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5.3 Common IGP Routing Protocol Issues,
Causes, and Solutions 5.3.7 Passive
interface The passive-interface command works
differently with the different IP routing protocols that
support it. RIP/IGRP – With RIP and IGRP, routing
updates are received, but are not sent. EIGRP – With
EIGRP, the router stops sending hello packets on passive
interfaces. When this happens, EIGRP cannot form neighbor
adjacencies on the interface, and routing updates can neither
be sent nor received. OSPF – With OSPF, routing
information is neither sent nor received on a passive
interface.The network address of the passive interface appears
as a stub network in the OSPF domain. This example shows two
routers running IGRP between each other. When an interface is
defined as passive under IGRP, IGRP will receive updates on
that interface but will not send any updates. The
passive-interface command is used to avoid sending
unnecessary updates to a neighbor that does not need to receive
any IGRP updates, such as a small router at the edge. A simple
default route is enough information for that router to talk to
the outside world. It is important to use the
passive-interface command only where needed. If not,
undesired results might occur. Debugs and Verification
Figure shows the output of the show ip protocols
command, which shows the outgoing interface is defined as
passive. Figure shows the configuration of router R1, which
shows that the outgoing interface is defined as passive.
Solution
Figures and confirm that the interface
Ethernet 0 is defined as passive, so router R1 is not sending
any updates on Ethernet 0. Sometimes, it is desirable for some
networks to be advertised and others to be filtered. In this
situation, a distribute-list out would be a better
solution. In this example, the assumption is that the
passive-interface was configured by mistake, and this
command needs to be removed to solve this problem. Figure shows
the new configuration to solve this problem. Figure shows the
routing table entry on router R2 after fixing the problem.
Commands used in this example and some of the commands that can
be used for other routing protocols include: - show
ip protocols
- debug ip routing
- debug ip rip
- debug ip igrp
transactions
- debug ip eigrp
- debug isis adj packets
- debug isis
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5.4 Troubleshooting RIP
5.4.1 Incompatible RIP version types When RIP is
configured on a router, by default, the software receives RIP
Version 1 (RIP v1) and RIP Version 2 (RIP v2) packets, but
sends only RIP v1 packets. To send and receive only RIP v1
packets, the router must be configured with the command
version 1 under router rip. To send and receive
only RIP v2 packets, the router must be configured with the
command version 2 under router rip. When the
version command is used, by default, updates from other
routers sending other than the specified version are ignored.
This example shows two routers running RIP between each other.
Debugs and Verification
Figure shows the
configuration of router R2. In this configuration, RIP is
configured to send and receive only Version 1 packets. The
default RIP configuration, without the version 1
command, will allow the router to receive both Version 1 and 2
packets. Figure shows the output of the debug ip rip
command. This command reveals that router R2 is receiving a RIP
packet from router R1, which is configured to send Version 2
updates. Figure shows the output of the show ip
protocols command, which indicates that the Ethernet 0
interface is sending and receiving RIP v1 packets. This means
that if a Version 2 packet is received on Ethernet 0 of R2, it
will be ignored because the interface can send and receive only
Version 1 packets. Figure shows the configuration of router R1.
This shows that the sender R1 is configured to only send and
receive Version 2 packets. Figure shows the output of the
show ip protocols command, which shows that sender R1 is
sending and receiving only Version 2 packets. This is because
of the version 2 command that is configured under router
RIP. Solution
An obvious solution is to configure
all routers to run RIP v2. However, there may be times when
this is not possible, and some routers can only run RIP v1.
Therefore, another solution is to configure the appropriate
interfaces to send and receive the appropriate RIP v1 or RIP v2
packets. If the receiver, R2, is configured to receive only RIP
v1 packets, it will ignore the RIP v2 updates. Router R1 must
be configured on the sender side so that it will send both
Version 1 and Version 2 packets. When R2 receives the Version 1
packet, it will install the routes in the routing table. R2
will ignore RIP v2 packets because it is configured for RIP v1.
Figure shows the new configuration for R1. In this
configuration the sender, the R1 Ethernet 0 interface, is
configured to send and receive both RIP v1 and RIP v2 packets.
Figure shows the output of show ip protocols, which
indicates that the Ethernet 0 interface is sending and
receiving Version 1 and Version 2 packets. The advantage to
sending both Version 1 and Version 2 updates is that, if any
devices on this Ethernet segment are running Version 1 only or
Version 2 only, those devices will be capable of communicating
with router R1 on this Ethernet segment. Figure shows router R2
routing table after the configuration change.
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5.4 Troubleshooting RIP
5.4.2 Mismatched authentication key One of the
options in RIP v2 is that RIP v2 updates can be authenticated
for increased security. When authentication is used, a password
must be configured on both sides. This password is called the
authentication key. If this key does not match the key on the
other side, the RIP v2 updates will be ignored on both sides.
This example shows two routers running RIP between each other.
Debugs and Verification
Figure shows the
configurations of routers R1 and R2. In this configuration, a
different RIP authentication key is configured on R1 and R2.
The R2 Ethernet interface is configured with the key cisco1,
whereas R1 is configured with the key cisco. These two keys do
not match, so they ignore each other’s updates, and the routes
will not be installed in the routing table. Figure shows the
output from the debug ip rip command on router R2 that
indicates that R2 is receiving a RIP packet that has invalid
authentication. This means that the authentication key between
sender and receiver does not match. Solution
When
using authentication in RIP, make sure that the sender and
receiver are configured with the same authentication key.
Sometimes, adding a space at the end of the key can cause the
invalid authentication problems because a space will be taken
as a literal key entry. As a result, this causes a problem that
cannot be corrected just by looking at the configurations.
Debugs will show that there is a problem with the
authentication key. To solve this problem, configure the same
keys on both sender and receiver, or retype the authentication
key, making sure that no space is being added at the end.
Figure shows the new configuration to correct this problem. The
authentication key is reconfigured on router R2 to match the
key on router R1. Figure shows the routing table of R2 after
the configuration change.
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5.4 Troubleshooting RIP
5.4.3 Reaches RIP hop count limit The RIP metric
maximum is 15 hops. If a network has more than 15 hops, RIP is
not a suitable protocol. Figure shows a network that produces a
RIP hop-count limit problem. Router R2 is receiving an update
for a RIP route, which is more than 15 hops away. R2 does not
install that route in the routing table, as demonstrated in the