Ospf neighbour and LSA question????

aueddonline

Ospf neighbour and LSA question???

Does OSPF consider its neighbors to be only those directly connected or the opposite router over a WAN link and not all the routers in the same area?

If this is correct does when a change is heard through a LSA does it only inform its neighbors of the change?

So if r1 r2 and r3 were connected like in my very simple example, if R3 sends a LSA does R2 then forward it to its neighbors i.e R1 and R3 or just R1

Or is R3 send the LSA to both R2 and R3

R1

---

R2

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R3

Hope that makes sense,

EdTheLad

Adjacencies are formed with directly connected neighbors in general, you could create a tunnel or virtual link to form a neighbor relationship with a router that is not directly connected but that is beyond the scope of your question.
In a single AREA configuration,ospf sends 2 types of LSAs, type 1 and type 2. Type 1 are router LSAs and these describe all local subnets on the route i.e. connected interfaces. Type 2 LSAs are network LSAs and are sent by the designated router, type 2 LSAs are beyond the scope of your question.So looking at type 1 LSAs, if R1 sends info to R2 about its directly connected interfaces this information has to make it to R3 by some fashion! The fact is LSAs are flooded throughout an area, this means R2 will receive the LSA from R1, perform a quick sanity check to see this LSA is not corupt and ensure the LSA is up to date, if the LSA passes the tests it is flooded to R3.R2 will flood this LSA to R3 very quickly without performing a local recomputation( in case this LSA was generated due to topology change on R1). So as you see R1 doesnt have an adjacenciy with R3 but R3 still receives the exact LSA that was flooded by R1.
So in an area there is alot of flooding goning on hence the need for additional areas to reduce the flooding.

aueddonline

I set up a ospf network with 3 routers all connected to a switch, also with r-alfa and r-beta connected with a serial cable, here's the config and the errors at the bottom are from r-beta

r-alfa configuration

config t
int s0/0
ip address 10.0.66.1 255.255.255.0
bandwidth 128
clock rate 128000
no shut
int e0/0
ip address 10.0.88.1 255.255.255.0
no shut
ip ospf priority 10
int loopback 0
ip address 10.10.10.10 255.255.255.0
exit
router ospf 101
network 10.0.66.1 0.0.0.0 area 0
network 10.0.88.1 0.0.0.0 area 0
ospf auto-cost reference-bandwidth 1000


$$$$$$$$$$$$$$$$$R-beta configuration

config t
int s0/0
ip address 10.0.66.2 255.255.255.0
bandwidth 128000
no shut
int e0/0
ip address 10.0.88.2 255.255.255.0
no shut
ip ospf priority 5
router ospf 101
network 10.0.88.2 0.0.0.0 area 0
network 10.0.66.2 0.0.0.0 area 0
ospf auto-cost reference-bandwidth 1000

$$$$$$$$$$$$$$$R-3600 ronfiguration

config t
int f2/0
ip address 10.0.88.3 255.255.255.0
no shut
ip ospf priority 0
router ospf 101
network 10.0.88.3 0.0.0.0 area 0
ospf auto-cost reference-bandwidth 1000

$$$$$$$$$$$$$$$$2950 configuration

config t
int vlan 1
ip address 10.0.88.10 255.255.255.0
ip default-gateway 10.0.88.1
no shut




*Mar 1 02:28:29.949: %OSPF-5-ADJCHG: Process 101, Nbr 10.0.88.3 on Ethernet0/0 from EXSTART to DOWN, Neighbor Down: Too many retransmissions
*Mar 1 02:28:43.103: %OSPF-5-ADJCHG: Process 101, Nbr 10.10.10.10 on Ethernet0/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired
*Mar 1 02:29:29.951: %OSPF-5-ADJCHG: Process 101, Nbr 10.0.88.3 on Ethernet0/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired




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