OSPF Designated router Infor

Dear all,

I am working on OSPF lab but I just need to confirm that why we normally have two designated routers in a single area.
If we make only one designated router then what will be happened.

Regrads,
Toufiq

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EdTheLad

The DR is not per area its per broadcast work, it sends type 2 LSA's, so rather than having every router on a broadcast segment flood type 1 LSAs, one router becomes designated and sends a type 2 LSA which contains info received from all other routers connected to the broadcast network.If you lose a DR on a broadcast net work, there is no communication between the rest of the routers until a new DR is elected, this can take afew minutes.By having a BDR already in place, this cuts down the protocols "lack of visability time".
By having only 1 DR, a link may fail and this failure will not be discovered by ospf for afew minutes until a new DR is elected, traffic could be unnecessarily block holed for a couple of minutes even if there is a backup route present in ospf.

toufiq

Thanks for this information.
Please also confirm below point.

If making a designated router so I will put high priority on DR interface then do we need to configure priority on remaining routers interfaces(less than DR) of OSPF network or we don't need to configure priority on remaining routers because of OSPF default priority(i.e. one) and I will make high priority of DR interface than a default priority.

Regards,
Toufiq

hoogen82

On a broadcast or a NBMA network if you want a router to be eligible to be selected as DR... You need to make sure it has a high priority... Now default in Juniper is 128 and not 1..(Cisco def priority is 1.. Range from 1-15).

So yes just by making the priority of the DR to be higher will ensure it becomes the DR... But you need to remember that, this would not be the case all the time. There is no preempt function here.. so if there is a DR selected and exists in the network.. A higher priority router will not displace it..

HTH
-Hoogen

toufiq

Many Thanks!
I just need simple definition of Type-7 LSA/NSSA with example, I mean why we need to implement it and its comparison with Type-5 LSAs.

Regards,
Toufiq

hoogen82

Not so stubby area... That should tell you almost everything. Our requirement is to have stub area ... But then a stub area cannot have external LSA's in it... So the concept of NSSA... This area has a ASBR in it which redistributes external routes.. so we allow it in as Type 7... which are then at the ABR converted into Type 5 and sent to backbone. And now any other Type 5 coming from the backbone is rejected when it comes into this NSSA. So it acts as a stub area plus it can have an ASBR

toufiq

Thanks Its easy to understand.
My another point is related to page number 181(JNCIP Book), here we are implementing policy to inject static routes into area 1 but we are also rejecting term 2 reject policy.

I mean if we have applied exact router-filter then it will defiantly allow the IPs related to this subnet and other will will not be allowed so why we need term 2 with reject.

In real exam, do we need to add static route with reject command under edit-routing options category or here we will add static route(without reject) and then apply the policy.

Please guide and thanks for usual support.

Regards,
Toufiq

hoogen82

The term 2 reject is just a caution explanation is give in page 182 first para.

The use of a discriminating route filter is used here to prevent the inadvertent redistribution of other static routes, such as the one associated with your fxp0 OoB network (in the event that you neglected to add the no-advertise tag when it was defined).

The static route presented was for example. They inject this route in R7 so that it doesn't enter area 10. reject keyword is just for the icmp packet handling

toufiq

Thanks Boss!
I am configuring NSSA but I have seen some policies on page 189.
can you guide me that what is the basic difference between external Type 1 and Type 2 commands because as per knowledge these are the metric types.

But I need some more clarification from you for better understanding and how these works, if applied accordingly.

Many Thanks for you cooperation.

Regards,
Toufiq

hoogen82

Toufiq these are things that are very clearly explained in CCNP and JNCIS... I suggest if you are feeling confused at this point to go back read the theory again. Theory really kicks a** sometimes during the exams and can be of real help if you know them well.

E1 or external type 1

R1--R2--R3

If R1 is redistributing routes with metric of X.. the route would have a cost of X + whatever the metric along its path until reaching R3.

E2 or External Type 2

If R1 is redistributing routes with metric of X.. the route retains its value on R3 also..i.e. X no addition.

toufiq

Thanks, yes you are right i missing theory part here and it needs to be cleared from my previous books and further I need to study about metrics in detail before going through this scenario.

Many thanks for you cooperation.
Let me see in book and I will contact you, if i need any help.

Regards,
Toufiq

toufiq

Hello, I need to confirm another point which is related to metric calculation with OSPF.

Its page 229 In JNCIP book and 4th point in which metric value of 10 need to advertised with 420 tag towards backbone.after applying configuration I found that the results were not according to requirement(I mean according to book).

