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Troubleshooting
When troubleshooting connectivity issues for an IP-based network, the network layer
(that is, Layer 3) of the OSI reference model is often an appropriate place to begin your
troubleshooting efforts. For example, if you are experiencing connectivity issues between
two hosts on a network, you could check Layer 3 by pinging between the hosts. If the
pings are successful, you can conclude that the issue resides at upper layers of the OSI reference
model (that is, Layers 4–7). However, if the pings fail, you can focus your troubleshooting
efforts on Layers 1–3. This section discusses fundamental routing concepts
and provides you with a collection of Cisco IOS Software commands that could prove to
be useful when troubleshooting routing issues.
Basic Routing Processes
To review basic routing processes, consider Figure 6-1. In this topology, PC1 needs to
send traffic to Server1. Notice that these hosts are on different networks. So, the question
becomes, how does a packet from a source IP address of 192.168.1.2 get routed to a destination
IP address of 192.168.3.2?
Consider the following walkthrough of this process, step-by-step:
Step 1. PC1 compares its IP address and subnet mask of 192.168.1.2/24 with the destination
IP address and subnet mask of 192.168.3.2/24. PC1 concludes that the
destination IP address resides on a remote subnet. Therefore, PC1 needs to
send the packet to its default gateway, which could have been manually configured
on PC1 or dynamically learned via DHCP. In this example, PC1 has a
default gateway of 192.168.1.1 (that is, router R1). In order to construct a
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IP Address: 192.168.1.2/24
MAC Address: 1111.1111.1111
Default Gateway: 192.168.1.1
IP Address: 192.168.3.2/24
MAC Address: 2222.2222.2222
Default Gateway: 192.168.3.1
S 1/1
192.168.2.1/30 S 1/1
192.168.2.2/30
Fa 0/0
192.168.3.1/24
BBBB.BBBB.BBBB
Fa 0/0
192.168.1.1/24
AAAA.AAAA.AAAA
SW1 R1 R2 SW2
PC1
Server1
192.168.1.1
PC1’s ARP Cache
ARP Request
Data Frame
Source IP Address: 192.168.1.2
Source MAC Address: 1111.1111.1111
Destination IP Address: 192.168.3.2
Destination MAC Address: AAAA.AAAA.AAAA
ARP Reply
AAAA.AAAA.AAAA
Figure 6-2 Basic Routing: Step #1
properly constructed Layer 2 frame, however, PC1 also needs the MAC address
of its default gateway. PC1 ARPs for router R1’s MAC address. Once PC1
receives an ARP Reply from router R1, PC1 adds router R1’s MAC address to
its ARP cache. PC1 now sends its data in a frame destined for Server1, as
shown in Figure 6-2.
Step 2. Router R1 receives the frame sent from PC1 and interrogates the IP header. An
IP header contains a Time-to-Live (TTL) field, which is decremented once for
each router hop. Therefore, router R1 decrements the packet’s TTL field. If the
value in the TTL field is reduced to zero, the router discards the frame and
sends a time exceeded Internet Control Message Protocol (ICMP) message
back to the source. Assuming the TTL is not decremented to zero, router R1
checks its routing table to determine the best path to reach network
192.168.3.0/24. In this example, router R1’s routing table has an entry stating
that network 192.168.3.0/24 is accessible via interface Serial 1/1. Note that
ARPs are not required for serial interfaces, because these interface types do
not have MAC addresses. Router R1, therefore, forwards the frame out of its
Serial 1/1 interface, as depicted in Figure 6-3.
Step 3. When router R2 receives the frame, it decrements the TTL in the IP header,
just as router R1 did. Again, assuming the TTL did not get decremented to
zero, router R2 interrogates the IP header to determine the destination network.
In this case, the destination network of 192.168.3.0/24 is directly
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Chapter 6: Introduction to Troubleshooting Routing Protocols 143
IP Address: 192.168.1.2/24
MAC Address: 1111.1111.1111
Default Gateway: 192.168.1.1
IP Address: 192.168.3.2/24
MAC Address: 2222.2222.2222
Default Gateway: 192.168.3.1
S 1/1
192.168.2.1/30 S 1/1
192.168.2.2/30
Fa 0/0
192.168.3.1/24
BBBB.BBBB.BBBB
Fa 0/0
192.168.1.1/24
AAAA.AAAA.AAAA
SW1 R1 R2 SW2
PC1
Server1
Source IP Address: 192.168.1.2
Source MAC Address: N/A
Destination IP Address: 192.168.3.2
Destination MAC Address: N/A
Data Frame
192.168.3.0/24
Router R1’s Route Entry
Serial 1/1
Figure 6-3 Basic Routing: Step #2
IP Address: 192.168.1.2/24
MAC Address: 1111.1111.1111
Default Gateway: 192.168.1.1
IP Address: 192.168.3.2/24
MAC Address: 2222.2222.2222
Default Gateway: 192.168.3.1
S 1/1
192.168.2.1/30 S 1/1
192.168.2.2/30
Fa 0/0
192.168.3.1/24
BBBB.BBBB.BBBB
Fa 0/0
192.168.1.1/24
AAAA.AAAA.AAAA
SW1 R1 R2 SW2
PC1
Server1
192.168.3.2
Router R2’s ARP Cache ARP Request
Source IP Address: 192.168.1.2
Source MAC Address: BBBB.BBBB.BBBB
Destination IP Address: 192.168.3.2
Destination MAC Address: 2222.2222.2222
Data Frame
2222.2222.2222 ARP Reply
Figure 6-4 Basic Routing: Step #3
attached to router R2’s Fast Ethernet 0/0 interface. Similar to how PC1 sent out
an ARP Request to determine the MAC address of its default gateway, router
R2 sends an ARP Request to determine the MAC address of Server1. Once an
ARP Reply is received from Server1, router R2 forwards the frame out of its
Fast Ethernet 0/0 interface to Server1, as illustrated in Figure 6-4.
