers are not fully meshed.

Configuration Example 2: Partially-meshed OSPF Neighbors on an NBMA Network

There is a scaling problem with a fully meshed broadcast network. The number of PVCs required grows exponentially with the number of routers in the mesh. The formula to determine the number of PVCs based on the number of routers (n) is given by this equation:

Therefore, five routers require 10 PVCs and 10 routers require 45 PVCs. As you can see, this can become expensive not only in terms of cost but also in terms of management complexity. If you add one router to a 10-router mesh, then an additional 11 PVCs need to be purchased and configured. The number of PVCs can be reduced if a hub-and-spoke topology is used as shown in Figure 19-11. Router A is the hub router and Routers B and C are spoke routers. Spoke routers only have a connection or PVC to the hub router. A broadcast network can be used with a partial mesh, but there are a number of concerns that need to be addressed as will be pointed out in this example. Remove the PVC between Routers B and C (see Figure 19-10) to produce the topology in Figure 19-11.

Figure 19-11. An NBMA Network Can Be Configured as a Broadcast Network Using a Partial Mesh Configuration. The Hub Router Should Always Be the DR

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Serial0/0   bandwidth 64   ip address 10.1.1.1 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.2 101 broadcast   frame-relay map ip 10.1.1.3 102 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi   !   router ospf 1   network 1.1.1.1 0.0.0.0 area 1   network 10.1.1.0 0.0.0.7 area 0  _________________________________________________________________ Router B  interface Loopback0   ip address 2.2.2.2 255.255.255.255   !   interface Serial0   ip address 10.1.1.2 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.1 110 broadcast    no frame-relay map ip 10.1.1.3 112 broadcast    no frame-relay inverse-arp   frame-relay lmi-type ansi   !   router ospf 1   network 2.2.2.2 0.0.0.0 area 2   network 10.1.1.0 0.0.0.7 area 0  _________________________________________________________________ Router C  interface Loopback0   ip address 3.3.3.3 255.255.255.255   !   interface Serial0   ip address 10.1.1.3 255.255.255.248   encapsulation frame-relay    ip ospf network broadcast    frame-relay map ip 10.1.1.1 120 broadcast    no frame-relay map ip 10.1.1.2 121 broadcast    no frame-relay inverse-arp   !   router ospf 1   network 3.3.3.3 0.0.0.0 area 3   network 10.1.1.0 0.0.0.7 area 0  

Use the command clear ip ospf process on Router A to reset OSPF. Check the status of the OSPF neighbors on Router A.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface  3.3.3.3           1   FULL/DR         00:00:35    10.1.1.3        Serial0/0  2.2.2.2           1   FULL/DROTHER    00:00:34    10.1.1.2        Serial0/0 

The first concern with a partial mesh broadcast network is the election of the DR. Router C was elected as the DR because it has the highest router ID. All routers on a broadcast network need to become adjacent with the DR. Router B cannot become adjacent with the DR because there is not a direct connection between Routers B and C. The solution is to ensure that the hub router, Router A, is elected as the DR. If Router A fails, then it does not make any difference if either Router B or C is the BDR because they will no longer have an IP path between them. Router A needs to be configured so it is always elected the DR. One way to accomplish this is to set the interface priority on Routers B and C to zero. Setting the OSPF priority to zero makes the router ineligible to become the DR on the network. The default interface priority is 1, so Router A will always be elected the DR.

Another way is to ensure that the router that you want to become DR has the highest OSPF router ID.

 Router B  interface Serial0   ip address 10.1.1.2 255.255.255.248   encapsulation frame-relay   ip ospf network broadcast    ip ospf priority 0    frame-relay map ip 10.1.1.1 110 broadcast   no frame-relay inverse-arp   frame-relay lmi-type ansi  _________________________________________________________________ Router C  interface Serial0   ip address 10.1.1.3 255.255.255.248   encapsulation frame-relay   ip ospf network broadcast    ip ospf priority 0    frame-relay map ip 10.1.1.1 120 broadcast   no frame-relay inverse-arp  

Reset the OSPF process on Router A.

 rtrA#  clear ip ospf process  Reset ALL OSPF processes? [no]:  y  

Verify that Router A is now the DR and that neither router B nor C is the BDR.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 3.3.3.3           0  FULL/DROTHER  00:00:32    10.1.1.3        Serial0/0 2.2.2.2           0  FULL/DROTHER  00:00:32    10.1.1.2        Serial0/0 _________________________________________________________________ rtrB#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1  FULL/DR  00:00:39    10.1.1.1        Serial0 _________________________________________________________________ rtrC#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1  FULL/DR  00:00:35    10.1.1.1        Serial0 

Each router is advertising its loopback network into OSPF. Check the routing tables on Routers A, B, and C to determine if the routes are being advertised to all routers.

