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OSPF Forward Metric

March 12, 2009 3 comments

This Post will help you understand the role of OSPF Forward Metric in route selection

To make it simple , lets start with OSPF External Routing. As we all know that there are two types of metric-types associated with OSPF External Routes. Metric Type-1 (E1) and Metric Type-2 (E2).

E1 considers the cost advertised by the ASBR for the particular route plus the cost to reach the ASBR.

While E2 considers only the cost advertised by the ASBR and neglects the cost to reach the ASBR.

We will refer to the following diagram to understand the usage of forward metric in ospf

forward-metirc

Note – OSPF is enabled on Fast Ethernet Interfaces of R1, R2 and R3. And Serial Interfaces of R1, R2 and R4

– The ospf cost values are 1 for Fastethernet interfaces and 64 for serial interfaces

If we redistribute the Loopback 0 interface in OSPF at R4 . By default R3 will receive an E2 route with the cost of 20. Changing the Metric-Type to E1 the cost will be 85 ( 1+ 64 + 20 ) provided that the default bandwidth values
are used throughout the network.

As you can see from the output of show ip route 100.100.100.100 from R3 both with E1 and E2 metric-types

With Metric-Type – 1 (E1)

R3#sh ip route 100.100.100.100
Routing entry for 100.100.100.100/32
Known via “ospf 100”, distance 110, metric 85, type extern 1
Last update from 10.100.123.2 on FastEthernet1/0, 00:00:09 ago
Routing Descriptor Blocks:
10.100.123.2, from 4.4.4.4, 00:00:09 ago, via FastEthernet1/0
Route metric is 85, traffic share count is 1
* 10.100.123.1, from 4.4.4.4, 00:00:09 ago, via FastEthernet1/0
Route metric is 85, traffic share count is 1

With Metric-Type – 2 (E2)

R3#sh ip route 100.100.100.100
Routing entry for 100.100.100.100/32
Known via “ospf 100”, distance 110, metric 20, type extern 2, forward metric 65
Last update from 10.100.123.2 on FastEthernet1/0, 00:00:13 ago
Routing Descriptor Blocks:
10.100.123.2, from 4.4.4.4, 00:00:13 ago, via FastEthernet1/0
Route metric is 20, traffic share count is 1
* 10.100.123.1, from 4.4.4.4, 00:00:13 ago, via FastEthernet1/0
Route metric is 20, traffic share count is 1

R3 is doing equal cost loadbalancing in both cases.
Notice the forward metric 65 in the output above. It is only used in Metric-Type 2 (E2).
Forward Metric is actually the cost to reach the ASBR.
So it doesn’t make sense to show it the first output as Metric-Type-1 already uses it in the calculation.

Now what makes this interesting is that when we have multiple ABRs with each having different costs to the ASBR.

In case of Metric-Type-1 (E1) simply the router will choose the one with lowest commulative cost (which includes
external metric + the cost to reach the ASBR)

But when Metric-Type-2 (E2) , which is the default , is used the the results are a bit different.
Let us see how ,

First we change the cost of Interface on R2 connecting to R4 to 100.

R2(config)#interface serial 0/0
R2(config-if)#ip ospf cost 100

we now check the routing table on R3

R3#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

100.0.0.0/32 is subnetted, 1 subnets
O E2 100.100.100.100 [110/20] via 10.100.123.1, 00:02:36, FastEthernet1/0
10.0.0.0/24 is subnetted, 3 subnets
O IA 10.0.14.0 [110/65] via 10.100.123.1, 00:02:41, FastEthernet1/0
O IA 10.0.24.0 [110/101] via 10.100.123.2, 00:02:41, FastEthernet1/0
C 10.100.123.0 is directly connected, FastEthernet1/0

As E2 only considers the external cost, we see only 20 as the cost, neglecting the cost to reach the ASBR.
But wait previously we had two exit points through R1 and R2

Why in this case we only see the route through R1.

Does the route through R2 has a higher cost … umm let us check .

