Day 15. Static and Default Route Configuration
CCNA 200-301 Exam Topics
Configure and verify IPv4 and IPv6 static routing
Key Topics
Today we focus on static and default routing for IPv4 and IPv6. Static routes are a common part of an enterprise’s routing policy. Static routes can be used to force traffic to use a specific path or to establish a default route out of the enterprise. The network administrator hard-codes static routes into the routing table. Thus, a network administrator must monitor and maintain static routes to ensure connectivity.
Static and Default Routing Overview
When a router configured with a dynamic routing protocol can learn routes from other routers without additional input from the network administrator, why would you use static routing? Situations vary, and other reasons might be unique to a particular implementation, but, in general, you use static routing in these cases:
In a small network that requires only simple routing
In a hub-and-spoke network topology
When you want to create a quick ad hoc route
As a backup when the primary route fails
In general, you do not use static routes in these cases:
In a large network
When the network is expected to scale
Static routes are commonly used when you are routing from a larger network to a stub network (a network that is accessed by a single link). Static routes can also be useful for specifying a default route or gateway of last resort. For example, in Figure 15-1, R2 is attached to a stub network.
Figure 15-1 Example of a Stub Network
In Figure 15-1, no other route out of the stub network exists except to send packets to HQ. Therefore, it makes sense to configure R2 with a default route pointing out the interface attached to HQ. Similarly, HQ has only one way to route packets destined for the stub network attached to R2. Therefore, it makes sense to configure HQ with a static route pointing out the interface attached to R2. Yes, you could configure both routers with a dynamic routing protocol, but that could introduce a level of complexity that might not be necessary in a stub network situation.
IPv4 Static Route Configuration
To configure a static route, use the ip route command with the following relevant syntax:
Router(config)# ip route network-address subnet-mask {ip-address | exit-interface}
[administrative-distance]
The explanation for each parameter follows:
network-address: The destination network address of the remote network to be added to the routing table.
subnet-mask: The subnet 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 are used:
ip-address: Commonly referred to as the next-hop router’s IP address
exit-interface: The outgoing interface used in forwarding packets to the destination network
In addition, the optional administrative-distance parameter is used when configuring a floating static route, as you see later in today’s review.
Figure 15-2 shows the topology we use today in reviewing IPv4 static and default routing.
Figure 15-2 IPv4 Static and Default Routing Topology
Table 15-1 shows the IPv4 addressing scheme used with the topology in Figure 15-2.
Table 15-1 IPv4 Addressing Scheme
Device |
Interface |
IP Address |
Subnet Mask |
Default Gateway |
HQ |
S0/0/0 |
10.10.10.1 |
255.255.255.0 |
— |
R1 |
G0/0 S0/0/0 |
172.16.3.1 172.16.2.2 |
255.255.255.0 255.255.255.0 |
— — |
R2 |
G0/0 S0/0/0 S0/0/1 S0/1/0 |
172.16.1.1 172.16.2.1 192.168.0.1 10.10.10.2 |
255.255.255.0 255.255.255.0 255.255.255.0 255.255.255.0 |
— — — — |
R3 |
G0/0 S0/0/1 |
192.168.1.1 192.168.0.2 |
255.255.255.0 255.255.255.0 |
— — |
PC1 |
NIC |
172.16.3.10 |
255.255.255.0 |
172.16.3.1 |
PC2 |
NIC |
172.16.1.10 |
255.255.255.0 |
172.16.1.1 |
PC3 |
NIC |
192.168.2.10 |
255.255.255.0 |
192.168.2.1 |
Assume that R1 is configured and knows about its own directly connected networks. Example 15-1 shows the routing table for R1 before any static routing is configured.
Example 15-1 R1 Routing Table Before Static Routes Are Configured
R1# show ip route
<output omitted>
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.16.2.0/24 is directly connected, Serial0/0/0
L 172.16.2.2/32 is directly connected, Serial0/0/0
C 172.16.3.0/24 is directly connected, GigabitEthernet0/0
L 172.16.3.1/32 is directly connected, GigabitEthernet0/0
R1#
R1 does not know about these remote networks:
172.16.1.0/24: The LAN on R2
192.168.0.0/24: The serial network between R2 and R3
192.168.1.0/24: The LAN on R3
10.10.10.0/24: The serial network between R2 and HQ
0.0.0.0/0: All other networks accessible through HQ
IPv4 Static Routes Using the Next-Hop Parameter
Using the next-hop parameter, R1 can be configured with three static routes—one for each network R1 does not yet know about. Example 15-2 shows the command syntax.
