OSPF DR- DROTHER exchange

dtimmerm

setup in GNS3

BDR .1/25---SWITCH--- .2/25 DR
|
|.3/25
DROTHER

3 routers share a common subnet 10.0.0.0/25 subnet
Lo0 DR: 192.168.254.84
Lo0 BDR: 192.168.254.83
Lo0 DROTHER: 192.168.254.82

Situation:

I configured first the DR and BDR (interface,loopback and ospf config)

example of DR config

interface Loopback0
ip address 192.168.254.84 255.255.255.0
!
interface FastEthernet0/0
ip address 10.0.0.2 255.255.255.128
duplex auto
speed auto

router ospf 100
log-adjacency-changes
network 10.0.0.0 0.0.0.127 area 0
network 192.168.254.0 0.0.0.255 area 0

Ok so far so good

problem:

I started Wireshark on DRother to see what happens when I configure DRother

what happens:

DRother sends a mcast to 224.0.0.5 with BDR/DR 0.0.0.0
BDR's hello timer expires and send a hello to 224.0.0.5, has DR and DRother in it's neighbo list
DR's hello timer expires and send a hello to 224.0.0.5, has BDR and DRother in it's neighbo list

=>biderectional communication is established with his neighbors

I expected that now and exchange would follow with the DR (DBD,LSR,LSU)


DROTHER --> DR via 224.0.0.6
DR --> DROTHER unicast to DR

and finally DR via 224.0.0.5 to all routers

but this is not the case.

I see unicast(in both directions) exchanges(DBD, LSupdate) with both DR and the BDR(???) in both directions?? even unicast exchanges(DBD/LSupdate,...) between DR and BDR (while stable)
(30 messages in 2 seconds)??

In this stream I see only 2 messages to 224.0.06?? While the same message is also sent from DROTHER has been sent to 224.0.0.5


Is this due to GNS3 and wireshark or do I really don't understand how the sync works

dtimmerm

update:

when situation of above is stable (synced)

I add new loopback to DROTHER and add the loopback to the OSPF routing process, same area as the rest


DROTHER --> 224.0.0.6 LSupdate (included: updated LSA1 with 3 links instead of 2)
DR -->224.0.0.5 LS update (included: LSA 1 of DROTHER)

so far so good

but then

BDR --> 224.0.0.5 LS update(old LSA type 1 of BDR)* [DROTHER knows this network long time]
DROTHER --> BDR LS update (included: updated LSA1 of DROTHER with 3 links instead of 2) WTF?(**)
BDR--> DROTHER LS ack (Ack for **)
BDR --> 224.0.0.5 LS Ack (Ack for **)
DROTHER-->224.0.0.6 (LS Ack)(Ack for *)
DR --> 224.0.0.5 LS Ack(Ack for *)



I understand that you won't understand this

But I'm just trying to follow the DBD, Update messages etc via Wireshark

Like the sequence
BDR --> 224.0.0.5 LS update(old LSA type 1 of BDR)*
DROTHER-->224.0.0.6 (LS Ack)(Ack for *)
DR --> 224.0.0.5 LS Ack(Ack for *)

While the network is stable:s

Route is not aged out,
why does the BDR send it to 224.0.05
...

dtimmerm

what I actually expect is the following

new router exchanges hello packets (neighbor discovery)
all other neighbors reply with the new neighbor's router ID in their neighbor list

==> bidirectional communication + new router has learnt about DR/BDR via the hello's of the neighbors

then

new router communicates with DR via 224.0.0.6 and DR unicast with the new router

>Exstart, Exchange Loading Full

and then DR sends a LSA type 2 out to 224.0.0.5


or how is it actually going?

Or does the DR sends out the new LSA 1 out to 224.0.0.5. But of what use is LSA type 2 then.

I'm confused

kryolla

DRother will send LSA1 to DR via .06 and the DR will flood LSA1 and 2 to the whole area via .05. Think about how a router will route packets within an area and you will understand what LSA1 & 2 are for.

dtimmerm

that's what I thought but the output in wireshark was quite different

the LSA 2 seems to be useless as it lists only the attached networks without any further detail.

kryolla

your not understanding the OSPF database and how route processing works. Do some more research. LSA 2 are extremely important for intra-area routing