We shut the Interface on R1 connecting to R4

R1(config)#interface serial 0/0
R1(config-if)#shut

and then check the routing table on R3

R3#sh ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

100.0.0.0/32 is subnetted, 1 subnets
O E2 100.100.100.100 [110/20] via 10.100.123.2, 00:00:01, FastEthernet1/0
10.0.0.0/24 is subnetted, 3 subnets
O IA 10.0.14.0 [110/165] via 10.100.123.2, 00:00:01, FastEthernet1/0
O IA 10.0.24.0 [110/101] via 10.100.123.2, 00:00:01, FastEthernet1/0
C 10.100.123.0 is directly connected, FastEthernet1/0

No. It also has the same metric. Then why does we not see both in the routing table with R3 doing equal cost loadbalancing.

Let us check the route

R3#sh ip route 100.100.100.100
Routing entry for 100.100.100.100/32
Known via “ospf 100”, distance 110, metric 20, type extern 2, forward metric 101
Last update from 10.100.123.2 on FastEthernet1/0, 00:01:35 ago
Routing Descriptor Blocks:
* 10.100.123.2, from 4.4.4.4, 00:01:35 ago, via FastEthernet1/0
Route metric is 20, traffic share count is 1

Okk.. so I think you got the answer. It is the forward metric.

R3 calculates route to the destination 100.100.100.100/32 with a cost 20 through R1 and R2. But it installs only the one through R1
as it has lower forward metric. We can see that

R1(config)#interface serial 0/0
R1(config-if)#no shut

R3#sh ip route 100.100.100.100
Routing entry for 100.100.100.100/32
Known via “ospf 100”, distance 110, metric 20, type extern 2, forward metric 65
Last update from 10.100.123.1 on FastEthernet1/0, 00:00:09 ago
Routing Descriptor Blocks:
* 10.100.123.1, from 4.4.4.4, 00:00:09 ago, via FastEthernet1/0
Route metric is 20, traffic share count is 1

So what we conclude is that although the metric will be 20 ( only external metric ). But the route with lower forward metric
will be installed in the routing table. But remember this is only for Metric-Type-2 (E2)

Happy Labbing

ccie@21 ..

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CCIE Scholarship By Internetwork Expert

March 6, 2009 3 comments

I have been extremely lucky to be chosen as single International Recipient of the Real CCIE’s, Real People 2008 Scholarship by Internetwork Expert.

As the scholarship winner below is the List of what I have received.

  • Internetwork Expert CCIE 2.0 Program
  • CCIE Rack Rental package compliments of Graded Labs
  • Onsite Bootcamp

I have checked out some Products and believe me these are the must not only for CCIE preparation but for gaining knowledge of individual technologies as well.

As I have been working on individual technologies first as a part of my preparation strategy. I used Volume I version 5 OSPF Section along with OSPF Lectures in Open-Lecture Series.

I strongly recommend the Open-Lecture Series , these are very different than Advance Technologies Class. For Example the Open-Lecture Series contain 6 OSPF Clasess that Include Advanced OSPF Design , OSPF Inter-Area Routing with Multiple ABRs, Traffic Engineering with Virtual-Links and etc .

I also used the Graded Labs Rack Rental to practice OSPF section of workbook I and did not find any issues with the setup.

I would like to end this post with Thanks to the whole Internetwork Expert team for choosing me as the scholarship winner. And a very special thanks to River Hopkins (Customer Success Manager) for her help and support.

Zeeshan

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About Me

March 6, 2009 2 comments

I am Zeeshan Sanaullah from Karachi, Pakistan. I ‘ll be writing about my LAB preparation in search of getting the digits. I work for a Cisco Premier Partner. My Journey started off in the Tech World when I got my 386 computer when I was class 5 and since then computers have become an essential part of my life.

Initially I Started with Web-Development and Graphics but eventually landed in the Networking field.

I was a CCNA at 15 and now at 21 Im CCDA, CCNP, CIPTD , CSSDS, JNCIA-ER, and JNCIA-EX .

I passed CCIE R&S Written Exam in September last year.

This Blog will focus on CCIE Lab Preparation and I will also write about the day – to – day stuff that I face in my Job as a Network Engineer

Hope the journey goes well….

Zeeshan

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