OSPF E1 and E2 route consideration

[swapan.samanta89]

"Both intra-area and inter-area cost still be considered in ospf path selection regardless of whether the rotute E1 and E2"

is it true.But we know that E1 route take consider internal as well as external cost but E2 route consider only external cost.

Explain this one

Regards 

swapan

[Richard Burts]

swapan

We do not know where you got that statement or anything about its context. Perhaps if we knew where it was from and what things it had been talking about we might understand it well enough to try to explain what it meant. But I agree with you that on the surface that statement seems to be not true.

HTH

Rick

[swapan.samanta89]

Hello Rick.Actually i have read a post from INE by Brian.http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/..Can u explain in details by hot potato and cold potato routing.with scenario.

[Julio E. Moisa]

Hi

E1 is always preferred than E2, both are used to mark external routes but the difference between them is the way to calculate the cost in few words the OSPF metric.

E2 will include the external cost only. It does not matter the internal cost except when a tie is detected. E2 is the default in OSPF for the received external routes through an ASBR.

E1 will include the external cost + the internal cost used to reach that subnet through an ASBR.

This link also could be useful about the OSPF path selection: https://networklessons.com/ospf/ospf-path-selection-explained/

[swapan.samanta89]

hi julio

"E2 will include the external cost only. It does not matter the internal cost except when a tie is detected"

can u explain what type of tie are u talking??

[Julio E. Moisa]

Hi

If you have multiple E2 routes to a specific destination and they have the same external cost, the internal cost to the ASBR is compared and used as tie breaker. 

http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/

:-)

[swapan.samanta89]

Hi julio

As i mentioned intra-area as well as inter-area routes considered regardless of E1 and E2 route.

can u explain about hot potato and cold potato routing with scenario

Regards 

swapan

[Julio E. Moisa]

Hi

The difference between hot and cold potatos is related to the processing of the packets. Hot potato will advertise a packet asap, in this case we can mention 2 E2 routes for specific destination subnet with different external cost, OSPF will prefer the best external cost (the processing will be quick) but it is different when 2 E2 or more routes to a specific destination that have the same external cost, OSPF to eliminate the tie will compare the internal cost and select the lowest cost to the ASBR in this case it is called Cold potate (it could take a little longer).

This link could be useful: https://routingmonkey.wordpress.com/2014/12/27/why-ospf-e1-route-preferred-over-e2-route/

How the link mentions, OSPF will prefer the intra-area (routes created at the same area) than inter-areas (routes received from different areas).

OSPF will prefer the routes originated under a OSPF environment than external routes advertised by other routing protocol. 

Please rate the comment if it is useful  :-)

[Masoud Pourshabanian]

Hello,

If there are two E2 routes with same cost, tie breaker will be the distance to ASBR. You can see the metric to ASBR by using Show ip route 10.10.10.10

In the output: forward metric is the metric to ASBR

show ip route 10.10.10.10
Routing entry for 10.1.6.0/24
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 10.1.13.3 on FastEthernet0/0.13, 00:12:03 ago
  Routing Descriptor Blocks:

If the are two E2 route with same cost, route with lowest forward metric sits in the routing table. It is the idea of Hot potato. If you carry a hop potato with a bear hand, what do you prefer to do? To get rid of it as fast as possible.It means forwarding it to the closest ASBR to send it out of your autonomous system.

If the route is E1, you do not see this Forward metric in the output of "Show IP route"

Forward metric is calculated by LSA type 4 and type 1.

Hope it helps,

Masoud

[Diana Karolina Rojas]

I highly recommend you review this post from Brian McGahan: http://blog.ine.com/2011/04/04/understanding-ospf-external-route-path-selection/ 

He explain specifically this point.

Regards,