 rtrA#  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       1.1.1.1 is directly connected, Loopback0  2.0.0.0/32 is subnetted, 1 subnets  O IA    2.2.2.2 [110/1563] via 10.1.1.2, 00:01:06, Serial0/0  3.0.0.0/32 is subnetted, 1 subnets  O IA    3.3.3.3 [110/1563] via 10.1.1.3, 00:01:06, Serial0/0  10.0.0.0/29 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0/0 _________________________________________________________________ rtrB#  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, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets  O IA    1.1.1.1 [110/65] via 10.1.1.1, 00:03:23, Serial0  2.0.0.0/32 is subnetted, 1 subnets  C       2.2.2.2 is directly connected, Loopback0  3.0.0.0/32 is subnetted, 1 subnets  O IA    3.3.3.3 [110/65] via 10.1.1.3, 00:03:23, Serial0  C    169.254.0.0/16 is directly connected, Ethernet0      10.0.0.0/29 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0 _________________________________________________________________ rtrC#  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, * - candidate default        U - per-user static route, o - ODR Gateway of last resort is not set      1.0.0.0/32 is subnetted, 1 subnets  O IA    1.1.1.1 [110/65] via 10.1.1.1, 00:03:43, Serial0  2.0.0.0/32 is subnetted, 1 subnets  O IA    2.2.2.2 [110/65] via 10.1.1.2, 00:03:44, Serial0  3.0.0.0/32 is subnetted, 1 subnets  C       3.3.3.3 is directly connected, Loopback0  10.0.0.0/29 is subnetted, 1 subnets C       10.1.1.0 is directly connected, Serial0 

The routes are being advertised, but can all routers reach them?

 rtrA#  ping 2.2.2.2  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/8 ms rtrA#  ping 3.3.3.3  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/8 ms _________________________________________________________________ rtrB#  ping 1.1.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms rtrB#  ping 3.3.3.3  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) _________________________________________________________________ rtrC#  ping 1.1.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms rtrC#  ping 2.2.2.2  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) 

Router A can ping Routers B and C because they are directly connected. Router B can ping A but not C and Router C can ping A but not B. The problem is that Routers B and C think they are directly connected because they are on the same IP subnet and the network type is broadcast. This line was highlighted previously in the routing table on Router B.

 O IA  3.3.3.3 [110/65] via 10.1.1.3, 00:03:23, Serial0 

For Router B to reach network 3.3.3.3 on Router C, it must send the packet to 10.1.1.3. Router B is not directly connected to Router C so the packet must first be sent to Router A. This is accomplished by an additional frame-relay map statement on Routers B and C.

 Router B  interface Serial0   ip address 10.1.1.2 255.255.255.248   encapsulation frame-relay   ip ospf network broadcast   ip ospf priority 0   frame-relay map ip 10.1.1.1 110 broadcast    frame-relay map ip 10.1.1.3 110 broadcast    no frame-relay inverse-arp   frame-relay lmi-type ansi  _________________________________________________________________ Router C  interface Serial0   ip address 10.1.1.3 255.255.255.248   encapsulation frame-relay   ip ospf network broadcast   ip ospf priority 0   frame-relay map ip 10.1.1.1 120 broadcast    frame-relay map ip 10.1.1.2 120 broadcast    no frame-relay inverse-arp  

19-2 Verification

Verify that all routers can ping the loopback interfaces on the other routers.

 rtrA#  ping 2.2.2.2  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms rtrA#  ping 3.3.3.3  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms _________________________________________________________________ rtrB#  ping 1.1.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/8 ms rtrB#  ping 3.3.3.3  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms _________________________________________________________________ rtrC#  ping 1.1.1.1  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/8 ms rtrC#  ping 2.2.2.2  Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 8/9/12 ms 

Troubleshooting

Step 1. Verify the IP address and netmask assignments used on the NBMA network interfaces.

Step 2. Verify that the proper DLCIs and IP addresses are used in the frame-relay map ip statements. These can be checked using the show frame-relay map command, as shown here:

 rtrA#  show frame-relay map  Serial0/0 (up): ip 10.1.1.2 dlci 101(0x65,0x1850), static,               broadcast,               CISCO, status defined, active Serial0/0 (up): ip 10.1.1.3 dlci 102(0x66,0x1860), static,               broadcast,               CISCO, status defined, active 

Step 3. Before configuring OSPF, check IP connectivity by pinging the other end of each Frame Relay link.

Step 4. If using a partial mesh with a broadcast network, ensure that the hub router is elected DR.

Step 5. If using a partial mesh with a broadcast network, ensure that the spoke routers have been configured with a frame relay map command to each spoke through the hub router.