Many operators currently run level-2 IS-IS in their external routers, and OSPF or other routing protocols for IP. In this case the external router must remain as the level-2 router for the SDH NEs, and so for a dual area must be a dual Integrated IS-IS router. However the router may be configured to route all IP packets using OSPF by configuring redistribution of IP routes between IS-IS and OSPF. In this way all IP packets will be OSPF routed, whilst CLNP packets continue to be level-2 IS-IS routed. This is shown in Figure III-4.
Figure III-4
Note that the Integrated IS-IS stack in the external routers will not be aware that the level-2 subdomain is meant only for CLNP packets. The OSPF learned routes must therefore be redistributed into Integrated IS-IS with a low default metric, to make them more attractive to IP packets than the level-2 sub-domain.
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