OSPF and neighbors 1

[bnorton916]

I believe I am misunderstanding something.

I am using packet tracer to better understand ospf for the CCNA

I have a network with about 10 routers and all in area 0.

Where I am confused is that different routers seem to have different DR and BDR.
i.e.

Router3#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.10.6.1 1 FULL/BDR 00:00:33 10.10.3.1 FastEthernet0/0
172.32.1.1 1 FULL/DR 00:00:33 10.10.7.2 FastEthernet0/1

Router5#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.10.9.1 1 FULL/DR 00:00:30 10.10.8.1 Ethernet0/0/0
10.10.5.1 1 FULL/BDR 00:00:30 10.10.5.1 FastEthernet0/0

Router5#show ip ospf database
OSPF Router with ID (10.10.8.2) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count
172.32.1.1 172.32.1.1 1046 0x80000002 0x00b6c5 1
10.10.6.1 10.10.6.1 1046 0x80000005 0x00bfb5 3
172.20.1.1 172.20.1.1 1051 0x80000003 0x00d6e5 2
10.10.8.2 10.10.8.2 1046 0x80000004 0x004843 2
10.10.9.1 10.10.9.1 1046 0x80000004 0x008b03 2
10.10.7.1 10.10.7.1 1046 0x80000005 0x00313a 3
10.10.5.1 10.10.5.1 1041 0x80000006 0x001d44 3
172.16.1.1 172.16.1.1 1041 0x80000003 0x00f8e2 2

Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.10.2.2 10.10.6.1 1051 0x80000001 0x00dc17
10.10.5.2 10.10.8.2 1046 0x80000001 0x007c46
10.10.1.1 172.16.1.1 1046 0x80000001 0x004a05
10.10.7.2 172.32.1.1 1046 0x80000001 0x005f26
10.10.3.2 10.10.7.1 1046 0x80000001 0x00d718
10.10.9.2 172.20.1.1 1051 0x80000001 0x005346
10.10.8.1 10.10.9.1 1046 0x80000001 0x003183

Where am I missing something?

Thanks,

Bill

 

[leedsit]

Hi,

Not fully sure of your Logical Layer 3 setup. However you have to remember that each Layer 2 network, or broadcast domain needs to have a DR and BDR to prevent to many neighborships forming which can have an affect on convergence times etc. Lets say you have a Layer 2 nework, with 10 router interfaces attached(different Routers), each of those interfaces will form ospf neighborships with each other, I forget the formular but on that Layer 2 network, thats nearly 100 neighborships formed. Thats why we have a DR and BDR per Layer 2 network or subnet. Non DR and BDR routers form neighborships with only those two routers vastly reducing the number of neighborships formed

Dont forget, If you have a router with 6 interfaces in different subnets(as they will be), each interface may be the DR or BDR for that subnet. Its best to think as DRs and BDRs from an Interface perspective, not a routers.

Hope that makes sense.

Lee.


LEEroy
CCNP,CCIP

 

[bnorton916]

OK, I am feeling a little bit stupid.

Don't you normally only have two routers in a subnet? I can see situations where you wouldn't but my thinking(though limited), is that would not be normal.

Let me ponder this in my mind.

Thanks for your help.

Bill

 

[bnorton916]

Ok, so there can only be 1 DR and 1 BDR per subnet, I thought it was per area. After thinking about it, it has to be this way since ospf are broadcast.
show ip ospf detail was useful for me.

Still a lingering question please look at pic:

Why does router5 output this:

Router#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
10.10.5.1 1 FULL/BDR 00:00:35 10.10.5.1 FastEthernet0/0
10.10.9.1 1 FULL/DR 00:00:35 10.10.8.1 Ethernet0/0/0

What am I missing?

Bill

 

[leedsit]

Hi,

It is a little tricky to get your head around at first, but the trick to remember is one DR and BDR per subnet/broadcast domain, as you progress you will see ospf does work differently for WAN technologies, for example on point to point links, no DR or BDR is elected as its a point to point link, only two devices can exist, the A Interface and the B interface. Frame relay is a little trickier... Its all down to the fact OSPF mainly uses Multicast packets to form these neighborships.

