This Tech Note explains an issue with the formation of OSPF adjacency when the dialer interfaces are configured as point-to-point links.
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The OSPF network-type on Primary Rate Interface (PRI), Basic Rate Interface (BRI), and dialer interfaces is point-to-point, which means that an interface can't form adjacency with more than one neighbor. A common problem when a PRI, BRI, or dialer interfaces try to form an OSPF adjacency is the neighbor gets stuck in the exstart/exchange process. Let's look at an example.
Using the show ip ospf neighbor command, we can see the neighbor state is stuck in "EXSTART".
RTR-A# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 188.8.131.52 1 EXSTART/ - 00:00:37 184.108.40.206 Serial6/0:23 220.127.116.11 1 EXSTART/ - 00:00:39 18.104.22.168 Serial6/0:23 RTR-B# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 22.214.171.124 1 EXSTART/ - 00:00:36 126.96.36.199 BRI0 RTR-C# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 188.8.131.52 1 EXSTART/ - 00:00:35 184.108.40.206 BRI0
RTR-Bs configuration shows the network-type is point-to-point:
RTR-B# show ip ospf interface bri0 BRI0 is up, line protocol is up (spoofing) Internet Address 220.127.116.11/24, Area 2 Process ID 1, Router ID 18.104.22.168, Network Type POINT_TO_POINT, Cost: 1562 Transmit Delay is 1 sec, State POINT_TO_POINT, Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 Hello due in 00:00:06 Index 1/1, 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 0 Suppress hello for 0 neighbor(s)
We can debug this situation using the debug ip ospf adj command. Let's look at some sample output taken while running this command on RTR-B in the figure above:
1: Send DBD to 22.214.171.124 on BRI0 seq 0xB41 opt 0x42 flag 0x7 len 32 2: Rcv DBD from 126.96.36.199 on BRI0 seq 0x1D06 opt 0x42 flag 0x7 len 32 mtu 1500 state EXSTART 3: First DBD and we are not SLAVE 4: Rcv DBD from 188.8.131.52 on BRI0 seq 0xB41 opt 0x42 flag 0x2 len 92 mtu 1500 state EXSTART 5: NBR Negotiation Done. We are the MASTER 6: Send DBD to 184.108.40.206 on BRI0 seq 0xB42 opt 0x42 flag 0x3 len 92 7: Database request to 220.127.116.11 8: sent LS REQ packet to 18.104.22.168, length 12 9: Rcv DBD from 22.214.171.124 on BRI0 seq 0x250 opt 0x42 flag 0x7 len 32 mtu 1500 state EXCHANGE 10: EXCHANGE - inconsistent in MASTER/SLAVE 11: Bad seq received from 126.96.36.199 on BRI0 12: Send DBD to 188.8.131.52 on BRI0 seq 0x2441 opt 0x42 flag 0x7 len 32 13: Rcv DBD from 184.108.40.206 on BRI0 seq 0x152C opt 0x42 flag 0x2 len 92 mtu 1500 state EXSTART 14: Unrecognized dbd for EXSTART 15: Rcv DBD from 220.127.116.11 on BRI0 seq 0xB42 opt 0x42 flag 0x0 len 32 mtu 1500 state EXSTART 16: Unrecognized dbd for EXSTART
Lines 1 - 3: RTR-B sends the first DBD to 18.104.22.168 (RTR-A) with seq 0xB41 and receives the first DBD from 22.214.171.124 (RTR-A) with seq# 0x1D06. Neighbor negotiation is still not complete.
Lines 4 - 6: RTR-B receives a reply from 126.96.36.199 (RTR-A) indicating that RTR-A received RTR-B's first DBD. Since RTR-B has the higher router ID, RTR-A elects itself slave. After receiving the acknowledgement from RTR-A, RTR-B declares itself master and sends the first DBD with data in it. Note the sequence number, which is 0xB42. Since RTR-B is the master, only it can increment the sequence number.
Line 7: RTR-B requests data from RTR-A since RTR-A indicated it has more data to send (flag set to 0x2 in last DBD received from RTR-A).
Line 8: RTR-B sends a link-state request packet to 188.8.131.52 (RTR-A). This is an OSPF packet type 3. This packet is usually sent to the neighbor's IP address. In this case, the neighbor's IP address is its router ID.
Lines 9 - 11: RTR-B receives a reply from slave (RTR-A) with a completely different sequence number and a flag of 0x7, which is the init flag. This DBD was intended for another router (most likely RTR-C), but RTR-B incorrectly received it. RTR-B declares there is a discrepancy because a flag of 0x7 means the slave has changed its status to master by setting the MS (Master/Slave) bit during the adjacency exchange. RTR-B also complains about the sequence number because it's out of order. The slave should always follow the master's sequence number.
Line 12: RTR-B re-initializes the adjacency by sending the first DBD to 184.108.40.206 to re-elect master and slave.
Lines 13 - 14: RTR-B receives a DBD from 220.127.116.11 (RTR-A), indicating that it's a slave, without recognizing RTR-B's sequence number. RTR-B declares that it doesn't recognize this DBD since the master and slave negotiation is not complete yet. This DBD packet was intended for another router.
Line 15: RTR-B receives a reply from 18.104.22.168 (RTR-A) for the old DBD, but it's too late because RTR-B has already re-initialized the adjacency process.
Line 16: RTR-B fails to recognize this DBD because it is for an "old" adjacency, which RTR-B has already torn down.
This process will repeat endlessly.
According to section 8.1 of RFC 2328 , OSPF sends a multicast packet for a point-to-point network-type even after the interface achieves the 2-way state. Since RTR-A is trying to form adjacencies with both RTR-B and RTR-C, RTR-B receives DBD packets meant for RTR-C and RTR-C receives DBD packets meant for RTR-B.
To solve this problem, change the network type on all routers to point-to-multipoint. This changes the behavior of OSPF to send unicast packets after the 2-way state. Now RTR-B receives only packets destined for itself and RTR-C receives packets destined for itself. Changing the network-type in this way ensures that the OSPF router will form adjacency on a PRI, BRI, or dialer interface.
To change the network-type, enter the following configuration commands, ending each line by pressing ENTER. We'll change RTR-B as an example.
RTR-B# configure terminal RTR-B(config)# int bri 0 RTR-B(config-if)# ip ospf network point-to-multipoint RTR-B(config-if)# end
Now if we look at the show commands for RTR-B, we can verify that the network-type is point-to-multipoint and the state is full.
RTR-B# show ip ospf interface bri0 BRI0 is up, line protocol is up (spoofing) Internet Address 22.214.171.124/24, Area 2 Process ID 1, Router ID 126.96.36.199, Network Type POINT_TO_MULTIPOINT, Cost: 1562 Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT, Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5 Hello due in 00:00:16 Index 1/1, 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 172.16.141.10 Suppress hello for 0 neighbor(s) RTR-B# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 172.16.141.10 1 FULL/ - 00:01:36 188.8.131.52 BRI0
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