OSPF network -types mismatch

[ramprasanthbe]

I have a setup of R1 -- R2. I configured the OSPF network type as broadcast in R1 and P2P in R2. 

I still see the neighbourship comes up. Will the network type is not checked in the initial hello message?

If so, what are the consequences of keeping different network types in OSPF links?. Thanks for your feedback

[Jaderson Pessoa]

Hello,

Look this good article:https://packetlife.net/blog/2008/jun/19/ospf-network-types/

In shot words.

P2P: do not scale well, if your network interface is /24 it will be propagated as /32

BROADCAST: will use multicast to neighbor statement and scale very well.

[luis_cordova]

Hi @ramprasanthbe 

For the adjacency to be successful, the hello and dead timer must be the same.
In the types of OSPF that you set these parameters are the same, so you have adjacency.
The other OSPF types these parameters are not equal, so you would not have adjacency.
Check those indicated in this link:

https://books.google.cl/books?id=of76QmGUnAoC&pg=PA416&lpg=PA416&dq=difference+between+ospf+network+types+frames&source=bl&ots=neT3Qo7WUO&sig=ACfU3U3iLrcOnlByvQooEGd8B6f_jgz94Q&hl=es&sa=X&ved=2ahUKEwjVkaGBw-flAhXXLLkGHXamA7I4FBDoATAAegQIBxAB#v=onepag...

Regards

[Jaderson Pessoa]

@luis_cordova 

Hello timer and dead timer in point-to-point and broadcast are the same.. in this case he will have adjacency.. but the difference as i said..  point-to-point will show in routing table a route /32 :)

[ramprasanthbe]

Thanks for your feedback. Let's say I have used P2P in R1 and broadcast in
R2.

R2 will send hello packets with hello-interval as 10 and dead interval as
40 and inturn that matches with R1 hello packets. I agree with that point.
R1 interface has an OSPF interface priority of 128 and it is eligible to
participate in the DR election, but P2P network type stops R1 to
participate in the DR election.

So hello's sent out from R1 will not have any DR address 0.0.0.0 and hellos
sent from R2 will have DR address. Hence R1 will not able to able to
participate in DR election and R1 neighbourship towards R2 will be in
Full/- state and R2 neighbourship towards R1 is also in Full state.

Then what is the use of DR field for the packet coming from R2. if the DR
and BDR election is not completed in ospf . will OSPF will complete
adjacency?

Will this is not enough for OSPF to deduct the network type mismatch?

I tested in other vendor routers and it is not allowing to bring the
neighborship
up.

Thanks for your help.

[Giuseppe Larosa]

Hello Ramprasanthibe,

in my experience:

a network type mismatch between broadcast and P2P can cause routing problems.

The broadcast side would like to elect a DR /BDR and will wait up to wait time 40 seconds to do that.

After that the R1 broadcast side will start to send OSPF hellos every 10 seconds stating:

DR = my own OSPF RID example 10.1.1.1.1

BDR= 0.0.0.0  ! means unknown

neighbor seen : R2 LAN IP address

The two devices can reach the two way status ( I see your IP address in respective hello messages), but they cannot reach full adjacency:

R1 will create an LSA type 2 with LSA id = R1's LAN interface to R2 with an emptly list of neighbors

R1 will create its own Router LSA for the area the link is in.

R2 will only create a Router LSA for the area the common link is in.

R1 expects to receive updates on ALLOSPFDR 224.0.0.6

R2 uses only allospf routers on link 224.0.0.5

on point to point links we see in show ip ospf neighbor

FULL/-

on broadcast links possible cases are:

the device is not a DR/BDR:

FULL/DR

FULL/BDR

two way

two way

.......

###############################

For a DR/BDR the show ip ospf neigh will tell:

FULL/BDR

FULL/DROTHER

FULL/DROTHER

.....

the use of point to point OSPF network type (both ends) started in service provider networks for single dedicated high speed links to avoid creation of un necessary L2 network type.

As a further step for IPv4 p2p links the right to use /31 subnets has been added.

The Choice of the DR and BDR in a broadcast environment must be careful and you need to play with OSPF priority to avoid to have the same devices elected as DR/BDR for their  highest OSPF RID.

Another way to say resources on user facing Vlans/subnets is to use

router ospf 10

passive-interface Vlan50

passive-interface gi5/2

......

Where Vlan50 is the SVI of a multilayer switch.

Edit:

For access multilayer switches :

the passive-interface default can be used

an NSSA area can be used for each site a L3 or L2 port channel can be used between the multilayer switches.

uplinks are in area 0.0.0.0 of course

Hope to help

Giuseppe

[Jaderson Pessoa]

Hello, @ramprasanthbe 

as i said, adjacency will be form without issue.  Like exemple below;

and routes will be available on both routers

but it is not common in environments.

Regards,

[chetannevergivesup]

Yes the neighbor adjacencies do form , OPSF do exchange LSA's but it wont run SPF and wont install the route into the routing table.