Router advertisements

Router advertisements are sent by routers to announce their availability. z/OS® Communications Server receives router advertisements, but it does not originate them. The router advertisement includes information that is used by z/OS Communications Server, including an indication of whether the sending router should be used as the default router.

Sending router should be a default router

If the router advertisement indicates that the sending router should be used as a default router, z/OS Communications Server takes the following actions:

If the dynamic default route that is to be added as the result of the received router advertisement has already been added by a previous advertisement, the length of time that that route remains valid is reset using the Lifetime value specified on the received advertisement. If no default route exists, a dynamic default route is added as a result of the received router advertisement.
If a default route exists that has equal or lower precedence than the route that is to be added, a dynamic route is added as a result of the received router advertisement. If a route with lower precedence exists, it is removed but is reinstated later if the dynamic default route that is added is removed. The following types of routes have equal or lower precedence:
- A router advertisement route that has a reachable gateway, an active interface, and the same preference value as the default router preference value that was received in the advertisement; this type of route has equal precedence
- A router advertisement route that has an unreachable gateway, an inactive interface, or a lower preference value than the default router preference value that was received in the advertisement; this type of route has lower precedence
- A replaceable static route; this type of route has lower precedence
The dynamic default route that is added has the following characteristics:
- The next-hop address is the source address of the advertisement.
- The interface is the interface on which the advertisement was received.
- The metric is set according to the preference value that was received in the advertisement. The setting 1 indicates high preference, 2 indicates medium preference, and 3 indicates low preference.
- The length of time that the route remains valid is equal to the Lifetime value set on the advertisement.
If a default route exists that has a higher precedence than the route that is to be added, a dynamic default route is not added as the result of the received router advertisement. A dynamic default route is added later if the route with the higher precedence is removed. The following types of routes have higher precedence:
- A router advertisement route that has a reachable gateway, an active interface, and a higher preference value than the default router preference value that was received in the advertisement
- A non-replaceable static route
- An IPv6 OSPF route
- An IPv6 RIP route
A neighbor cache entry is created or updated for the sending router. The neighbor cache entry contains the following information obtained from the router advertisement:
- An indication that the neighbor is a router
- An indication that the neighbor is a default router
- The link-local and link-layer addresses of the neighbor

Sending router should not be a default router

If the router advertisement indicates that the sending router should not be used as a default router, z/OS Communications Server takes the following actions:

If an IPv6 dynamic default route exists that has the advertisement's source as its next hop and the receiving interface as its interface, and that route was added as the result of a received router advertisement (but not, for example, as the result of IPv6 OSPF or IPv6 RIP), that route is deleted.
A neighbor cache entry is created or updated for the sending router. The neighbor cache entry contains the following information obtained from the router advertisement:
- An indication that the neighbor is a router
- An indication that the neighbor is not a default router
- The link-local and link-layer addresses of the neighbor