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The previous steps identified two router data structures:
■ IP routing table: When a router needed to route an IP packet, it consulted its IP
routing table to find the best match. The best match is the route that has the longest
prefix. Specifically, a route entry with the longest prefix is the most specific network.
For example, imagine that a router has an entry for network 10.0.0.0/8 and for network
10.1.1.0/24. Also, imagine that the router is seeking the best match for a destination
address of 10.1.1.1/24. The router would select the 10.1.1.0/24 route entry as the
best entry, because that route entry has the longest prefix.
■ Layer 3 to Layer 2 mapping: In the previous example, router R2’s ARP cache contained
Layer 3 to Layer 2 mapping information. Specifically, the ARP cache had a
mapping that said a MAC address of 2222.2222.2222 corresponded to an IP address
of 192.168.3.2. While an ARP cache is the Layer 3 to Layer 2 mapping data structure
used for Ethernet-based networks, similar data structures are used for Frame Relay
and Asynchronous Transfer mode (ATM) point-to-multipoint links. For point-to-point
links, however, an egress interface might be shown in the IP routing table, as opposed
to a next-hop IP address. For these types of links (for example, point-to-point Frame
Relay or ATM Permanent Virtual Circuits [PVC], High-level Data Link Control
[HDLC], or Point-to-Point Protocol [PPP] links), the information required to construct
an outgoing frame can be gleaned from the egress interface, thus not requiring a nexthop
IP address.
Continually querying a router’s routing table and its Layer 3 to Layer 2 mapping data
structure (for example, an ARP cache) is less than efficient. Fortunately, Cisco Express
Forwarding (CEF), as introduced in Chapter 5, makes lookups much more efficient. CEF
gleans its information from the router’s IP routing table and Layer 3 to Layer 2 mapping
tables. Then, CEF’s data structures can be referenced when forwarding packets. The two
primary CEF data structures are as follows:
■ Forwarding Information Base (FIB): The FIB contains Layer 3 information, similar
to the information found in an IP routing table. Additionally, a FIB contains information
about multicast routes and directly connected hosts.
■ Adjacency table: When a router is performing a route lookup using CEF, the FIB
references an entry in the adjacency table. The adjacency table entry contains the
frame header information required by the router to properly form a frame. Therefore,
an egress interface and a next-hop IP address would be in an adjacency entry for a
multipoint interface, whereas a point-to-point interface would require only egress interface
information. Note that if a host is adjacent to a router, the router can reach
that host over a single Layer 2 hop (that is, traffic would not have to be routed to
reach an adjacency).
As a reference, Figure 6-5 shows the router data structures previously discussed.
Troubleshooting Basic Routing
When troubleshooting some routing issues, you might want to examine a router’s IP routing
table. If the traffic’s observed behavior is not conforming to information in the IP
routing table, however, recall that the IP routing table is maintained by a router’s control
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Chapter 6: Introduction to Troubleshooting Routing Protocols 145
IP Routing Table
CEF Forwarding
Information Base
Layer 3 to Layer 2
Mappings
CEF Adjacency
Table
Figure 6-5 A Router’s Data Structures
Table 6-2 Troubleshooting Layer 3 Forwarding Information
show ip route ip-address Displays a router’s best route to the specified IP address.
show ip route network
subnet-mask
Displays a router’s best route to the specified network, if the
specific route (with a matching subnet mask length) is found in
the router’s IP routing table.
show ip route network
subnet-mask longerprefixes
Displays all routes in a router’s IP routing table that are encompassed
by the specified network address and subnet mask.
(NOTE: This command is often useful when troubleshooting
route summarization issues.)
show ip cef ip-address Displays information (for example, next-hop IP address and
egress interface) required to forward a packet, similar to the output
of the show ip route ip-address command. (NOTE: The
output of this command comes from CEF. Therefore, routing
protocol information is not presented in the output.)
show ip cef network
subnet-mask
Displays information from a router’s FIB showing the information
needed to route a packet to the specified network with the
specified subnet mask.
plane. CEF, however, operates in the data plane. Therefore, you might also want to view
CEF information, because CEF’s data structures (that is, the FIB and the adjacency table)
contain all the information required to make packet forwarding decisions. For your reference,
Table 6-2 contains a collection of commands useful for verifying both IP routing
table information (that is, control plane information) and CEF information (that is, data
plane information).
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Table 6-2 Troubleshooting Layer 3 Forwarding Information
Command Description
show ip cef exact-route
source-ip-address
destination-ipaddress
Displays the adjacency that will be used to forward a packet
from the specified source IP address to the specified destination
IP address. (NOTE: This command is useful if the router is
load balancing across multiple adjacencies, and you want to see
which adjacency will be used for a certain combination of
source and destination IP addresses.)
