3

I know the difference between E1 and E2 metric-types. I know the preference level given to those routes. I know difference between Metric and Forwarding Metric and how it works.

What I don't understand - why this two type of E1/E2 exist ? Is there any special reason for those two types of metric to exist ? If forwarding metric is used for to find the best path to get the ASBR. then the E1 routes carries the same kind of information ( route cost at distribution point + best path cost for the ASBR).

Can someone give me a real world example where should we use Type-E1 over E2 ? or wise versa.

If I redistribute into OSPF via multiple ASBR, default E2 Cost 20 will remain through entire OSPF domain. Then the local router will use the Forwarding distance + Metric(20) to reach the external LSA 5. Ultimately I would achieve the same result if I redistribute the same route via multiple ASBR as a Type E1.

Am I missing something ?

Sajith Happy Routing!

  • You should take the time to read the RFCs (RFC 2328). – Ron Trunk Apr 23 '18 at 17:26
6

RFC 2328, OSPF Version 2 defines the Type 1 and Type 2 external metrics. Type 1 metrics are used if the primary cost is internal to your AS, but Type 2 metrics are used if the primary cost is external to your AS.

OSPF supports two types of external metrics. Type 1 external metrics are expressed in the same units as OSPF interface cost (i.e., in terms of the link state metric). Type 2 external metrics are an order of magnitude larger; any Type 2 metric is considered greater than the cost of any path internal to the AS. Use of Type 2 external metrics assumes that routing between AS'es is the major cost of routing a packet, and eliminates the need for conversion of external costs to internal link state metrics.


Think about years ago. Your internal links were probably 100 Mbps, but your External links were probably T1 (1.5 Mbps) or maybe T3 (45 Mbps). If you had two external paths, one on a T1, and one on a T3. You would want to always prefer the external path using the T3, so you would assign fixed costs to each of the external paths, preferring the T3 path. It really makes no sense to count the internal cost because of the great discrepancy between the primary and secondary external paths. The internal cost is relatively minor compared to the external speeds.

Contrast that with today, where you may have 1 Gbps internal links, and 1 Gbps (or maybe even 10 Gbps external links). It then makes sense that you also count the internal cost because the internal cost is greater than the external cost.


The E1/E2 notation is primarily used by Cisco.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.