Please note that I am using olive for my Lab.

Router-1 Config:
============
[edit policy-options policy-statement external]
root# show
term 1 {
from {
route-filter 10.0.5.0/24 exact;
}
then {
metric 10;
tag 420;
external {
type 1;
}
accept;
}
}
term 2 {
then reject;
======================================
Results on R5.
[edit]
root# run show route 10.0.5.0/24

inet.0: 20 destinations, 20 routes (20 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.5.0/24 *[OSPF/150] 00:00:02, metric 12, tag 420
> to 10.0.2.2 via em2.0

According to book metric should be 36 here because of R5 Cost reflect towards ASBR.

hoogen82

Toufiq... Youre reference bandwidth is 10^8.. which means all interfaces would probable me with a metric of 1...

So on R1 .redistributed as 10 reaches R3 as 11 and R5 as 12

If you increase your reference bandwidth.. and then use the bandwidth keyword to change the metric calculation..
Like R1-R3 is fast ethernet.. have bandwidth as 100m.. and then R3-R5 is atm.. so have bandwidth of 155m... you will see the difference..

toufiq

Thanks for your response but i am using olive thats why i cant play with ATM and FE interfaces.My question is that I am using here metric type-1 on R1 router and redistributing metric 10 towards backbone and in which every hop adding a metric 1 in the total received value of metric and I am getting metric 12 on R5 because of two hops(i.e. R1 & R3).

edit]
root# run show route 10.0.5.0/24

inet.0: 24 destinations, 24 routes (24 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.5.0/24 *[OSPF/150] 04:24:57, metric 12, tag 420
> to 10.0.2.2 via em2.0

again If I check this route on R3 then metric value will be 11 because of single hop.

[edit]
root# run show route 10.0.5.0/24

inet.0: 27 destinations, 27 routes (27 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.5.0/24 *[OSPF/150] 04:27:53, metric 11, tag 420
> to 10.0.4.14 via em0.0

hoogen82

Yes that's what I said.. if you don't want it to change you have to use metric type 2.. I know you are using olive.. what you can do is if it is possible set the bandwidth to reflect those interfaces in olive.. set interface em0.0 bandwidth 100m --> Fast ethernet

So what is your question... I guess I have explained it..

toufiq

Thanks for this response.
can you please guide that, why we use sub interfacing on FE interfaces for routers connectivity.for example interface fe-0/0/1.200

I mean, we can also use fe-0/0/1 (without.200) for connectivity.
This configuration is found on JNCIP Book.

Regards,
Toufiq

Aldur

toufiq wrote: »

Thanks for this response.
can you please guide that, why we use sub interfacing on FE interfaces for routers connectivity.for example interface fe-0/0/1.200

I mean, we can also use fe-0/0/1 (without.200) for connectivity.
This configuration is found on JNCIP Book.

Regards,
Toufiq

Subinterfaces/logical units will be used for many reasons. Mainly they are used to line up logical unit with circuit identifier. Such as in the case of frame relay, DLCI 200 will normally be applied to an interface with the logical unit of 200. With ethernet vlan 200 will line up with logical unit 200, or with ATM VCI 200 will line up with logical unit 200.

This makes for much easier troubleshooting. If you look at fe-0/0/1.200 you know that it is using vlan 200 just from the naming of it.

Granted you could set vlan 200 on logical unit 0 but this would cause troubleshooting issues in the future, the very near future

toufiq

Thanks as its clear now.
another issue is that I have configured metric type-1 on R1 and requirement is to receive metric 36 on R5 but I am receiving only 12 on R5.

Some one told me that, I need to configure reference-bandwidth on R1 & R2 routers then i will receive required metric on R5.

R1 Config:
=======

policy-options {
policy-statement external {
term 1 {
from {
route-filter 10.0.5.0/24 exact;
}
then {
metric 10;
tag 420;
external {
type 1;

Result on R5
=========
edit]
root# run show route 10.0.5.1

inet.0: 22 destinations, 23 routes (22 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

10.0.5.0/24 *[OSPF/150] 00:00:06, metric 13, tag 420
> to 10.0.2.2 via em2.0

hoogen82

Toufiq along with the reference bandwidth try setting your interfaces em0.0 or em0.1 which is path from R1-R3-R5.. In the book R1-R3 is fastethernet... so set the links with bandwidth as 100m. R3-R5 is ATM.. so set that link as 155m .. You will get the output ...