Cisco NCS 5500 Series

• Floating Static Routes , on page 193
• Fully Specified Static Routes , on page 202
• Recursive Static Routes , on page 202
Static Route Functional Overview
Networking devices forward packets using route information that is either manually configured or dynamically
learned using a routing protocol. Static routes are manually configured and define an explicit path between
two networking devices. Unlike a dynamic routing protocol, static routes are not automatically updated and
must be manually reconfigured if the network topology changes. The benefits of using static routes include
security and resource efficiency. Static routes use less bandwidth than dynamic routing protocols, and no CPU
cycles are used to calculate and communicate routes. The main disadvantage to using static routes is the lack
of automatic reconfiguration if the network topology changes.
Static routes can be redistributed into dynamic routing protocols, but routes generated by dynamic routing
protocols cannot be redistributed into the static routing table. No algorithm exists to prevent the configuration
of routing loops that use static routes.
Static routes are useful for smaller networks with only one path to an outside network and to provide security
for a larger network for certain types of traffic or links to other networks that need more control. In general,
most networks use dynamic routing protocols to communicate between networking devices but may have one
or two static routes configured for special cases.
Default Administrative Distance
Static routes have a default administrative distance of 1. A low number indicates a preferred route. By default,
static routes are preferred to routes learned by routing protocols. Therefore, you can configure an administrative
distance with a static route if you want the static route to be overridden by dynamic routes. For example, you
could have routes installed by the Open Shortest Path First (OSPF) protocol with an administrative distance
of 120. To have a static route that would be overridden by an OSPF dynamic route, specify an administrative
distance greater than 120.
Directly Connected Routes
The routing table considers the static routes that point to an interface as “directly connected.” Directly connected
networks are advertised by IGP routing protocols if a corresponding interface command is contained under
the router configuration stanza of that protocol.
In directly attached static routes, only the output interface is specified. The destination is assumed to be directly
attached to this interface, so the packet destination is used as the next hop address. The following example
shows how to specify that all destinations with address prefix 2001:0DB8::/32 are directly reachable through
interface TenGigE 0/0/0/0:
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 TenGigE 0/0/0/0
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.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
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Implementing Static Routes
Static Route Functional Overview