CCNA 2 Chapter 2 Static Routing

Static Routes and Packet Forwarding

1. The packet arrives on the GigabitEthernet 0/0 interface of R1. 2. R1 does not have a specific route to the destination network, 192.168.2.0/24; therefore, R1 uses the default static route. 3. R1 encapsulates the packet in a new frame. Because the link to R2 is a point-to-point link, R1 adds an "all 1s" address for the Layer 2 destination address. 4. The frame is forwarded out of the Serial 0/0/0 interface. The packet arrives on the Serial 0/0/0 interface on R2. 5. R2 de-encapsulates the frame and looks for a route to the destination. R2 has a static route to 192.168.2.0/24 out of the Serial 0/0/1 interface. 6. R2 encapsulates the packet in a new frame. Because the link to R3 is a point-to-point link, R2 adds an "all 1s" address for the Layer 2 destination address. 7. The frame is forwarded out of the Serial 0/0/1 interface. The packet arrives on the Serial 0/0/1 interface on R3. 8. R3 de-encapsulates the frame and looks for a route to the destination. R3 has a connected route to 192.168.2.0/24 out of the GigabitEthernet 0/0 interface. 9. R3 looks up the ARP table entry for 192.168.2.10 to find the Layer 2 Media Access Control (MAC) address for PC3. If no entry exists, R3 sends an Address Resolution Protocol (ARP) request out of the GigabitEthernet 0/0 interface, and PC3 responds with an ARP reply, which includes the PC3 MAC address. 10. R3 encapsulates the packet in a new frame with the MAC address of the GigabitEthernet 0/0 interface as the source Layer 2 address and the MAC address of PC3 as the destination MAC address. 11. The frame is forwarded out of GigabitEthernet 0/0 interface. The packet arrives on the network interface card (NIC) interface of PC3.

default static route

A default route is a route that matches all packets and is used by the router if a packet does not match any other, more specific route in the routing table. A default route can be dynamically learned or statically configured. A default static route is simply a static route with 0.0.0.0/0 as the destination IPv4 address. Configuring a default static route creates a Gateway of Last Resort. Default static routes are used: When no other routes in the routing table match the packet destination IP address. In other words, when a more specific match does not exist. A common use is when connecting a company's edge router to the ISP network. When a router has only one other router to which it is connected. In this situation, the router is known as a stub router.

Default Static IPv6 Route

A default route is a static route that matches all packets. Instead of routers storing routes for all of the networks in the Internet, they can store a single default route to represent any network that is not in the routing table. A default route does not require any left-most bits to match between the default route and the destination IPv6 address. Routers commonly use default routes that are either configured locally, or learned from another router using a dynamic routing protocol. They are used when no other routes match the packet's destination IP address in the routing table. In other words, if a more specific match does not exist, then use the default route as the Gateway of Last Resort. Default static routes are commonly used when connecting: A company's edge router to a service provider network. A router with only an upstream neighbor router. The router has no other neighbors and is therefore, referred to as a stub router. As shown in the figure, the command syntax for a default static route is similar to any other static route, except that the ipv6-prefix/prefix-length is::/0, which matches all routes. The basic command syntax of a default static route is: ipv6 route ::/0 {ipv6-address | exit-intf}

Automatically Installed Host Routes

A host route is an IPv4 address with a 32-bit mask or an IPv6 address with a 128-bit mask. There are three ways a host route can be added to the routing table: Automatically installed when an IP address is configured on the router (as shown in Figures 1 and 2) Configured as a static host route Host route automatically obtained through other methods (discussed in later courses) Cisco IOS automatically installs a host route, also known as a local host route, when an interface address is configured on the router. A host route allows for a more efficient process for packets that are directed to the router itself, rather than for packet forwarding. This is in addition to the connected route, designated with a C in the routing table for the network address of the interface. When an active interface on a router is configured with an IP address, a local host route is automatically added to the routing table. The local routes are marked with "L" in the output of the routing table. The IP addresses assigned to the Branch Serial0/0/0 interface are 198.51.100.1/30 for IPv4 and 2001:DB8:ACAD:1::1/64 for IPv6. The local routes for the interface are installed by the IOS in the routing table as shown in the output in Figure 1 for IPv4 and Figure 2 for IPv6. Note: For IPv4, the local routes marked with "L" were introduced with IOS version 15.