Example 15-2 Static Route Configuration with the Next-Hop Parameter
R1(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1 R1(config)# ip route 192.168.0.0 255.255.255.0 172.16.2.1 R1(config)# ip route 192.168.1.0 255.255.255.0 172.16.2.1 R1(config)# ip route 10.10.10.0 255.255.255.0 172.16.2.
The interface that routes to the next hop must be up and up before the static routes can be entered in the routing table. Example 15-3 verifies that the static routes are now in the routing table.
Example 15-3 R1 Routing Table After Static Routes Are Configured
R1# show ip route
<output omitted>
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
S 10.10.10.0/24 [1/0] via 172.16.2.1
172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
S 172.16.1.0/24 [1/0] via 172.16.2.1
C 172.16.2.0/24 is directly connected, Serial0/0/0
L 172.16.2.2/32 is directly connected, Serial0/0/0
C 172.16.3.0/24 is directly connected, GigabitEthernet0/0
L 172.16.3.1/32 is directly connected, GigabitEthernet0/0
S 192.168.0.0/24 [1/0] via 172.16.2.1
S 192.168.1.0/24 [1/0] via 172.16.2.1
R1#
When using the next-hop parameter, the router must have a route in the table to the network that the next-hop address belongs to. In the highlighted line in Example 15-3, we see that R1 does indeed have a route to the 172.16.2.0/24 network, which includes the next-hop address 172.16.2.1. However, configuring a next-hop address requires the router to perform a recursive lookup to find the exit interface before it can send the packet out the Serial 0/0/0 interface.
IPv4 Static Routes Using the Exit Interface Parameter
To avoid a recursive lookup and have a router immediately send packets to the exit interface, configure the static route using the exit-interface parameter instead of the ip-address next-hop parameter.
For example, on R2, we can configure static routes to the R1 and R3 LANs by specifying the exit interface:
R2(config)# ip route 172.16.3.0 255.255.255.0 serial 0/0/0 R2(config)# ip route 192.168.1.0 255.255.255.0 serial 0/0/1
Any previous static routes to this network using a next-hop IP address should be removed. R2 now has two static routes in its routing table (see Example 15-4) that it can use immediately to route to the 172.16.3.0/24 and 192.168.1.0/24 networks without having to do a recursive route lookup.
Example 15-4 R2 Routing Table After the Static Route Is Configured
R2# show ip route <output omitted> Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks C 10.10.10.0/24 is directly connected, Serial0/1/0 L 10.10.10.2/32 is directly connected, Serial0/1/0 172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks C 172.16.1.0/24 is directly connected, GigabitEthernet0/0 L 172.16.1.1/32 is directly connected, GigabitEthernet0/0 C 172.16.2.0/24 is directly connected, Serial0/0/0 L 172.16.2.1/32 is directly connected, Serial0/0/0 S 172.16.3.0/24 is directly connected, Serial0/0/0 192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks C 192.168.0.0/24 is directly connected, Serial0/0/1 L 192.168.0.1/32 is directly connected, Serial0/0/1 S 192.168.1.0/24 is directly connected, Serial0/0/1 R2#
IPv4 Default Route Configuration
A default route is a special kind of static route used to represent all routes with zero or no bits matching. In other words, when no routes have a more specific match in the routing table, the default route is a match.
The destination IP address of a packet can match multiple routes in the routing table. For example, consider having the following two static routes in the routing table:
172.16.0.0/24 is subnetted, 3 subnets S 172.16.1.0 is directly connected, Serial0/0/0 S 172.16.0.0/16 is directly connected, Serial0/0/1
A packet destined for 172.16.1.10, the packet’s destination IP address, matches both routes. However, the 172.16.1.0 route is the more specific route because the destination matches the first 24 bits, whereas the destination matches only the first 16 bits of the 172.16.0.0 route. Therefore, the router uses the route with the most specific match.
A default route is a route that matches all packets. Commonly called a quad-zero route, a default route uses 0.0.0.0 (thus the term quad-zero) for both the network-address and the subnet-mask parameters, as in this syntax:
Router(config)# ip route 0.0.0.0 0.0.0.0 {ip-address | exit-interface}
Referring to the topology in Figure 15-2, assume that HQ has a connection to the Internet. From the perspective of R2, all default traffic can be sent to HQ for routing outside the domain known to R2.
