IP Static Routing

IP Static Routing

• Static routes are typically implemented on a network, even when a dynamic routing protocol is configured.
• IPv4 and IPv6 support standard, default, floating, and summary static routes.
• Static routes use the ip route and ipv6 route global configuration commands.

Next-Hop static route

• Consists of an IP address, exit interface, or both.
• Next-hop route specifies only the next-hop IP address.
• Directly connected static route specifies only the router exit interface.
• Fully specified static route specifies both the IP address and exit interface

IPv4 Static Route

• Configured using the global configuration command Router(config)# ip route network-address subnet-mask { ip-address | exit-intf [ip-address]} [distance]
• Either the ip-address, exit-intf, or the ip-address and exit-intf parameters must be configured.

IPv6 Static Route

• Configured using the global configuration command Router(config)# ipv6 route ipv6-prefix/prefix-length {ipv6-address | exit-intf [ipv6-address]} [distance]
• Most parameters are identical to the IPv4 version.

Initial Router State

• A router has entries for only directly connected networks and associated local addresses
• Before static routes are configured R1 can ping R2, but cannot ping the R3 LAN

IPv4 Next-Hop Route Configuration

• Next-hop IP address is specified, the exit interface is derived from the next hop
• Command example using the IP address of the next hop, R2:
  • R1(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.2
  • R1(config)# ip route 192.168.1.0 255.255.255.0 172.16.2.2
  • R1(config)# ip route 192.168.2.0 255.255.255.0 172.16.2.2

IPv6 Next-Hop Route Configuration

• Commands to configure IPv6 static routes to the three remote networks.
• First enable IPv6 unicast routing using the command: R1(config)# ipv6 unicast-routing
  • R1(config)# ipv6 route 2001:db8:acad:1::/64 2001:db8:acad:2::2
  • R1(config)# ipv6 route 2001:db8:cafe:1::/64 2001:db8:acad:2::2
  • R1(config)# ipv6 route 2001:db8:cafe:2::/64 2001:db8:acad:2::2

IPv4 Directly Connected Static Route Configuration

• Use the exit interface to specify the next-hop address.
• A next-hop address is generally recommended.
• Directly connected static routes should only be used with point-to-point serial interfaces
  • R1(config)# ip route 172.16.1.0 255.255.255.0 s0/1/0
  • R1(config)# ip route 192.168.1.0 255.255.255.0 s0/1/0
  • R1(config)# ip route 192.168.2.0 255.255.255.0 s0/1/0

IPv6 Directly Connected Static Route Configuration

• Three directly connected IPv6 static routes are configured using the exit interface.
• The use of a next-hop address is generally recommended for static routes
  • R1(config)# ipv6 route 2001:db8:acad:1::/64 s0/1/0
  • R1(config)# ipv6 route 2001:db8:cafe:1::/64 s0/1/0
  • R1(config)# ipv6 route 2001:db8:cafe:2::/64 s0/1/0

IPv4 Fully Specified Static Route Configuration

• Configured when the exit interface is a multi-access interface where it is necessary to explicitly identify the next hop.
• An exit interface is optional, however it is necessary to use a next-hop address.
• When the exit interface is an Ethernet network, use a static route that includes the next-hop address.
  • R1(config)# ip route 172.16.1.0 255.255.255.0 GigabitEthernet 0/0/1 172.16.2.2
  • R1(config)# ip route 192.168.1.0 255.255.255.0 GigabitEthernet 0/0/1 172.16.2.2
  • R1(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet 0/0/1 172.16.2.2

IPv6 Fully Specified Static Route Configuration

• Used when used IPv6 link-local address as the next-hop address
• Command Example:
  • R1(config)# ipv6 route 2001:db8:acad:1::/64 fe80::2
  • The interface has to be specified for a link-local nexthop
  • R1(config)# ipv6 route 2001:db8:acad:1::/64 s0/1/0 fe80::2

Verify Route Configuration

• Use show ip route, show ipv6 route, ping and traceroute
  • show ip route static
  • show ip route network
  • show running-config | section ip route
• Replace ip with ipv6 for the IPv6 versions