Example 6-1 provides sample output from the show ip route ip-address command. The
output shows that the next-hop IP address to reach an IP address of 192.168.1.11 is
192.168.0.11, which is accessible via interface Fast Ethernet 0/0. Because this information
is coming from the control plane, it includes information about the routing protocol,
which is OSPF in this case.
Example 6-1 show ip route ip-address Command Output
Example 6-2 provides sample output from the show ip route network subnet-mask command.
The output indicates that network 192.168.1.0/24 is accessible out of interface Fast
Ethernet 0/0, with a next-hop IP address of 192.168.0.11.
Example 6-2 show ip route network subnet_mask Command Output
Example 6-3 provides sample output from the show ip route network subnet-mask
[longer-prefixes] command, with and without the longer-prefixes option. Notice that the
router responds that the subnet 172.16.0.0 255.255.0.0 is not in the IP routing table. However,
after adding the longer-prefixes option, two routes are displayed, because these
routes are subnets of the 172.16.0.0/16 network.
R2# show ip route 192.168.1.11
Routing entry for 192.168.1.0/24
Known via “ospf 1”, distance 110, metric 11, type intra area
Last update from 192.168.0.11 on FastEthernet0/0, 00:06:45 ago
Routing Descriptor Blocks:
* 192.168.0.11, from 10.1.1.1, 00:06:45 ago, via FastEthernet0/0
Route metric is 11, traffic share count is 1
R2# show ip route 192.168.1.0 255.255.255.0
Routing entry for 192.168.1.0/24
Known via “ospf 1”, distance 110, metric 11, type intra area
Last update from 192.168.0.11 on FastEthernet0/0, 00:06:57 ago
Routing Descriptor Blocks:
* 192.168.0.11, from 10.1.1.1, 00:06:57 ago, via FastEthernet0/0
Route metric is 11, traffic share count is 1
(Continued)
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Chapter 6: Introduction to Troubleshooting Routing Protocols 147
Example 6-3 show ip route network subnet-mask [longer-prefixes] Command Output
Example 6-4 provides sample output from the show ip cef ip-address command. The output
indicates that, according to CEF, an IP address of 192.168.1.11 is accessible out of
interface Fast Ethernet 0/0, with a next-hop IP address of 192.168.0.11.
Example 6-4 show ip cef ip-address Command Output
Example 6-5 provides sample output from the show ip cef network subnet_mask command.
The output indicates that network 192.168.1.0/24 is accessible off of interface Fast
Ethernet 0/0, with a next-hop IP address of 192.168.0.11.
Example 6-5 show ip cef network subnet-mask Command Output
R2# show ip route 172.16.0.0 255.255.0.0
% Subnet not in table
R2# show ip route 172.16.0.0 255.255.0.0 longer-prefixes
Codes: 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
Gateway of last resort is not set
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, Serial1/0.1
C 172.16.2.0 is directly connected, Serial1/0.2
R2# show ip cef 192.168.1.11
192.168.1.0/24, version 42, epoch 0, cached adjacency 192.168.0.11
0 packets, 0 bytes
via 192.168.0.11, FastEthernet0/0, 0 dependencies
next hop 192.168.0.11, FastEthernet0/0
valid cached adjacency
R2# show ip cef 192.168.1.0 255.255.255.0
192.168.1.0/24, version 42, epoch 0, cached adjacency 192.168.0.11
0 packets, 0 bytes
via 192.168.0.11, FastEthernet0/0, 0 dependencies
next hop 192.168.0.11, FastEthernet0/0
valid cached adjacency
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Table 6-3 Troubleshooting Layer 3 to Layer 2 Mapping Information
Command Description
show ip arp Displays a router’s ARP cache, containing IP address to MAC address
mappings. (NOTE: By default, a router’s ARP cache stores information
for four hours. Therefore, you might need to execute a clear ip
arp command to allow a router to relearn information after you make
a topology change.)
show frame-relay
map
Displays Frame Relay DLCIs associated with different next-hop IP
addresses.
show adjacency
detail
Displays the frame headers in a router’s CEF adjacency table used to
encapsulate a frame being sent to an adjacency.
Example 6-6 provides sample output from the show ip cef exact-route source-ip-address
destination-ip-address command. The output indicates that a packet sourced from an IP
address of 10.2.2.2 and destined for an IP address of 192.168.1.11 will be sent out of interface
Fast Ethernet 0/0 to a next-hop IP address of 192.168.0.11.
Example 6-6 show ip cef exact-route source-ip-address destination-ip-address
Command Output
For a multipoint interface (for example, a point-to-multipoint Frame Relay/ATM PVC or
an Ethernet interface), after a router knows the next-hop address for a packet, it needs appropriate
Layer 2 information (for example, next-hop MAC address, Data Link Connection
Identifier [DLCI], or Virtual Path Identifier/Virtual Circuit Identifier [VPI/VCI]) to
properly construct a frame (or a cell, in the case of ATM). Table 6-3 outlines some useful
commands for viewing a router’s Layer 3 to Layer 2 mapping information.
Example 6-7 provides sample output from the show ip arp command. The output shows
the learned or configured MAC addresses along with their associated IP addresses.
Example 6-7 show ip arp Command Output
Example 6-8 provides sample output from the show ip frame-relay map command. The
output shows the Frame Relay subinterfaces that correspond to DLCIs (that is, Frame
Relay PVC identifiers) known to the router. Notice that these subinterfaces are point-topoint
subinterfaces, meaning that next-hop IP address information is not required to
properly construct a frame.
