| SureshHomepage 2003-09-12, 8:40 pm |
| In an routing table if you look for the entries that were installed by OSPF you would see some 4 types of routes namely O, IA, E1, E2, N1. The last 3 are of bit significant as they are not actually learned by OSPF but got 'redistributed' from some other routing protocol like RIP, EIGRP, IS-IS, etc.
This type 5 is of comes in 2 flavours E1 and E2!
In OSPF, the cost is measured in terms of bandwidth.
E1
If the cost of reaching a destination network is calculated or measured right from the origination of the route then it is termed as E1. This would be by adding both internal as well as external costs.
Internal cost here means the cost from the point of redistribution and external cost means cost to destination network just from that point of redist.
E2
On the other hand, if the cost is measured only from the point of redistribution in order to reach the destination network barely ignoring the external cost and considering only the internal cost, then it would be termed as E1. E2 is the default one.
Example:
R1------R2------R3
Consider,
R1 is where you are sitting and viewing the routing table. You run OSPF between R1 and R2. Between R2 and R3 it is some other protocol, could be RIP.
R2 is the redistribution point where the RIP routes are converted into OSPF routes by the process of redistribution.
At R1, if see the E1 routes there, then it has included the cost associated with reaching R3 all the way through R2.
If you see E2 there, its just the cost associated with reaching R2 alone. Cost to R3 from R2 (external cost) is ignored.
Hope this would help a bit! |