The following command configures R2 with a default route pointing to HQ:
R2(config)# ip route 0.0.0.0 0.0.0.0 serial 0/1/0
R2 now has a gateway of last resort listed in the routing table—a candidate default route indicated by the asterisk (*) next to the S code (see Example 15-5).
Example 15-5 R2 Routing Table After the Default Route Is Configured
R2# show ip route
<some codes omitted>
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, Serial0/1/0
L 10.10.10.2/32 is directly connected, Serial0/1/0
172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C 172.16.1.0/24 is directly connected, GigabitEthernet0/0
L 172.16.1.1/32 is directly connected, GigabitEthernet0/0
C 172.16.2.0/24 is directly connected, Serial0/0/0
L 172.16.2.1/32 is directly connected, Serial0/0/0
S 172.16.3.0/24 is directly connected, Serial0/0/0
192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.0.0/24 is directly connected, Serial0/0/1
L 192.168.0.1/32 is directly connected, Serial0/0/1
S 192.168.1.0/24 is directly connected, Serial0/0/1
S* 0.0.0.0/0 is directly connected, Serial0/1/0
R2#
From R1’s and R3’s perspective, R2 is the default route. The following commands configure R1 and R3 with a default route pointing to R2:
R1(config)# ip route 0.0.0.0 0.0.0.0 serial 0/0/0 ! R3(config)# ip route 0.0.0.0 0.0.0.0 serial 0/0/1
Again, we can verify that the default route is now in the routing table for R1 (see Example 15-6).
Example 15-6 R1 and R3 Routing Tables After the Default Route Is Configured
!R1!!!!!!!!!!!
R1# show ip route
<some codes omitted>
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
C 172.16.2.0/24 is directly connected, Serial0/0/0
L 172.16.2.2/32 is directly connected, Serial0/0/0
C 172.16.3.0/24 is directly connected, GigabitEthernet0/0
L 172.16.3.1/32 is directly connected, GigabitEthernet0/0
S* 0.0.0.0/0 is directly connected, Serial0/0/0
R1#
!
!R3!!!!!!!!!!!!
R3# show ip route
<some codes omitted>
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.0.0/24 is directly connected, Serial0/0/1
L 192.168.0.2/32 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, GigabitEthernet0/0
L 192.168.1.1/32 is directly connected, GigabitEthernet0/0
S* 0.0.0.0/0 is directly connected, Serial0/0/1
R3#
After evaluating the complete routing tables for R1, R2, and R3 shown in Examples 15-5 and 15-6, you can see that R1 and R3 need only one route out—a default route. R2 acts as a hub router to the R1 and R3 spokes. Therefore, it needs two static routes pointing to the R1 and R3 LANs. R2 also has a route out to HQ for any destinations it does not know about. But what about HQ? Currently, HQ does not have routes back to any of the networks accessible through R2. Any traffic from PC1, PC2, and PC3 is thus currently confined to the R1, R2, and R3 networks. None of these PCs can ping the HQ interface address 10.10.10.1. In the traceroute output in Example 15-7, failure occurs after R2 responds. This is because HQ receives the ICMP requests from PC1 but does not have a route back to the 172.16.3.0/24 network. Therefore, HQ drops the packets.
Example 15-7 A Failed traceroute from PC1 to HQ
C:> tracert 10.10.10.1 Tracing route to 10.10.10.1 over a maximum of 30 hops: 1 0 ms 0 ms 1 ms 172.16.3.1 2 0 ms 0 ms 1 ms 172.16.2.1 3 * * * Request timed out. 4 ^C C:>
In the next section, we configure HQ with static routes to complete the static route configuration for the topology in Figure 15-2.
IPv4 Summary Static Route Configuration
Before configuring five separate static routes for each of the networks in Figure 15-2, notice that the 172.16 networks can be summarized into one route and that the 192.168 networks can be summarized into one route. Example 15-8 shows the five routes in binary, with the bits in common highlighted.
Example 15-8 Summary Route Calculation for HQ Static Routes
Summary calculation for the 172.16 networks: 10101100.00010000.00000001.00000000 10101100.00010000.00000010.00000000 10101100.00010000.00000011.00000000 Summary calculation for the 192.168 networks: 11000000.10101000.00000000.00000000 11000000.10101000.00000001.00000000
The summary route for the 172.16 networks is 172.16.0.0/22 because the three network addresses have 22 bits in common. Although this summary static route is not part of the current addressing scheme, it also includes the route 172.16.0.0/24. The summary route for the 192.168 networks is 192.168.0.0/23 because the two network addresses have 23 bits in common.