R2# show ip cef exact-route 10.2.2.2 192.168.1.11
10.2.2.2 -> 192.168.1.11 : FastEthernet0/0 (next hop 192.168.0.11)
R2# show ip arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 192.168.0.11 0 0009.b7fa.d1e1 ARPA FastEthernet0/0
Internet 192.168.0.22 - c001.0f70.0000 ARPA FastEthernet0/0
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Example 6-8 show frame-relay map Command Output
Example 6-9 provides sample output from the show adjacency detail command. The output
shows the CEF information used to construct frame headers for the various router
interfaces and subinterfaces.
Example 6-9 show adjacency detail Command Output
R2# show adjacency detail
Protocol Interface Address
IP Serial1/0.1 point2point(25)
0 packets, 0 bytes
2C510800
CEF expires: 00:02:18
refresh: 00:00:19
Epoch: 0
IP Serial1/0.2 point2point(25)
0 packets, 0 bytes
2C610800
CEF expires: 00:02:18
refresh: 00:00:19
Epoch: 0
IP FastEthernet0/0 192.168.0.11(9)
0 packets, 0 bytes
0009B7FAD1E1C0010F7000000800
ARP 04:02:59
Epoch: 0
EIGRP Troubleshooting
The Cisco proprietary Enhanced Interior Gateway Routing Protocol (EIGRP) is considered
to be a balanced hybrid routing protocol (or an advanced distance-vector routing protocol).
Specifically, EIGRP advertises routes to directly attached neighbors, like a distancevector
routing protocol, while using a series of tables, similar to a link-state routing
protocol.
EIGRP also offers the benefit of fast convergence after a link failure. Load balancing is
supported over both equal-cost paths (a default behavior) and unequal-cost paths
(through the variance feature).
This section discusses strategies for troubleshooting an EIGRP-based network. Before
considering the specifics of EIGRP troubleshooting, however, this section takes a more
R2# show frame-relay map
Serial1/0.2 (up): point-to-point dlci, dlci 182(0xB6,0x2C60), broadcast
status defined, active
Serial1/0.1 (up): point-to-point dlci, dlci 181(0xB5,0x2C50), broadcast
status defined, active
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Incoming Route Information Outgoing Route Information
Data
Structure
of IP
Routing
Protocol
IP
Routing
Table
Redistributed Routes
Injected Routes
Route Installation
Figure 6-6 Interaction between IP Routing Protocol Data Structures and IP Routing Tables
generic look at how the data structures of various IP routing protocols interact with a
router’s IP routing table.
Data Structures of IP Routing Protocols
As traffic is routed through a network, the routers encountered along the way from the
source to the destination need consistency in how they route traffic. For example, if one
router selected the best path based on hop count, and another router selected the best
path based on a link’s bandwidth, a routing loop could conceivably occur. Fortunately,
having a common routing protocol configured on all routers within a topology helps
ensure consistency in routing decisions.
That is not to say that a topology could not have more than one routing protocol. You
could strategically redistribute routes between routing protocols. Also, you could use
static routes in conjunction with dynamic routing protocols. However, you must take care
in environments with redundant links and multiple routing protocols to avoid potential
routing loops.
To better troubleshoot specific dynamic routing protocols, consider, generically, how
dynamic routing protocols’ data structures interact with a router’s IP routing table.
Figure 6-6 shows the interaction between the data structures of an IP routing protocol and
a router’s IP routing table. Realize, however, that not all routing protocols maintain their
own data structures. For example, Routing Information Protocol (RIP) is a routing protocol
that works directly with an IP routing table in a router, rather than maintaining a separate
data structure.
As a router receives route information from a neighboring router, that information is
stored in the data structures of the IP routing protocol (if the IP routing protocol uses
data structures). A data structure might also be populated by the local router. For example,
a router might be configured for route redistribution where route information is redistributed
by a routing information source (for example, a dynamic routing protocol, a static
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Chapter 6: Introduction to Troubleshooting Routing Protocols 151
Connected interface 0
Static route 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5
External Border Gateway Protocol (BGP) 20
Internal EIGRP 90
IGRP 100
OSPF 110
Intermediate System-to-Intermediate System (IS-IS) 115
Routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Unknown* 255
route, or a connected route). Also, the router might be configured to have specific interfaces
participate in an IP routing protocol.
The data structure analyzes all the information it receives to select the best route to certain
networks. This best route is determined by looking for the route with the best metric.
The data structure of an IP routing protocol will then inject that best route into the
router’s IP routing table, if that same route information has not already been learned by a
more believable routing source. Specifically, different routing protocols have different
administrative distances. An administrative distance of a routing protocol can be
thought of as the believability of that routing protocol. As an example, RIP has an administrative
distance of 120, whereas OSPF has an administrative distance of 110. Therefore,
if both RIP and OSPF had knowledge of a route to a specific network, the OSPF route
would be injected into the router’s IP routing table because OSPF has a more believable
administrative distance. Therefore, the best route selected by an IP routing protocol’s data
structure is only a candidate to be injected into the router’s IP routing table. As a reminder,
the following is a list of the administrative distances. The lower the administrative
distance, the more preferred the route.
If an IP routing protocol’s data structure identifies more than one route to a destination
network, multiple routes might be injected into a router’s IP routing table if those multiple
routes have an equal metric. In some cases, however, a routing protocol (for example,
EIGRP) might support load balancing across unequal-cost paths. In such an instance, multiple
routes might be injected into a router’s IP routing table, even though those routes
have different metrics.
