Aug 12, 2009

What is the difference between OSPF E2 Routes and E1 Routes

Look at the figure above, when using OSPF dynamic routing, all routers exchange Link-State Packets between each Router. Router A is Autonomous System Border Router (ASBR) because it advertises external link to internal Routers. With RIP, IGRP, EIGRP the external network is in one type, but with OSPF the external network is redistributed by 2 types: E1 and E2

R1#show ip route
Codes: C - connected, S - static, I - IGRP, 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, E - EGP

E2 is the default routes type for routes learned via redistribution. For example, Router C and other routers in the routing domain will learn external networks that Router A redistribute with cost from Router A to external network.

If Router A is configurable with E1. At this time, Router C in the routing domain will learn external networks via Router A with cost is cost of Router C to Router A and Router A to destination network
RA#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RA(config)#router ospf 1
RA(config-router)#redistribute rip subnets metric-type 1
After configure external type 1, the cost from RouterC to External network is larger the cost from RouterC to external network with E2 configuration.

Author : Lâm Viết Thảo // 4:15 PM
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