Default Static Routes

• A static route that matches all packets for networks not in the routing table.
• Routers use default routes configured locally or learned from another router to identify the Gateway of Last Resort
• Default static routes are used to connect an edge router to a service provider network, or a stub router

IPv4 Default Static Route Configuration

• Similar to any other IPv4 static route, except that the network address and subnet mask is 0.0.0.0
• The 0.0.0.0 0.0.0.0 in the route will match any network address., or quad-zero route.
• Command example Router(config)# ip route 0.0.0.0 0.0.0.0 {ip-address | exit-intf}

IPv6 Default Static Route Configuration

• Similar to any other IPv6 static route configuration except that the ipv6-prefix/prefix-length is ::/0, matching all routes.
• Command example: Router(config)# ipv6 route ::/0 {ipv6-address | exit-intf}

Configuring a Default Static Route

• For IPv4, any packets not matching more specific route entries are forwarded to R2 at 172.16.2.2
  • R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.2.2
• For IPv6, any packets not matching more specific IPv6 route entries are forwarded to R2 at 2001:db8:acad:2::2
  • R1(config)# ipv6 route ::/0 2001:db8:acad:2::2

Verify Default Static Route Configuration

• The show ip route static command output displays the contents of the static routes in the routing table.
  • The asterisk (*) next to the route with code ‘S’ indicates that this static route is a candidate default route, which is why it is selected as the Gateway of Last Resort.
• The static default route uses the /0 mask, or ::/0 prefix for IPv6, meaning none of the bits are required to match. As long as a more specific match does not exist, the default static route matches all packets.

Floating Static Route

• Static routes that provide a backup path to a primary static or dynamic route and only used when the primary route is not available.
• Achieved by configuring the route with a higher administrative distance than the primary route.
  • Higher administrative distance makes the route less desirable than that of another static route or a route learned through a dynamic routing protocol.
• By default, static routes have an administrative distance of 1, making them preferable to routes learned from dynamic routing protocols.

IPv4 and IPv6 Floating Static Route Configuration

• Command examples:
  • R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.2.2
  • R1(config)# ip route 0.0.0.0 0.0.0.0 10.10.10.2 5
  • R1(config)# ipv6 route ::/0 2001:db8:acad:2::2
  • R1(config)# ipv6 route ::/0 2001:db8:feed:10::2 5

Floating Route Behavior

• When used, Router 1 automatically generates syslog messages for the link going down.
• Inspecting the routing table would show the secondary route being used.

Host Routes

• An IPv4 address with a 32-bit mask, or an IPv6 address with a 128-bit mask.
• Can be added to the routing table automatically when an IP address is configured on the router
• Can be configured as a static host route

Automatically Installed Host Routes

• Cisco IOS automatically installs a host route, also known as a local host route, when an interface address is configured on the router.
• This allows a more efficient process for packets directed to the router itself, rather than for packet forwarding.
• This route is designated with a C in the routing table for the network address of the interface.
• The local routes are marked with L in the output of the routing table.

Static Host Route Configuration

• Manually configured static route to direct traffic to a specific destination device, such as a server.
• Destination IP address and a 255.255.255.255 (/32) mask.
• For IPv6 a /128 prefix length is configured
• Command Examples:
  • Branch(config)# ip route 209.165.200.238 255.255.255.255 198.51.100.2
  • Branch(config)# ipv6 route 2001:db8:acad:2::238/128 2001:db8:acad:1::2

Static Host Route Verification

• A review of both the IPv4 and IPv6 route tables verifies that the routes are active.

IPv6 Static Host Route Configuration with Link-Local Next-Hop Configuration

• The next-hop address can be the link-local address.
  • Also need to to specify an interface type and an interface number
• First, the original IPv6 static host route is removed, then a fully specified route configured with the IPv6 address of the server and the IPv6 link-local address of the ISP router
• Command Examples:
  • Branch(config)# no ipv6 route 2001:db8:acad:2::238/128 2001:db8:acad:1::2
  • Branch (config)# ipv6 route 2001:db8:acad:2::238/128 serial 0/1/0 fe80::2