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Table 6-4 EIGRP Data Structures
Data Structure Description
EIGRP interface table All of a router’s interfaces that have been configured to participate in
an EIGRP routing process are listed in this table. However, if an interface
has been configured as a passive interface (that is, an interface
that does not send routing information), that interface does not
appear in this table.
EIGRP neighbor table This table lists a router’s EIGRP neighbors (that is, neighboring
routers from whom an EIGRP Hello message has been received). A
neighbor is removed from this table if the neighbor has not been
heard from for a period of time defined as the hold-time. Also, if an
interface, from which a neighbor is known, is removed from the
EIGRP interface table because it goes down, the neighbor is removed
from this table unless there is a multiple link and one of the interfaces
is still up. In that case, the second interface will still provide the
neighborship.
EIGRP topology table This table contains routes learned by a router’s EIGRP routing
process. The best route for a network in this table becomes a candidate
to be injected into the router’s IP routing table. If multiple
routes in this table have an equal metric, or if EIGRP’s variance feature
is configured, more than one route might become candidates for
injection into the IP routing table, but only to a maximum of 4 by
default.
Depending on the IP routing protocol in use, a router will periodically advertise all of its
routes, or updates to its routing information, to its neighbors. Also be aware that some
routing protocols need to establish a relationship with a neighboring router before exchanging
route information with that neighbor. This relationship is called an adjacency or
a neighborship.
Data Structures of EIGRP
Now that you have reviewed, from a generic perspective, how an IP routing protocol’s
data structure interacts with a router’s IP routing table, consider the specific data structures
used by EIGRP, as described in Table 6-4.
EIGRP Operation
Like most high-end routing protocols, EIGRP supports variable-length subnet masking
(VLSM), and advertisements are sent via multicast (that is, to an address of 224.0.0.10). By
default, EIGRP automatically performs route summarization. This could be an issue for a
topology containing discontiguous subnets of the same major classful network. To turn
off automatic summarization, you can issue the no auto-summary command in router
configuration mode for an EIGRP autonomous system.
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EIGRP also supports load balancing across unequal-cost cost paths using the variance feature.
By default, the variance value for an EIGRP routing process defaults to a variance of
one, meaning the load balancing will only occur over equal-cost paths. You can, however,
issue the variance multiplier command in router configuration mode to specify a range of
metrics over which load balancing will occur. For example, imagine that a route had a metric
of 200000, and you configured the variance 2 command for the EIGRP routing process.
This would cause load balancing to occur over routes with a metric in the range of 200000
through 400000 (2 * 200000). As you can see, a route could have a metric as high as 400000
(that is, the variance multiplier multiplied by the best metric) and still be used.
Upon learning of a neighbor, due to the receipt of an EIGRP Hello packet, an EIGRP
router will perform a full exchange of routing information with the newly established
neighbor. Once the neighborship has been formed, however, only updated route information
will be exchanged with that neighbor.
Routing information learned from EIGRP neighbors is inserted into the EIGRP topology
table. The best route for a specific network in the IP EIGRP topology table becomes a
candidate to be injected into the router’s IP routing table. If that route is indeed injected
into the IP routing table, that route becomes known as the successor route. This is the
route that is then advertised to neighboring routers.
The following parameters are used to determine the best route:
■ Advertised Distance (AD): The distance from a neighbor to the destination network
■ Feasible Distance (FD): The AD plus the metric to reach the neighbor advertising
the AD
EIGRP’s metric is calculated with the following formula:
EIGRP metric = [K1 * bandwidth + ((K2 * bandwidth) / (256 – load)) + K3 * delay] *
[K5 / (reliability + K4)]
By default, the K values are as follows:
■ K1 = 1
■ K2 = 0
■ K3 = 1
■ K4 = 0
■ K5 = 0
As a result of these default K values, EIGRP’s default metric can be calculated as
default EIGRP metric = bandwidth + delay
where:
■ bandwidth = 10,000,000 / minimum bandwidth in kbps * 256
■ delay = sum of delays of all interfaces in path in tens of milliseconds * 256
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EIGRP Troubleshooting Commands
With an understanding of EIGRP’s data structures, and an understanding of how data structures
play a role in populating a router’s IP routing table, you can now strategically use Cisco
IOS show and debug commands to collect information about specific steps in the routing
process. Table 6-5 shows a collection of such commands, along with their description, and the
step of the routing process or EIGRP data structure each command can be used to investigate.
154 CCNP TSHOOT 642-832 Official Certification Guide
Key
Topic
Table 6-5 EIGRP Troubleshooting Commands
Command Routing Component
or Data Structure
Description
show ip eigrp
interfaces
EIGRP interface table This command displays all of a router’s interfaces
configured to participate in an EIGRP routing
process (with the exception of passive interfaces).
show ip eigrp
neighbors
EIGRP neighbor table This command shows a router’s EIGRP neighbors.
show ip eigrp
topology
EIGRP topology table This command displays routes known to a
router’s EIGRP routing process. These routes are
contained in the EIGRP topology table.
show ip route
eigrp
IP routing table This command shows routes known to a router’s
IP routing table that were injected by the router’s
EIGRP routing process.
debug ip
routing
IP routing table This command displays updates that occur in a
router’s IP routing table. Therefore, this command
is not specific to EIGRP.
debug eigrp
packets
Exchanging EIGRP information
with neighbors
This command can be used to display all EIGRP
packets exchanged with a router’s EIGRP neighbors.
