Problem: Candidate Default Is Not Being Advertised-Cause: ip default-network Command Is Missing

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Problem: Candidate Default Is Not Being Advertised ‚ Cause: ip default-network Command Is Missing

In a classless environment, when a router needs to send a packet to a particular destination, it performs the following check in this order:

  1. Is the destination address one of my connected interface addresses? If yes, use ARP for the address and then encapsulate the packet in an Ethernet frame and send it to the destination.

  2. If no, do I have a route in my routing table for this destination address? If yes, use that route from the routing table and send the packet.

  3. If no, check whether the gateway of last resort is set. If it is set, send the packet to the address mentioned in the gateway of last resort. (In Example 5-74, the packets will be sent to 131.108.1.1. If there is no gateway of last resort, the packet is dropped.)

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 Set
 R1#  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 Verification

Example 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 Configured
 R1#  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 Table
 R2#  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 

Solution

IGRP 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 155.155.0.0
 R2#  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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Troubleshooting IP Routing Protocols
Troubleshooting IP Routing Protocols (CCIE Professional Development Series)
ISBN: 1587050196
EAN: 2147483647
Year: 2002
Pages: 260

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