We can now configure HQ with two summary static routes instead of five individual static routes:
HQ(config)# ip route 172.16.0.0 255.255.252.0 serial 0/0/0 HQ(config)# ip route 192.168.0.0 255.255.254.0 Serial0/0/0
Now PC1 can successfully trace a route to the HQ interface, as Example 15-9 shows.
Example 15-9 A Successful traceroute from PC1 to HQ
C:> tracert 10.10.10.1 Tracing route to 10.10.10.1 over a maximum of 30 hops: 1 1 ms 0 ms 0 ms 172.16.3.1 2 0 ms 1 ms 2 ms 172.16.2.1 3 1 ms 2 ms 1 ms 10.10.10.1 Trace complete. C:>
The trace is successful because HQ now has a route back to PC1’s network, as shown in Example 15-10.
Example 15-10 HQ Routing Table with IPv4 Summary Static Routes
HQ# show ip route
<output omitted>
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.10.10.0/24 is directly connected, Serial0/0/0
L 10.10.10.1/32 is directly connected, Serial0/0/0
172.16.0.0/22 is subnetted, 1 subnets
S 172.16.0.0/22 is directly connected, Serial0/0/0
S 192.168.0.0/23 is directly connected, Serial0/0/
HQ#
IPv6 Static Routing
Static routing with IPv6 is similar to static routing with IPv4. We can use the same topology but change the addressing to IPv6, as shown in Figure 15-3.
Figure 15-3 Static and Default Routing IPv6 Topology
Table 15-2 shows the IPv6 addressing scheme used with the topology in Figure 15-3.
Table 15-2 IPv6 Addressing Scheme
Device |
Interface |
IPv6 Address/Prefix |
Default Gateway |
HQ |
S0/0/0 Link-local |
2001:DB8:1:F::1/64 FE80::F |
— |
R1 |
G0/0 S0/0/0 |
2001:DB8:1:3::1/64 2001:DB8:1:2::2/64 FE80::1 |
— — |
R2 |
G0/0 S0/0/0 S0/0/1 S0/1/0 Link-local |
2001:DB8:1:1::1/64 2001:DB8:1:2::1/64 2001:DB8:1:A0::1/64 2001:DB8:1:F::2/64 FE80::2 |
— — — — |
R3 |
G0/0 S0/0/1 Link-local |
2001:DB8:1:A1::1/64 2001:DB8:1:A0::2/64 FE80::3 |
— — — |
PC1 |
NIC |
2001:DB8:1:3:209:7CFF:FE9A:1A87/64 |
FE80::1 |
PC2 |
NIC |
2001:DB8:1:1:204:9AFF:FEE3: C943/64 |
FE80::2 |
PC3 |
NIC |
2001:DB8:1:A1:201:C9FF:FEE5: D3A/64 |
FE80::3 |
IPv6 Static Route Configuration
The command syntax for IPv6 static routing is similar to the syntax for IPv4:
Router(config)# ipv6 route ipv6-prefix/prefix-length {ipv6-address |
exit-interface} [administrative-distance]
Therefore, the following commands configure R2 with static routes to the R1 and R3 LANs:
R2(config)# ipv6 route 2001:DB8:1:3::/64 Serial0/0/0 R2(config)# ipv6 route 2001:DB8:1:A1::/64 Serial0/0/1
As highlighted in the output from the show ipv6 route command in Example 15-11, R2 now has routes in the routing table to the R1 and R3 LANs.
Example 15-11 R2 IPv6 Routing Table After Static Routes Are Configured
R2# show ipv6 route
IPv6 Routing Table - 11 entries
<code output omitted>
C 2001:DB8:1:1::/64 [0/0]
via ::, GigabitEthernet0/0
L 2001:DB8:1:1::1/128 [0/0]
via ::, GigabitEthernet0/0
C 2001:DB8:1:2::/64 [0/0]
via ::, Serial0/0/0
L 2001:DB8:1:2::1/128 [0/0]
via ::, Serial0/0/0
S 2001:DB8:1:3::/64 [1/0]
via ::, Serial0/0/0
C 2001:DB8:1:F::/64 [0/0]
via ::, Serial0/1/0
L 2001:DB8:1:F::2/128 [0/0]
via ::, Serial0/1/0
C 2001:DB8:1:A0::/64 [0/0]
via ::, Serial0/0/1
L 2001:DB8:1:A0::1/128 [0/0]
via ::, Serial0/0/1
S 2001:DB8:1:A1::/64 [1/0]
via ::, Serial0/0/1
L FF00::/8 [0/0]
via ::, Null0
IPv6 Default Route Configuration
The following is the command syntax for an IPv6 default route:
Router(config)# ipv6 route ::/0 {ipv6-address | exit-interface}
Just as with the quad-zero in IPv4, the double colon (::) means all 0s or any address, and the /0 means any prefix length.