However, the focus of the command can be
narrowed to only display specific EIGRP packet
types (for example, EIGRP Hello packets).
debug ip eigrp Exchanging EIGRP information
with neighbors
This command shows information contained in
EIGRP packets and reveals how an EIGRP routing
process responds to that information.
R2# show ip eigrp interfaces
IP-EIGRP interfaces for process 100
Example 6-10 provides sample output from the show ip eigrp interfaces command. Although
three interfaces are configured to participate in EIGRP autonomous system 100,
only two of those interfaces have a peer, because the other interface is a loopback interface.
Example 6-10 show ip eigrp interfaces Command Output
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Chapter 6: Introduction to Troubleshooting Routing Protocols 155
Example 6-11 provides sample output from the show ip eigrp neighbors command. In the
output, notice that two neighbors are known to this router. Also notice that the output
shows off of which interface each neighbor resides.
Example 6-11 show ip eigrp neighbors Command Output
Example 6-12 provides sample output from the show ip eigrp topology command. The
output displays the routes known to the EIGRP topology table. Also, notice that the output
contains the feasible distance (FD) for each route. Recall that the FD is the advertised
distance (AD) from a neighbor, plus the metric required to reach that neighbor. Finally, notice
that the state of each route is passive. This is the desired state. A route that stays in
the active state, however, could indicate a problem. A state of active means that the router
is actively searching for a route. If a route remains in this state, the resulting condition is an
EIGRP stuck-in-active (SIA) error.
Example 6-12 show ip eigrp topology Command Output
Xmit Queue Mean Pacing Time Multicast Pending
Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes
Lo0 0 0/0 0 0/1 0 0
Se1/0.1 1 0/0 880 0/15 4155 0
Fa0/0 1 0/0 193 0/2 828 0
R2# show ip eigrp neighbors
IP-EIGRP neighbors for process 100
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
1 192.168.0.11 Fa0/0 12 00:01:34 193 1158 0 3
0 172.16.1.1 Se1/0.1 12 00:01:39 880 5000 0 23
R2# show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(10.2.2.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.3.0/30, 1 successors, FD is 2681856
via 172.16.1.1 (2681856/2169856), Serial1/0.1
P 10.2.2.2/32, 1 successors, FD is 128256
via Connected, Loopback0
P 10.3.3.3/32, 1 successors, FD is 2297856
via 172.16.1.1 (2297856/128256), Serial1/0.1
P 10.1.2.0/24, 1 successors, FD is 2195456
via 172.16.1.1 (2195456/281600), Serial1/0.1
P 10.0.0.0/8, 1 successors, FD is 128256
via Summary (128256/0), Null0
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P 10.1.1.1/32, 1 successors, FD is 409600
via 192.168.0.11 (409600/128256), FastEthernet0/0
P 10.4.4.4/32, 1 successors, FD is 2323456
via 172.16.1.1 (2323456/409600), Serial1/0.1
P 192.168.0.0/24, 1 successors, FD is 281600
via Connected, FastEthernet0/0
P 192.168.1.0/24, 1 successors, FD is 284160
via 192.168.0.11 (284160/28160), FastEthernet0/0
P 172.16.0.0/16, 1 successors, FD is 2169856
via Summary (2169856/0), Null0
P 172.16.1.0/30, 1 successors, FD is 2169856
via Connected, Serial1/0.1
Example 6-13 provides sample output from the show ip route eigrp command. The output
of this command only shows entries in the IP routing table that were learned via
EIGRP.
Example 6-13 show ip route eigrp Command Output
R2# show ip route eigrp
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
D 172.16.0.0/16 is a summary, 00:01:36, Null0
10.0.0.0/8 is variably subnetted, 7 subnets, 4 masks
D 10.1.3.0/30 [90/2681856] via 172.16.1.1, 00:01:36, Serial1/0.1
D 10.3.3.3/32 [90/2297856] via 172.16.1.1, 00:01:36, Serial1/0.1
D 10.1.2.0/24 [90/2195456] via 172.16.1.1, 00:01:36, Serial1/0.1
D 10.0.0.0/8 is a summary, 00:01:36, Null0
D 10.1.1.1/32 [90/409600] via 192.168.0.11, 00:01:36, FastEthernet0/0
D 10.4.4.4/32 [90/2323456] via 172.16.1.1, 00:01:36, Serial1/0.1
D 192.168.1.0/24 [90/284160] via 192.168.0.11, 00:01:36, FastEthernet0/0
Example 6-14 provides sample output from the debug ip routing command. The output
shown reflects a loopback interface on a neighboring router being administratively shut
down and then brought back up. Specifically, the loopback interface has an IP address of
10.1.1.1, and it is reachable via an EIGRP neighbor with an IP address of 192.168.0.11.
Example 6-14 debug ip routing Command Output
R2# debug ip routing
IP routing debugging is on
*Mar 1 00:20:11.215: RT: delete route to 10.1.1.1 via 192.168.0.11, eigrp metric
[90/409600]
*Mar 1 00:20:11.219: RT: SET_LAST_RDB for 10.1.1.1/32
OLD rdb: via 192.168.0.11, FastEthernet0/0
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Chapter 6: Introduction to Troubleshooting Routing Protocols 157
Example 6-15 provides sample output from the debug eigrp packets command. This command
can produce a large volume of output by default. However, you can specify specific
EIGRP packet types you want to see in the output.
