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Implementing Static Routes on Cisco IOS XR Software
Information About Implementing Static Routes on Cisco IOS XR Software
RC-422
Cisco IOS XR Routing Configuration Guide
OL-14356-01
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 will be 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:
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 Gigethernet0/0/0/0
2001:0DB8:3000::1
A fully specified route is valid (that is, a candidate for insertion into the routing table) when the specified
interface, IPv4 or IPv6, is enabled and up.