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Problem: Candidate Default Is Not Being Advertised ‚ Cause: ip default-network Command Is MissingIn a classless environment, when a router needs to send a packet to a particular destination, it performs the following check in this order:
Example 5-74 shows that the gateway of last resort is set to 131.108.1.1. This means that if a router does not have an entry in the routing table, it will send the packet to 131.108.1.1. Example 5-74 Verifying That a Gateway of Last Resort Is SetR1# show ip route Gateway of last resort is 131.108.1.1 to network 0.0.0.0 In any routing protocol except IGRP, the way to set the gateway of last resort is to define a static route 0.0.0.0 with the mask of 0.0.0.0 and a next -hop address, as shown in Example 5-75; however, IGRP cannot understand 0.0.0.0, so there is a separate way to set the gate-way of last resort in IGRP. Example 5-75 Configuring a Default Route to Set the Gateway of Last Resort R1(config-term)# ip route 0.0.0.0 0 0.0.0.0 131.108.1.1 Figure 5-31 shows the flowchart to follow to fix this problem. Figure 5-31. Problem-Resolution Flowchart
Debugs and VerificationExample 5-76 shows the configuration of R1. No default-network statement is configured. Example 5-76 R1's Configuration Reveals That a Candidate Default Route Has Not Been ConfiguredR1# interface Loopback1 ip address 131.108.2.1 255.255.255.0 ! interface Loopback3 ip address 155.155.155.1 255.255.255.0 ! interface Ethernet0 ip address 131.108.1.1 255.255.255.0 ! router igrp 1 network 131.108.0.0 network 155.155.0.0 Example 5-77 shows the routing table in Router R2, which R2 is receiving 155.155.155.0/24, but it is not a candidate for default because it is not configured as a candidate for default route. Example 5-77 R2's Routing TableR2# show ip route igrp I 155.155.0.0/24 [100/8976] via 131.108.1.1, 00:00:22, Serial0 131.108.0.0/24 is subnetted, 3 subnets I 131.108.2.0 [100/8976] via 131.108.1.1, 00:00:22, Serial0 SolutionIGRP is incapable of carrying the 0.0.0.0/0 (also known as default route), as explained in the problem section. Instead, it follows the default-network command to mark a network as a candidate for default. In this example, R1 is sending 155.155.155.0/24, and it is desirable to make R1 a candidate for default. To do that, change the configuration on R1 and establish the 155.155.0.0 network as the default network. Upon doing this, IGRP will automatically start treating 155.155.155.0/24 as the candidate for default and will set the gateway of last resort on R2. Example 5-78 shows the configuration for default-network on R1. This default-network statement must always point toward a major network, not a subnet; otherwise , it will not set the gateway of last resort. Example 5-78 Configuring 155.155.0.0 as the Default Network R1(config-term)# ip default-network 155.155.0.0 Example 5-79 illustrates that after the configuration change on R1, the debug ip igrp transaction output shows IGRP treating 155.155.155.0/24 route as an exterior route because it is marked as a candidate for default route. Example 5-79 IGRP Treats 155.155.0.0 as an Exterior Route IGRP: received update from 131.108.1.1 on Serial1 subnet 131.108.3.0, metric 8976 (neighbor 501) subnet 131.108.1.0, metric 10476 (neighbor 8476) exterior network 155.155.0.0, metric 8976 (neighbor 501) Example 5-80 now shows that the gateway of last resort is set and that 155.155.155.0/24 is marked as a candidate for default. Also, the * next to the I in the routing table entry means that this entry is a candidate for a default route. Example 5-80 R2's Routing Table Indicates the Candidate for Default and Shows That the Gateway of Last Resort Is Set to 155.155.0.0R2# show ip route Gateway of last resort is 131.108.1.1 to network 155.155.0.0 137.99.0.0/24 is subnetted, 1 subnets C 137.99.3.0 is directly connected, Loopback0 I* 155.155.0.0/16 [100/8976] via 131.108.1.1, 00:01:17, Serial1 |
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