Example 6-15 debug eigrp packets Command Output
*Mar 1 00:20:11.227: RT: no routes to 10.1.1.1
*Mar 1 00:20:11.227: RT: NET-RED 10.1.1.1/32
*Mar 1 00:20:11.231: RT: delete subnet route to 10.1.1.1/32
*Mar 1 00:20:11.235: RT: NET-RED 10.1.1.1/32
*Mar 1 00:20:17.723: RT: SET_LAST_RDB for 10.1.1.1/32
NEW rdb: via 192.168.0.11
*Mar 1 00:20:17.723: RT: add 10.1.1.1/32 via 192.168.0.11, eigrp metric
[90/409600]
*Mar 1 00:20:17.723: RT: NET-RED 10.1.1.1/32
R2# debug eigrp packets
EIGRP Packets debugging is on
(UPDATE, REQUEST, QUERY, REPLY, HELLO, IPXSAP, PROBE, ACK, STUB, SIAQUERY,
SIAREPLY)
*Mar 1 00:20:48.151: EIGRP: Received HELLO on FastEthernet0/0 nbr 192.168.0.11
*Mar 1 00:20:48.155: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
*Mar 1 00:20:48.187: EIGRP: Sending HELLO on FastEthernet0/0
*Mar 1 00:20:48.191: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0
...OUTPUT OMITTED...
*Mar 1 00:20:59.091: EIGRP: Received QUERY on FastEthernet0/0 nbr 192.168.0.11
*Mar 1 00:20:59.095: AS 100, Flags 0x0, Seq 6/0 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
...OUTPUT OMITTED...
*Mar 1 00:20:59.287: EIGRP: Received REPLY on Serial1/0.1 nbr 172.16.1.1
*Mar 1 00:20:59.291: AS 100, Flags 0x0, Seq 25/16 idbQ 0/0 iidbQ un/rely 0/0
peerQ un/rely 0/0
*Mar 1 00:20:59.295: EIGRP: Enqueueing ACK on Serial1/0.1 nbr 172.16.1.1
...OUTPUT OMITTED...
*Mar 1 00:21:06.915: EIGRP: Sending UPDATE on FastEthernet0/0
*Mar 1 00:21:06.915: AS 100, Flags 0x0, Seq 19/0 idbQ 0/0 iidbQ un/rely 0/0
serno 17-17
*Mar 1 00:21:06.919: EIGRP: Enqueueing UPDATE on Serial1/0.1 iidbQ un/rely 0/1
serno 17-17
*Mar 1 00:21:06.923: EIGRP: Enqueueing UPDATE on Serial1/0.1 nbr 172.16.1.1 iidbQ
un/rely 0/0 peerQ un/rely 0/0 serno 17-17
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Example 6-16 provides sample output from the debug ip eigrp command. This command
provides real-time tracking of EIGRP messages, much like the debug eigrp packets command.
However, the debug ip eigrp command focuses more on showing what the EIGRP
routing process is doing in response to the messages, as illustrated in the highlighted text.
Example 6-16 debug ip eigrp Command Output
Trouble Ticket: EIGRP
This trouble ticket focuses on EIGRP. You are presented with baseline data, a trouble
ticket, and information collected while investigating the reported issue. You are then challenged
to identify the issue and create an action plan to resolve that issue.
Trouble Ticket #3
You receive the following trouble ticket:
EIGRP has just been configured as the routing protocol for the network. After configuring
EIGRP on all routers and instructing all router interfaces to participate in
EIGRP, router R2 does not appear to be load balancing across its links to BB1 and
BB2 when sending traffic to network 10.1.2.0/24.
R2# debug ip eigrp
IP-EIGRP Route Events debugging is on
*Mar 1 00:21:30.123: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming
QUERY packet
*Mar 1 00:21:30.127: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32 M
4294967295 - 0 4294967295 SM 4294967295 - 0 4294967295
*Mar 1 00:21:30.147: IP-EIGRP(Default-IP-Routing-Table:100): 10.1.1.1/32 - don’t
advertise out Serial1/0.1
*Mar 1 00:21:30.155: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32
metric 4294967295 - 0 4294967295
*Mar 1 00:21:30.335: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming
REPLY packet
*Mar 1 00:21:30.339: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32 M
4294967295 - 0 4294967295 SM 4294967295 - 0 4294967295
*Mar 1 00:21:30.343: IP-EIGRP(Default-IP-Routing-Table:100): 10.1.1.1/32 routing
table not updated thru 192.168.0.11
*Mar 1 00:21:30.903: IP-EIGRP(Default-IP-Routing-Table:100): 10.1.1.1/32 - not in
IP routing table
*Mar 1 00:21:30.907: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32
metric 4294967295 - 0 4294967295
*Mar 1 00:21:36.739: IP-EIGRP(Default-IP-Routing-Table:100): Processing incoming
UPDATE packet
*Mar 1 00:21:36.739: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32 M
409600 - 256000 153600 SM 128256 - 256 128000
*Mar 1 00:21:36.743: IP-EIGRP(Default-IP-Routing-Table:100): route installed for
10.1.1.1 ()
*Mar 1 00:21:36.775: IP-EIGRP(Default-IP-Routing-Table:100): Int 10.1.1.1/32
metric 409600 - 256000 153600
*Mar 1 00:21:36.779: IP-EIGRP(Default-IP-Routing-Table:100): 10.1.1.1/32 - don’t
advertise out Serial1/0.1
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Chapter 6: Introduction to Troubleshooting Routing Protocols 159
S 1/0.2
.1
Lo 0
10.3.3.3/32
.1
Fa 0/0
S 1/0.2
.1
DLCI = 182
DLCI = 811
S 1/0.1
.1
Lo 0
10.1.1.1/32
Lo 0
10.2.2.2/32
172.16.1.0/30
Fa 0/0
DLCI = 881
.11
FXS
1/0/0
FXS
1/0/1
R2
192.168.1.0/24
192.168.0.0/24
.11
Fa 0/1
172.16.2.0/30
S 1/0.1
.2
DLCI = 882
Fa 0/0
.22
10.1.3.0/30
10.1.2.0/24
Gig 0/8 Fa 5/46
Lo 0
10.4.4.4/32
S 1/0.2
.2
DLCI = 821 .2
Fa 0/0
Gig 0/9 Fa 5/47
Fa 5/45
x3333
Gig 0/10 Fa 5/48
100 Mbps
10 Mbps
R1
BB2
BB1
R2 FRSW
x1111 x2222
SW1 SW2
S 1/0.1
.2
DLCI = 181
Figure 6-7 Trouble Ticket #3 Topology
This trouble ticket references the topology shown in Figure 6-7.