Reach Remote Networks

A router can learn about remote networks in one of two ways: Manually - Remote networks are manually entered into the route table using static routes. Dynamically - Remote routes are automatically learned using a dynamic routing protocol. A network administrator can manually configure a static route to reach a specific network. Unlike a dynamic routing protocol, static routes are not automatically updated and must be manually reconfigured any time the network topology changes.

Verify IPv6 static routes

Along with ping and traceroute, useful commands to verify static routes include: show ipv6 route show ipv6 route static show ipv6 route network show ipv6 route static command. The output reflects the use of static routes using next-hop global unicast addresses. show ip route 2001:DB8:ACAD:3:: command. the ipv6 route configuration in the running configuration.

Floating Static Route

Another type of static route is a floating static route. Floating static routes are static routes that are used to provide a backup path to a primary static or dynamic route, in the event of a link failure. The floating static route is only used when the primary route is not available. To accomplish this, the floating static route is configured with a higher administrative distance than the primary route. The administrative distance represents the trustworthiness of a route. If multiple paths to the destination exist, the router will choose the path with the lowest administrative distance. For example, assume that an administrator wants to create a floating static route as a backup to an EIGRP-learned route. The floating static route must be configured with a higher administrative distance than EIGRP. EIGRP has an administrative distance of 90. If the floating static route is configured with an administrative distance of 95, the dynamic route learned through EIGRP is preferred to the floating static route. If the EIGRP-learned route is lost, the floating static route is used in its place.

Test the IPv4 Floating Static Route

Because the default static route on R1 to R2 has an administrative distance of 1, traffic from R1 to R3 should go through R2. The output in Figure 1 confirms that traffic between R1 and R3 flows through R2. What would happen if R2 failed? To simulate this failure both serial interfaces of R2 are shut down, as shown in Figure 2. Notice in Figure 3 that R1 automatically generates messages indicating that the serial interface to R2 is down. A look at the routing table verifies that the default route is now pointing to R3 using the floating static default route configured with an AD value of 5 and a next-hop of 10.10.10.2. The output in Figure 4 confirms that traffic now flows directly between R1 and R3.

Standard Static Route

Both IPv4 and IPv6 support the configuration of static routes. Static routes are useful when connecting to a specific remote network.

Configure an IPv4 Floating Static Route

Configure a default route using the next-hop address 192.168.1.2. R3(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2 Configure a default route using the next-hop address 10.10.10.1 with an administrative distance of 5. R3(config)# ip route 0.0.0.0 0.0.0.0 10.10.10.1 5 Exit configuration mode and display the routing table. R3(config)# exit *Feb 21 16:13:54.159: %SYS-5-CONFIG_I: Configured from console by console R3# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is 192.168.1.2 to network 0.0.0.0 S* 0.0.0.0/0 [1/0] via 192.168.1.2 R3# You successfully configured a floating static route on R3.

Configure a Fully Specified Static IPv6 Route

Configure a fully specified static IPv6 route to the R1 LAN using the R1 link-local address as the next-hop address. R2(config)# ipv6 route 2001:DB8:ACAD:1::/64 s0/0/0 FE80::1 Exit configuration mode and display the IPv6 routing table. R2(config)# exit R2# show ipv6 route IPv6 Routing Table - default - 9 entries Codes: C - Connected, L - Local, S - Static, U - Per-user Static route B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2 IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 S 2001:DB8:ACAD:1::/64 [1/0] via FE80::1, Serial0/0/0 <output omitted> R2# You successfully configured a fully specified static IPv6 route on R2.