Continuing with the example in Figure 15-3, we can configure R1, R2, and R3 with the following default routes:
R1(config)# ipv6 route ::/0 serial 0/0/0 R2(config)# ipv6 route ::/0 serial 0/1/0 R3(config)# ipv6 route ::/0 serial 0/0/1
The highlights in Example 15-12 show the default routes for R1, R2, and R3.
Example 15-12 Default Routes in the Routing Tables for R1, R2, and R3
!R1!!!!!!!!!!! R1# show ipv6 route IPv6 Routing Table - 6 entries <code output omitted> S ::/0 [1/0] via ::, Serial0/0/0 <output for connected and local routes omitted> !R2!!!!!!!!!!! R2# show ipv6 route IPv6 Routing Table - 12 entries <code output omitted> S ::/0 [1/0] via ::, Serial0/1/0 S 2001:DB8:1:3::/64 [1/0] via ::, Serial0/0/0 S 2001:DB8:1:A1::/64 [1/0] via ::, Serial0/0/1 <output for connected and local routes omitted> !R3!!!!!!!!!!! R3# show ipv6 route IPv6 Routing Table - 6 entries <code output omitted> S ::/0 [1/0] via ::, Serial0/0/1 <output for connected and local routes omitted
IPv6 Summary Static Route Configuration
Much as in the IPv4 static routing scenario, HQ can be configured with two summary static routes to the R1, R2, and R3 LANs. Example 15-13 shows the first four hextets (64 bits) of the five routes in binary, with the bits in common highlighted.
Example 15-13 Summary Route Calculation for HQ Static Routes
Summary calculation for the first four hextets of 2001:DB8:1:1::/64, 2001:DB8:1:2::/64, and 2001:DB8:1:3::/64 networks: 0010000000000001:0000110110111000:0000000000000001:0000000000000001:: 0010000000000001:0000110110111000:0000000000000001:0000000000000010:: 0010000000000001:0000110110111000:0000000000000001:0000000000000011:: Summary calculation for the first four hextets of 2001:DB8:1:A0::/64 and 2001:DB8:1:A1::/64 networks: 0010000000000001:0000110110111000:0000000000000001:0000000010100000:: 0010000000000001:0000110110111000:0000000000000001:0000000010100001::
The first summary route is 2001:DB8:1::/62 because the three network addresses have 62 bits in common. Although this summary static route is not part of the current addressing scheme, it also includes the network 2001:DB8:1::/64. The second summary route is 2001:DB8:1:A0::/63 because the two network addresses have 63 bits in common.
You can now configure HQ with the following two summary static routes:
HQ(config)# ipv6 route 2001:DB8:1::/62 Serial0/0/0 HQ(config)# ipv6 route 2001:DB8:1:A0::/63 Serial0/0/0
Now HQ has two summary routes, as you can see in the highlighted entries in Example 15-14.
Example 15-14 HQ Routing Table with IPv6 Summary Static Routes
HQ# show ipv6 route IPv6 Routing Table - 5 entries <output omitted> S 2001:DB8:1::/62 [1/0] via ::, Serial0/0/0 C 2001:DB8:1:F::/64 [0/0] via ::, Serial0/0/0 L 2001:DB8:1:F::1/128 [0/0] via ::, Serial0/0/0 S 2001:DB8:1:A0::/63 [1/0] via ::, Serial0/0/0 L FF00::/8 [0/0] via ::, Null0 HQ#
Study Resources
For today’s exam topics, refer to the following resources for more study.
Resource |
Module or Chapter |
Switching, Routing, and Wireless Essentials |
15 |
CCNA 200-301 Official Cert Guide, Volume 1 |
16 |
18 |
|
25 |
|
Portable Command Guide |
15 |