20 - OSPF - CBT VID 11 - LAB config Flashcards
E2, router ID config
IF - neighbor relation have already formed, & we change router id name, what happens ?
OSPf process has to be reset for the Router id to change, because neighbors have already formed.
When Ospf redistribute route in, it marks them as two types. which are ?
E1 or E2
What is route type E1 & E2 ?
& what is the default ?
E1 - increment the metric
E.g add on to the route metric cost from path learnt in the Ospf network.
E.g 200 plus 10 1hop + 20 for another hop
E2 - does not increment the metric
E.g whatever the metric cost was of the static redistribute into Ospf area, this will remain the cost and the Ospf network won’t add on to this metric
Default is E2.
Why would you ever mark a route as only “E2” ?
If there is only one route out to that external network why add onto the metric cost ?
But if there was a 2nd route out to these external network we can tweak it to our desire.
To clear the router ID the command is
> clear ip OSPF process ? Y
How do you spot OSPF routes in the table ?
> show ip route
O IA 172.30.10.0 [110/210] via …..
O = Ospf IA = intra area route (route learn from Ospf different area)
Also :
O E2 172.30.10.0 [110/200] via…….
These are E2 external routes learnt.
The 200 metric cost in bracket will never increase coz there E2.
loopback interface routes advertised as /24 willl show up on the routing table as ?
Ospf route table knows automatically this is a loopback interface, even if you put the loopback interface as /24.
How do you make loopback interface look like mask of /24 in OSPF routing table?
To change this, meaning to make loopback interface with /24 subnet look like a /24 on the routing table and to not look like loopback interfaces /32 you have to go to the router with the loopback interface and type
> int loopback 1
ip ospf network point-to-point
The above command will make sure all routers receive this loopback address like a normal network subnet and not a /32 single host.
How to change the router “Priority” ?
> int fa0/0
ip ospf priority 200
(Default priority is 1)
“How to stop router from becoming DR & BDR” ?
If you have 3 routers (A,B,C) on a shared segment and want router A, to become the DR, and the rest to be nothing then you have to do the below:
Router A - priority 200 (could be anything high)
Router B, C - priority 0
Making the router priority Zero, stop it from ever becoming DR, or BDR.
Most importantly it make neighbour relationship between them and the new DR full, and between router B & C in two way state.
Router A - with Router B,C - full
Router B - with Router C - two way
This is because only DR send change updates.
How to implement summarisation routes on a ABRs router.?
Below on router2
> router ospf 1
area 10 range 10.10.0.0 255.255.252.0
Above Command explained below
- area 10 to state what area the routes coming from.
- Range 10… to state the routes we wish to advertise in summary form
To verify do show up route
0 IA 10.10.0.0/22
How to implement summarisation routes on a ASBR router. ?
Summarisation on ASBR routers are different because you are not advertising routes into another ospf area.
Also you are not receiving routes from another ospf
Instead we are receiving/advertising routes into
another system : rip,EIGRP,BGP etc
Below on router-1
> router ospf 1
summary-address 172.16.0.0 255.255.252.0 (? See options)
- not-advertise(Not needed for now)
- tag (Not needed for now)
The above will shout all routes into ospf area 0 summarized.
By default ospf will inherit the metric value of those routes
(Need to watch beginning to see how he added these re-distributed routes)
Make ospf calculate gigabit Ethernet accurately into the ospf cost. And not the default of 1 for gigabit. ?
The command has to be done on all routers.
The below command below is called “auto cost bandwidth”
> router ospf 1
auto-cost reference-bandwidth (?) followed by a number
(Infinite matric = is when ospf calculation of a route is soo high it marks it as not reachable or down.)
So when configuring for gigabit we have to be careful because it could mark our very slow link as down when it does it calculation with our new cost ruler.
1gb should be = 1000
So we should do
> auto-cost reference-bandwidth 1000
The above has to be done on all routers !!!
To verify go to all routers and type
> show up route and we will see the know ospf metric value on existing routes