Configure a Directly Connected Static Route

Configure a static route to the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 networks using exit interface S0/0/1. R3(config)# ip route 172.16.1.0 255.255.255.0 S0/0/1 R3(config)# ip route 172.16.2.0 255.255.255.0 S0/0/1 R3(config)# ip route 172.16.3.0 255.255.255.0 S0/0/1 Exit configuration mode and display the routing table. R3(config)# exit *Feb 21 18:01:14.055: %SYS-5-CONFIG_I: Configured from console by console R3# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 is directly connected, Serial0/0/1 S 172.16.2.0 is directly connected, Serial0/0/1 S 172.16.3.0 is directly connected, Serial0/0/1 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, Serial0/0/1 L 192.168.1.1/32 is directly connected, Serial0/0/1 192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.2.0/24 is directly connected, GigabitEthernet0/0 L 192.168.2.1/32 is directly connected, GigabitEthernet0/0 R3# You successfully configured the directly attached static routes on R3.

Configure a Fully Specified Static Route

Configure a static route to the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 networks using the exit interface S0/0/1 and next-hop address 192.168.1.2. R3(config)# ip route 172.16.1.0 255.255.255.0 s0/0/1 192.168.1.2 R3(config)# ip route 172.16.2.0 255.255.255.0 s0/0/1 192.168.1.2 R3(config)# ip route 172.16.3.0 255.255.255.0 s0/0/1 192.168.1.2 Exit configuration mode and display the routing table. R3(config)# exit *Feb 21 18:07:51.319: %SYS-5-CONFIG_I: Configured from console by console R3# show ip route R3#Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 [1/0] via 192.168.1.2, Serial0/0/1 S 172.16.2.0 [1/0] via 192.168.1.2, Serial0/0/1 S 172.16.3.0 [1/0] via 192.168.1.2, Serial0/0/1 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, Serial0/0/1 L 192.168.1.1/32 is directly connected, Serial0/0/1 192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.2.0/24 is directly connected, GigabitEthernet0/0 L 192.168.2.1/32 is directly connected, GigabitEthernet0/0 R3# You successfully configured the fully specified static routes on R3.

Configure a Next-Hop Static Route

Configure a static route to the 172.16.1.0/24, 172.16.2.0/24, and 172.16.3.0/24 networks using the next-hop address 192.168.1.2. R3(config)# ip route 172.16.1.0 255.255.255.0 192.168.1.2 R3(config)# ip route 172.16.2.0 255.255.255.0 192.168.1.2 R3(config)# ip route 172.16.3.0 255.255.255.0 192.168.1.2 Exit configuration mode and display the routing table. R3(config)# exit *Feb 21 17:57:13.699: %SYS-5-CONFIG_I: Configured from console by console R3# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 [1/0] via 192.168.1.2 S 172.16.2.0 [1/0] via 192.168.1.2 S 172.16.3.0 [1/0] via 192.168.1.2 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.1.0/24 is directly connected, Serial0/0/1 L 192.168.1.1/32 is directly connected, Serial0/0/1 192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.2.0/24 is directly connected, GigabitEthernet0/0 L 192.168.2.1/32 is directly connected, GigabitEthernet0/0 R3# You successfully configured the next-hop static routes on R3.

Configure a Directly Connected Static IPv6 Route

Configure an IPv6 route to the 2001:DB8:ACAD:1::/64, 2001:DB8:ACAD:2::/64, and 2001:DB8:ACAD:4::/64 networks using exit interface S0/0/1. R3(config)# ipv6 route 2001:DB8:ACAD:1::/64 S0/0/1 R3(config)# ipv6 route 2001:DB8:ACAD:2::/64 S0/0/1 R3(config)# ipv6 route 2001:DB8:ACAD:4::/64 S0/0/1 Exit configuration mode and display the IPv6 routing table. R3(config)# exit R3# show ipv6 route IPv6 Routing Table - default - 8 entries Codes: C - Connected, L - Local, S - Static, U - Per-user Static route B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2 IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 S 2001:DB8:ACAD:1::/64 [1/0] via Serial0/0/1, directly connected S 2001:DB8:ACAD:2::/64 [1/0] via Serial0/0/1, directly connected C 2001:DB8:ACAD:3::/64 [0/0] via GigabitEthernet0/0, directly connected L 2001:DB8:ACAD:3::1/128 [0/0] via GigabitEthernet0/0, receive S 2001:DB8:ACAD:4::/64 [1/0] via Serial0/0/1, directly connected C 2001:DB8:ACAD:5::/64 [0/0] via Serial0/0/1, directly connected L 2001:DB8:ACAD:5::1/128 [0/0] via Serial0/0/1, receive L FF00::/8 [0/0] via Null0, receive R3# You successfully configured directly attached static IPv6 routes on R3.

