Settings that must match OSPF Neighborship

[alex_ciobanu]

Hello,

According to official certification guide for CCIE v4, it is written that one of the conditions that 2 routers can become neighbors is "OSPF Hello and Dead timers must be equal".

As it is known, the timers can be configured in 2 ways, on interface level: "ip ospf hello-interval <sec>" and "ip ospf dead-interval <sec>". The other way is to configure hello-interval through the dead-interval command using the "ip ospf dead-interval minimal hello-multiplier <xxx>" option. With this command, the dead-interval is automatically set to 1 and the hello-interval is set to a fraction of a second, based on the multiplier. If xxx = 10, the hello-interval is going to be 1/10 = 100 msec (10 hello messages per second)

I have 2 routers configured with different multipliers --> so we have different hello-intervals, and still the neighbors are UP, and even if I clear the ospf process, the adjaceny still loads.

Is this a small exception, stating the the hello-intervals must be equal only if they are higher than 1 sec for adjaceny to form, or just a bug ?

R2(config-if)#do sh run int fa0/0

Building configuration...

Current configuration : 144 bytes

!

interface FastEthernet0/0

ip address 10.0.0.2 255.255.255.0

ip ospf dead-interval minimal hello-multiplier 10

R2(config-if)#do sh ip ospf inte fa0/0

FastEthernet0/0 is up, line protocol is up

  Internet Address 10.0.0.2/24, Area 0

  Process ID 2, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 10

  Transmit Delay is 1 sec, State DR, Priority 1

  Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2

  Backup Designated router (ID) 1.1.1.1, Interface address 10.0.0.1

  Timer intervals configured, Hello 100 msec, Dead 1, Wait 1, Retransmit 5

R1#sh run int fa0/0

Building configuration...

Current configuration : 143 bytes

!

interface FastEthernet0/0

ip address 10.0.0.1 255.255.255.0

ip ospf dead-interval minimal hello-multiplier 6

duplex auto

speed auto

R1#show ip ospf interface fa0/0

FastEthernet0/0 is up, line protocol is up

  Internet Address 10.0.0.1/24, Area 0

  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1000

  Transmit Delay is 1 sec, State BDR, Priority 1

  Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2

  Backup Designated router (ID) 1.1.1.1, Interface address 10.0.0.1

  Timer intervals configured, Hello 166 msec, Dead 1, Wait 1, Retransmit 5

R1#clear ip ospf process

Reset ALL OSPF processes? [no]: yes

R1#

*Mar  1 01:30:05.463: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

*Mar  1 01:30:05.727: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from LOADING to FULL, Loading Done

R1#show ip os

R1#show ip ospf ne

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           1   FULL/DR         912 msec    10.0.0.2        FastEthernet0/0

R1#

Thanks in advance for replies !

[Peter Paluch · Accepted solution]

Hello Alexandru,

This document describes it nicely:

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/fasthelo.html#wp1036997

When fast hello packets are configured on the interface, the hello  interval advertised in the hello packets that are sent out this  interface is set to 0. The hello interval in the hello packets received  over this interface is ignored.

The dead interval must be consistent on a segment, whether it is set to 1  second (for fast hello packets) or set to any other value. The hello  multiplier need not be the same for the entire segment as long as at  least one hello packet is sent within the dead interval.

From the viewpoint of Hello messages, in Cisco OSPF Fast Hello implementation, the Hello time is advertised as 0 while the Dead interval is advertised as 1. The hello multiplier value is not indicated in Hello messages and hence cannot be compared. That is the reason your OSPF adjacencies come up despite having different hello multipliers set - because the Hello/Dead values in Hello packets are identical.

Under normal circumstances, the Hello/Dead intervals must be identical for all OSPF speakers on a common network segment.

Best regards,

Peter

[alex_ciobanu]

Thank you for reply Peter, now I understand the whole logic about this. I thought its a bug in OSPF or I am missing something. I did also one capture of a Hello packet now and saw that the "Hello Interval" was set to 0.