Imagine a VLAN/Switch with 3 sets/pair of firewalls connected, a couple of routers in an aggregation layer. The firewalls may be active active for all 6 participating on ospf, both the routers, thats 8 devices on one subnet all talking ospf.

Found the formular, so with 8 devices, n*(n-1)/2 n being devices 8*(8-1)/2 would equal 28 adjacencies/relationships.(think thats right but you get the jist)

Using a DBR and DR reduces the number significantly.

I cannot see you image for some reason, just a boxed X.

Lee.


LEEroy
CCNP,CCIP

 

[leedsit]

Managed to get the pic up on my phone.

Ok so...

Neighbor ID Pri State Dead Time Address Interface
10.10.5.1 1 FULL/BDR 00:00:35 10.10.5.1 FastEthernet0/0
10.10.9.1 1 FULL/DR 00:00:35 10.10.8.1 Ethernet0/0/0

Remember that the 10.10.5.1 and the 10.10.9.1 are the OSPF ROUTER ID`s, not the IP addresses, it just happends that if you dont set the router Id it will use the highest active Loopback or Physical Interface on that router as the ID.

Its saying that
Router(Router 1) OSPF Router ID of 10.10.5.1 is connected as a BDR which must be Interface on Router 1 e0/0/0 (10.10.5.1)
Router(Router 6) OSPF Router ID of 10.10.9.1 is connected as a DR which must be Interface on Router 6 FA0/0 (10.10.8.1)

That make sense?


LEEroy
CCNP,CCIP

 

[leedsit]

Thinking about this, I think I see where you may be confused.

With the output of below, it looks like the whole network has only 1 BDR and 1 DR which is not true, this is from this routers perspective on each interface....

Neighbor ID Pri State Dead Time Address Interface
10.10.5.1 1 FULL/BDR 00:00:35 10.10.5.1 FastEthernet0/0
10.10.9.1 1 FULL/DR 00:00:35 10.10.8.1 Ethernet0/0/0

and my text of

Router(Router 1) OSPF Router ID of 10.10.5.1 is connected as a BDR which must be Interface on Router 1 e0/0/0 (10.10.5.1)
Router(Router 6) OSPF Router ID of 10.10.9.1 is connected as a DR which must be Interface on Router 6 FA0/0 (10.10.8.1)

Lets Look at the Top Entry,

Because there are only two routers on this link(from your diagram)....

10.10.5.1 1 FULL/BDR 00:00:35 10.10.5.1 FastEthernet0/0 because the neighbor is a BDR, THIS Routers(Router 5) Interface must be a DR
10.10.9.1 1 FULL/DR 00:00:35 10.10.8.1 Ethernet0/0/0 becuse the neighbor is a DR, THIS Routers (Router 5) Interface must be a BDR.

If I remember rightly, if priority is not set for DR or BDR its the highest OSPF Router ID on the link.

Router 1 OSPF RID = 10.10.5.1
ROuter 6 OSPF RID = 10.10.9.1
Router 5 OSPF RID = 10.10.8.2

Between Router 5 and Router 1, Router 5 Has the highest RID, so this interface will be the DR
Between Router 5 and Router 6, Router 6 Has the highest RID, so this interface will be the BDR

which matches your output above.

Lee.




LEEroy
CCNP,CCIP

 

[bnorton916]

OK, this is making sense.

So you only have 1 DR/BDR per broadcast domain.

Then the router ID will be the highest IP address on the router, or which one can set through router-id command.

I think if I would set router ids for my routers I wouldn't have been quite so confused(I will go do that to solidify my understanding).

Thanks alot for your help.

Bill

 

[leedsit]

Yup Spot on, I found it very confusing at first. If you set your Router-ID to the same value as your Router in your lab.

I.E

Router 1 = 1.1.1.1
Router 2 = 2.2.2.2
.
.
.
Router 9 = 9.9.9.9

It makes reading all of your outputs a little easier and you know instantly which router it is.

Good Luck.



LEEroy
CCNP,CCIP