Configure a Next-Hop Static IPv6 Route

Configure an IPv6 route to the 2001:DB8:ACAD:1::/64, 2001:DB8:ACAD:2::/64, and 2001:DB8:ACAD:4::/64 networks using the next-hop address 2001:DB8:ACAD:5::2. R3(config)# ipv6 route 2001:DB8:ACAD:1::/64 2001:DB8:ACAD:5::2 R3(config)# ipv6 route 2001:DB8:ACAD:2::/64 2001:DB8:ACAD:5::2 R3(config)# ipv6 route 2001:DB8:ACAD:4::/64 2001:DB8:ACAD:5::2 Exit configuration mode and display the IPv6 routing table. R3(config)# exit R3# show ipv6 route IPv6 Routing Table - default - 8 entries Codes: C - Connected, L - Local, S - Static, U - Per-user Static route B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2 IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external ND - ND Default, NDp - ND Prefix, DCE - Destination, NDr - Redirect O - OSPF Intra, OI - OSPF Inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 S 2001:DB8:ACAD:1::/64 [1/0] via 2001:DB8:ACAD:5::2 S 2001:DB8:ACAD:2::/64 [1/0] via 2001:DB8:ACAD:5::2 C 2001:DB8:ACAD:3::/64 [0/0] via GigabitEthernet0/0, directly connected L 2001:DB8:ACAD:3::1/128 [0/0] via GigabitEthernet0/0, receive S 2001:DB8:ACAD:4::/64 [1/0] via 2001:DB8:ACAD:5::2 C 2001:DB8:ACAD:5::/64 [0/0] via Serial0/0/1, directly connected L 2001:DB8:ACAD:5::1/128 [0/0] via Serial0/0/1, receive L FF00::/8 [0/0] via Null0, receive R3# You successfully configured next-hop static IPv6 routes on R3.

use static routes to

Connect to specific network Connect a stub router Summarise routing table entries Create a backup route

Configure IPv4 and IPv6 Static Host Routes

Display the routing tables on the Branch router. Issue the command to view the IPv4 routing table. Issue the command to view the IPv6 routing table. Branch# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 198.51.100.0/24 is variably subnetted, 2 subnets, 2 masks C 198.51.100.0/30 is directly connected, Serial0/0/1 L 198.51.100.2/32 is directly connected, Serial0/0/1 Branch# show ipv6 route IPv6 Routing Table - 3 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP U - Per-user Static route, M - MIPv6 I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 D - EIGRP, EX - EIGRP external C 2001:DB8:ACAD::/64 [0/0] via Serial0/0/1, directly connected L 2001:DB8:ACAD::2/128 [0/0] via Serial0/0/1, receive L FF00::/8 [0/0] via Null0, receive Enter Global Configuration mode to configure the following: A static IPv4 route to a host at address 209.165.200.238 and an exit interface of Serial0/0/1. A static IPv6 route to a host at address 2001:DB8:ACAD::2 and an exit interface of Serial0/0/1. Exit Global Configuration mode. Branch# configure terminal Branch(config)# ip route 209.165.200.238 255.255.255.255 Serial0/0/1 Branch(config)# ipv6 route 2001:DB8:ACAD::2/128 Serial0/0/1 Branch(config)# exit Display the routing tables on the Branch router. Issue the command to view the IPv4 routing table. Issue the command to view the IPv6 routing table. Branch# show ip route Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set 198.51.100.0/24 is variably subnetted, 2 subnets, 2 masks C 198.51.100.0/30 is directly connected, Serial0/0/1 L 198.51.100.2/32 is directly connected, Serial0/0/1 209.165.200.0/32 is subnetted, 1 subnets S 209.165.200.238/32 is directly connected, Serial0/0/1 Branch# show ipv6 route IPv6 Routing Table - 4 entries Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP U - Per-user Static route, M - MIPv6 I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2 ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2 D - EIGRP, EX - EIGRP external S 2001:DB8:ACAD::2/128 [1/0] via Serial0/0/1, directly connected C 2001:DB8:CAFE::/64 [0/0] via Serial0/0/1, directly connected L 2001:DB8:CAFE::2/128 [0/0] via Serial0/0/1, receive L FF00::/8 [0/0] via Null0, receive You have successfully configured static host routes.

