01-10-2014 06:51 AM - edited 03-04-2019 10:02 PM
Hello
I've created a small redistribution lab:
If London1 pings New-Yorks4 loopback 172.30.1.1 it goes via New-York2 > New-York4, but the return traffic goes via Milan3, I'm used metric-type 1 so it will calculate the cost to the ASBR and onwards, but how does it get the cost of 30?
I then changed the metric cost to the default of 2 and it stays the same I would of thought the return traffic would go loopback > New-York4 > New-York2 > London1?
Thanks
01-10-2014 07:15 AM
The reason why it is using the cost to Milian, is if you look at your image, you are using an Ethernet link, whereas the other link is using Serial (default 1544), so when you change the default Metric Type from 2 to 1, it will take into consideration the cost to the ASBR and add that to the cost redistributed. Therefore, it will prefer the lin kto Milan since it's Ethernet (as long as the default bandwidth values are in place) which is what I'm assuming.
OSPF Cost is calculated as below.
Reference_Bandwidth/Interface_Bandwidth
The OSPF Reference Bandwidth is 100000000bps or 100Mbps.
So 100Mbps/10Mbps = 10
The default cost for redistributed routed into OSPF is 20, that's why you are getting the cost of 30.
I hope that helps some, feel free to ask more questions
01-10-2014 07:42 AM
When you changed the metric back to Type-2 did you do this for both redistribution poins or one?
There is a order of preference with OSPF, which doesn't care about cost/prefx length etc
1. Intra-area routes
2. Inter-area routes
3. External Type-1
4. External Type-2
01-10-2014 07:51 AM
It was on both ASBRs
01-10-2014 07:43 AM
Ah, so the cost with metric 2 is 10 (Reference_Bandwidth/Interface_Bandwidth) plus the default of 20 from the ASBR to the London1 router?
I thought from the ASBR it would also use the Reference_Bandwidth/Interface_Bandwidth on the 2 serial links to to the London1 router?
Does that make sense?
so 10 + 64 + 64 = 138
I used 100000 / 1544 to get the 64 cost.
Is there a command I can see these other costs that are not shown in in the routing table like EIGRP does with the feasible successor?
Thanks
Message was edited by: Andy White
01-10-2014 07:56 AM
When the route is redistributed into OSPF, the default cost for ALL redistributed routes is 20. So for a 100Mbps link, that givses the following formula
Reference_BW/Interface_Cost (Remember by default, Reference_BW always = 100Mbps in bps form) This can be changed...
So 100000000/10000000 = 10 + (redistributed costof 20) = 30
And you are right that 1544 = a cost of 64 with default reference bandwidth.
It does that that from the serial links to the original route, but it's different whn the cost is redistributed. Since the
default cost is 20.
Once again, feel free to ask questions, I'll asnwer all that I can.
01-10-2014 08:18 AM
Hello
Just for clarification - I can see NY4 router has two area's via to NY2 & Milan.
what area are you advertiseing the loopback of NY4 into- Area 24 or 34 ?
Depending on what area you originally advertise that loopback into, That would take preference to which router its get advertised into and effect the return path from London1
Also as stated the next hop from NY4 will be changed of you advertise the metric-type of ospf from milan to use type 2 whilest the metric-type one is advertised by NY2,
res
Paul
Please don't forget to rate any posts that have been helpful.
Thanks.
01-11-2014 11:44 AM
This is what I have on New-York4, I am advertising the loopback (172.30.1.x) into ospf.
router ospf 24
log-adjacency-changes
network 172.30.1.0 0.0.0.255 area 0
network 192.168.3.0 0.0.0.255 area 24
network 192.168.4.0 0.0.0.255 area 34
With regards to metric-type 1 and 2 then, so with 2 the default metric is always 20 so it doesn't care about link speeds and metric 1 uses the metric 20 still but calculates the the cost only to the ASBR and then it just adds the default of 20?
Thanks
01-11-2014 01:28 PM
Hello
Ospf usually doesn't add the cost of intra-area metrics in calculation of its redistributed Type E2 routes unlike in Type E1 routes. - Also in the ospf path selection order Type E1 is always preferred over Type E2 routes.
However after reading this excellent blog there can be times when both intra & inter area metrics are calculated no matter if they are type E1 or E2
http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/#more-4993
Res
Paul
Sent from Cisco Technical Support iPad App
01-13-2014 10:55 AM
Thanks, so:
E1 routes contains Internal cost + external cost . total cost?
E2 routes contains only external cost. Which means ASBR to external Networks and uses the default of 20?
Regards
01-13-2014 11:01 AM
Hello
Yes thats correct
res
Paul
Please don't forget to rate any posts that have been helpful.
Thanks.
01-13-2014 12:49 PM
So E1 will ingore using the seed of 20 completely and calculate the internal cost and external cost using 100000/bandwidth?
Sorry I think I have, E1s will calculate the internal cost to the ASBR then add the default seed metric of 20?
Message was edited by: Andy White
01-13-2014 01:21 PM
Hello
Isnt that what you just asked? If not i must of misread you last question
E1 resdistributed.cost plus intra area.cost
E2 redistrubted cost
Andy did you check out that link I posted?
Res
Paul
Sent from Cisco Technical Support Android App
01-13-2014 01:52 PM
Pdriver is correct.
With E1 (Metric-Type 1) routes, the router that receives the external route (OE 1) will add the cost that was included in the Type 5 LSA (External Route), as well as the cost to reach the nearest ASBR, which is including in Type 4 LSAs.
With E2 (Metric-Type 2) routes, the router that receives the external route (OE 2) will use the cost that was included in the Type 5 LSA (External Route), b default a cost of 20.
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