Recursive Static Routes - Cisco NCS 6000 Series Configuration Manual

Implementing Static Routes
shows how to specify that all destinations with address prefix 2001:0DB8::/32 are directly reachable through
interface GigabitEthernet 0/5/0/0:
RP/0/RP0/CPU0:router(config)# router static
RP/0/RP0/CPU0:router(config-static)# address-family ipv6 unicast
RP/0/RP0/CPU0:router(config-static-afi)# 2001:0DB8::/32 gigabitethernet 0/5/0/0
Directly attached static routes are candidates for insertion in the routing table only if they refer to a valid
interface; that is, an interface that is both up and has IPv4 or IPv6 enabled on it.

Recursive Static Routes

In a recursive static route, only the next hop is specified. The output interface is derived from the next hop.
The following example shows how to specify that all destinations with address prefix 2001:0DB8::/32 are
reachable through the host with address 2001:0DB8:3000::1:
RP/0/RP0/CPU0:router(config)# router static
RP/0/RP0/CPU0:router(config-static)# address-family ipv6 unicast
RP/0/RP0/CPU0:router(config-static-afi)# 2001:0DB8::/32 2001:0DB8:3000::1
A recursive static route is valid (that is, it is a candidate for insertion in the routing table) only when the
specified next hop resolves, either directly or indirectly, to a valid output interface, provided the route does
not self-recurse, and the recursion depth does not exceed the maximum IPv6 forwarding recursion depth.
A route self-recurses if it is itself used to resolve its own next hop. If a static route becomes self-recursive,
RIB sends a notification to static routes to withdraw the recursive route.
Assuming a BGP route 2001:0DB8:3000::0/16 with next hop of 2001:0DB8::0104, the following static route
would not be inserted into the IPv6 RIB because the BGP route next hop resolves through the static route and
the static route resolves through the BGP route making it self-recursive:
RP/0/RP0/CPU0:router(config)# router static
RP/0/RP0/CPU0:router(config-static)# address-family ipv6 unicast
RP/0/RP0/CPU0:router(config-static-afi)# 001:0DB8::/32 2001:0DB8:3000::1
This static route is not inserted into the IPv6 routing table because it is self-recursive. The next hop of the
static route, 2001:0DB8:3000:1, resolves through the BGP route 2001:0DB8:3000:0/16, which is itself a
recursive route (that is, it only specifies a next hop). The next hop of the BGP route, 2001:0DB8::0104,
resolves through the static route. Therefore, the static route would be used to resolve its own next hop.
It is not normally useful to manually configure a self-recursive static route, although it is not prohibited.
However, a recursive static route that has been inserted in the routing table may become self-recursive as a
result of some transient change in the network learned through a dynamic routing protocol. If this occurs, the
fact that the static route has become self-recursive will be detected and it will be removed from the routing
table, although not from the configuration. A subsequent network change may cause the static route to no
longer be self-recursive, in which case it is re-inserted in the routing table.
Fully Specified Static Routes
In a fully specified static route, both the output interface and next hop are specified. This form of static route
is used when the output interface is multiaccess and it is necessary to explicitly identify the next hop. The
next hop must be directly attached to the specified output interface. The following example shows a definition
of a fully specified static route:
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Recursive Static Routes
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