Dynamic vs. Static

Dynamic - constantly changing Static - stays the same

Next-Hop Options

Each router has entries only for directly connected networks and their associated local addresses. None of the routers have any knowledge of any networks beyond their directly connected interfaces. For example, R1 has no knowledge of networks: 2001:DB8:ACAD:2::/64 - LAN on R2 2001:DB8:ACAD:5::/64 - Serial network between R2 and R3 2001:DB8:ACAD:3::/64 - LAN on R3 The next hop can be identified by an IPv6 address, exit interface, or both. How the destination is specified creates one of three route types: Next-hop static IPv6 route - Only the next-hop IPv6 address is specified Directly connected static IPv6 route - Only the router exit interface is specified Fully specified static IPv6 route - The next-hop IPv6 address and exit interface are specified

Static Routing Advantages

Easy to implement in a small network. Very secure. No advertisements are sent, unlike with dynamic routing protocols. It is very predictable, as the route to the destination is always the same. No routing algorithm or update mechanisms are required. Therefore, extra resources (CPU and memory) are not required.

ipv6 route command

Enable IPv6 unicast routing. R3(config)# ipv6 unicast-routing You successfully enabled IPv6 unicast routing on R3. Most of parameters are identical to the IPv4 version of the command. An IPv6 static route can also be implemented as: Standard IPv6 static route Default IPv6 static route Summary IPv6 static route Floating IPv6 static route As with IPv4, these routes can be configured as recursive, directly connected, or fully specified. The ipv6 unicast-routing global configuration command must be configured to enable the router to forward IPv6 packets.

Solve a Connectivity Problem

Finding a missing (or misconfigured) route is a relatively straightforward process, if the right tools are used in a methodical manner. For instance, in this example, the user at PC1 reports that he cannot access resources on the R3 LAN. This can be confirmed by pinging the LAN interface of R3 using the LAN interface of R1 as the source (see Figure 1). The results show that there is no connectivity between these LANs. A traceroute in Figure 2 reveals that R2 is not responding as expected. For some reason, R2 forwards the traceroute back to R1. R1 returns it to R2. This loop would continue until the time to live (TTL) value decrements to zero, in which case, the router would then send an Internet Control Message Protocol (ICMP) destination unreachable message to R1. The next step is to investigate the routing table of R2, because it is the router displaying a strange forwarding pattern. The routing table in Figure 3 reveals that the 192.168.2.0/24 network is configured incorrectly. A static route to the 192.168.2.0/24 network has been configured using the next-hop address 172.16.2.1. Using the configured next-hop address, packets destined for the 192.168.2.0/24 network are sent back to R1. It is clear from the topology that the 192.168.2.0/24 network is connected to R3, not R1. Therefore, the static route to the 192.168.2.0/24 network on R2 must use next-hop 192.168.1.1, not 172.16.2.1. Figure 4 shows output from the running configuration that reveals the incorrect ip route statement. The incorrect route is removed and the correct route is then entered. Figure 5 verifies that R1 can now reach the LAN interface of R3. As a last step in confirmation, the user on PC1 should also test connectivity to the 192.168.2.0/24 LAN.

floating static route

Floating static routes are static routes that are used to provide a backup path to a primary static or dynamic route, in the event of a link failure. The floating static route is only used when the primary route is not available. To accomplish this, the floating static route is configured with a higher administrative distance than the primary route. The administrative distance represents the trustworthiness of a route. If multiple paths to the destination exist, the router will choose the path with the lowest administrative distance. For example, assume that an administrator wants to create a floating static route as a backup to an EIGRP-learned route. The floating static route must be configured with a higher administrative distance than EIGRP. EIGRP has an administrative distance of 90. If the floating static route is configured with an administrative distance of 95, the dynamic route learned through EIGRP is preferred to the floating static route. If the EIGRP-learned route is lost, the floating static route is used in its place.

Floating Static Routes

Floating static routes are static routes that have an administrative distance greater than the administrative distance of another static route or dynamic routes. They are very useful when providing a backup to a primary link, as shown in the figure. By default, static routes have an administrative distance of 1, making them preferable to routes learned from dynamic routing protocols. For example, the administrative distances of some common dynamic routing protocols are: EIGRP = 90 IGRP = 100 OSPF = 110 IS-IS = 115 RIP = 120 The administrative distance of a static route can be increased to make the route less desirable than that of another static route or a route learned through a dynamic routing protocol. In this way, the static route "floats" and is not used when the route with the better administrative distance is active. However, if the preferred route is lost, the floating static route can take over, and traffic can be sent through this alternate route.

Verify a Static Route

Issue the command to display only the static routes in the routing table. R3# show ip route static Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2 i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2 ia - IS-IS inter area, * - candidate default, U - per-user static route o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP + - replicated route, % - next hop override Gateway of last resort is not set 172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 [1/0] via 192.168.1.2 S 172.16.2.0 [1/0] via 192.168.1.2 S 172.16.3.0 [1/0] via 192.168.1.2, R3# Issue the command to display only the route information for the 172.16.3.1 address. R3# show ip route 172.16.3.1 Routing entry for 172.16.3.0/24 Known via "static", distance 1, metric 0 Routing Descriptor Blocks: * 192.168.1.2 Route metric is 0, traffic share count is 1 R3# Issue the command to display the 'ip route' configurations in the running-config using the section filter. R3# show running-config | section ip route ip route 172.16.1.0 255.255.255.0 192.168.1.2 ip route 172.16.2.0 255.255.255.0 192.168.1.2 ip route 172.16.3.0 255.255.255.0 192.168.1.2 R3# You successfully verified the static route settings on R3.

Troubleshoot a Missing Route

Networks are subject to forces that can cause their status to change quite often: An interface fails A service provider drops a connection Links become oversaturated An administrator enters a wrong configuration When there is a change in the network, connectivity may be lost. Network administrators are responsible for pinpointing and solving the problem. To find and solve these issues, a network administrator must be familiar with tools to help isolate routing problems quickly. Common IOS troubleshooting commands include: ping traceroute show ip route show ip interface brief show cdp neighbors detail

Configure a Default Static IPv6 Route

R1 can be configured with three static routes to reach all of the remote networks in our topology. However, R1 is a stub router because it is only connected to R2. Therefore, it would be more efficient to configure a default static IPv6 route. R1(config)# ipv6 route::/0 2001:DB8:ACAD:4::2 R1(config)#

Configure a Default Static Route

R1 can be configured with three static routes to reach all of the remote networks in the example topology. However, R1 is a stub router because it is only connected to R2. Therefore, it would be more efficient to configure a default static route.

default static route

Routers commonly use default routes that are either configured locally or learned from another router, using a dynamic routing protocol. A default route does not require any left-most bits to match between the default route and the destination IPv4 address. A default route is used when no other routes in the routing table match the destination IP address of the packet. In other words, if a more specific match does not exist, then the default route is used as the Gateway of Last Resort. Default static routes are commonly used when connecting: An edge router to a service provider network A stub router (a router with only one upstream neighbor router) As shown in the figure, the command syntax for a default static route is similar to any other static route, except that the network address is 0.0.0.0 and the subnet mask is 0.0.0.0. Note: An IPv4 default static route is commonly referred to as a quad-zero route.

ip route command

Static routes are configured using the ip route global configuration command. The basic syntax for the command is shown in the figure. The following parameters are required to configure static routing: network-address - Destination network address of the remote network to be added to the routing table, often this is referred to as the prefix. subnet-mask - Subnet mask, or just mask, of the remote network to be added to the routing table. The subnet mask can be modified to summarize a group of networks. One or both of the following parameters must also be used: ip-address - The IP address of the connecting router to use to forward the packet to the remote destination network. Commonly referred to as the next hop. exit-intf - The outgoing interface to use to forward the packet to the next hop. The distance parameter is used to create a floating static route by setting an administrative distance that is higher than a dynamically learned route.

When to Use Static Routes

Static routing has three primary uses: Providing ease of routing table maintenance in smaller networks that are not expected to grow significantly. Routing to and from stub networks. A stub network is a network accessed by a single route, and the router has only one neighbor. Using a single default route to represent a path to any network that does not have a more specific match with another route in the routing table. Default routes are used to send traffic to any destination beyond the next upstream router.

Why Use Static Routing?

Static routing provides some advantages over dynamic routing, including: Static routing has the following disadvantages: Initial configuration and maintenance is time-consuming. Configuration is error-prone, especially in large networks. Administrator intervention is required to maintain changing route information. Does not scale well with growing networks; maintenance becomes cumbersome. Requires complete knowledge of the whole network for proper implementation. dynamic and static routing features are compared. Notice that the advantages of one method are the disadvantages of the other. Static routes are useful for smaller networks with only one path to an outside network. They also provide security in a larger network for certain types of traffic or links to other networks that need more control. It is important to understand that static and dynamic routing are not mutually exclusive. Rather, most networks use a combination of dynamic routing protocols and static routes. This may result in the router having multiple paths to a destination network via static routes and dynamically learned routes. However, recall that the administrative distance (AD) value is a measure of the preference of route sources. Route sources with low AD values are preferred over routes sources with higher AD values. The AD value for a static route is 1. Therefore, a static route will take precedence over all dynamically learned routes, which will have higher AD values.

Static Routing Disadvantages

Suitable for simple topologies or for special purposes such as a default static route. Configuration complexity increases dramatically as the network grows. Managing the static configurations in large networks can become time consuming. If a link fails, a static route cannot reroute traffic. Therefore, manual intervention is required to re-route traffic.

Next-Hop Options

The next hop can be identified by an IP address, exit interface, or both. How the destination is specified creates one of the three following route types: Next-hop route - Only the next-hop IP address is specified Directly connected static route - Only the router exit interface is specified Fully specified static route - The next-hop IP address and exit interface are specified

summary static route

To reduce the number of routing table entries, multiple static routes can be summarized into a single static route if: The destination networks are contiguous and can be summarized into a single network address. The multiple static routes all use the same exit interface or next-hop IP address.

Verify a Default Static Route

show ip route static command output displays the contents of the static routes in the routing table. Note the asterisk (*)next to the route with code 'S'. As displayed in the Codes table in the figure, the asterisk indicates that this static route is a candidate default route, which is why it is selected as the Gateway of Last Resort. The key to this configuration is the /0 mask. The subnet mask in a routing table determines how many bits must match between the destination IP address of the packet and the route in the routing table. A binary 1 indicates that the bits must match. A binary 0 indicates that the bits do not have to match. A /0 mask in this route entry indicates that none of the bits are required to match. The default static route matches all packets for which a more specific match does not exist.

Static Route Applications

static routes are most often used to connect to a specific network or to provide a Gateway of Last Resort for a stub network. They can also be used to: Reduce the number of routes advertised by summarizing several contiguous networks as one static route Create a backup route in case a primary route link fails The following types of IPv4 and IPv6 static routes will be discussed: Standard static route Default static route Summary static route Floating static route

Verify a Default Static Route

the show ipv6 route static command output displays the contents of the routing table. Unlike IPv4, IPv6 does not explicitly state that the default IPv6 is the Gateway of Last Resort. The key to this configuration is the ::/0 mask. Remember that the IPv6 prefix-length in a routing table determines how many bits must match between the destination IP address of the packet and the route in the routing table. The ::/0 mask indicates that none of the bits are required to match. As long as a more specific match does not exist, the default static IPv6 route matches all packets.