As you investigate this issue, you examine baseline data collected after EIGRP was initially
configured. Example 6-17 confirms that router R2’s IP routing table contains only a single
path to get to the backbone network of 10.1.2.0/24.
Example 6-17 Baseline IP Routing Table on Router R2
R2# show ip route
Codes: 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
Gateway of last resort is not set
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, Serial1/0.1
C 172.16.2.0 is directly connected, Serial1/0.2
10.0.0.0/8 is variably subnetted, 6 subnets, 3 masks
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You then view the EIGRP topology table on router R2 to see if EIGRP has learned more
than one route to reach the 10.1.2.0/24 network. The output, shown in Example 6-18, indicates
that the EIGRP topology table knows two routes that could be used to reach the
10.1.2.0/24 network.
Example 6-18 EIGRP Topology Table on Router R2
Key
Topic
C 10.2.2.2/32 is directly connected, Loopback0
D 10.1.3.0/30 [90/3072000] via 172.16.2.2, 00:00:34, Serial1/0.2
D 10.3.3.3/32 [90/2713600] via 172.16.2.2, 00:00:34, Serial1/0.2
D 10.1.2.0/24 [90/2585600] via 172.16.2.2, 00:00:34, Serial1/0.2
D 10.1.1.1/32 [90/409600] via 192.168.0.11, 00:00:46, FastEthernet0/0
D 10.4.4.4/32 [90/2688000] via 172.16.2.2, 00:00:34, Serial1/0.2
C 192.168.0.0/24 is directly connected, FastEthernet0/0
D 192.168.1.0/24 [90/284160] via 192.168.0.11, 00:18:33, FastEthernet0/0
R2# show ip eigrp topology
IP-EIGRP Topology Table for AS(1)/ID(10.2.2.2)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
r - reply Status, s - sia Status
P 10.1.3.0/30, 1 successors, FD is 3072000
via 172.16.2.2 (3072000/2169856), Serial1/0.2
via 172.16.1.1 (4437248/2169856), Serial1/0.1
P 10.2.2.2/32, 1 successors, FD is 128256
via Connected, Loopback0
P 10.1.2.0/24, 1 successors, FD is 2585600
via 172.16.2.2 (2585600/281600), Serial1/0.2
via 172.16.1.1 (3950848/281600), Serial1/0.1
P 10.3.3.3/32, 1 successors, FD is 2713600
via 172.16.2.2 (2713600/409600), Serial1/0.2
via 172.16.1.1 (4053248/128256), Serial1/0.1
P 10.1.1.1/32, 1 successors, FD is 409600
via 192.168.0.11 (409600/128256), FastEthernet0/0
P 10.4.4.4/32, 1 successors, FD is 2688000
via 172.16.2.2 (2688000/128256), Serial1/0.2
via 172.16.1.1 (4078848/409600), Serial1/0.1
P 192.168.0.0/24, 1 successors, FD is 281600
via Connected, FastEthernet0/0
P 192.168.1.0/24, 1 successors, FD is 284160
via 192.168.0.11 (284160/28160), FastEthernet0/0
P 172.16.1.0/30, 1 successors, FD is 3925248
via Connected, Serial1/0.1
P 172.16.2.0/30, 1 successors, FD is 2560000
via Connected, Serial1/0.2
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Chapter 6: Introduction to Troubleshooting Routing Protocols 161
Finally, you examine the EIGRP configuration on router R1, as presented in Example 6-19.
Example 6-19 EIGRP Configuration on Router R2
Take a moment to look through the show command output and the topology. Then, hypothesize
the underlying cause, explaining why router R2’s IP routing table only shows
one route to network 10.1.2.0/24, even though the EIGRP topology table knows of two
routes to that network. Finally, on a separate sheet of paper, write out a proposed action
plan for resolving the reported issue.
Suggested Solution
Upon examination of router R2’s EIGRP topology table (as previously shown in Example
6-18), it becomes clear that the reason router R2 is only injecting one of the 10.1.2.0/24
routes into the IP routing table, is that the feasible distances of the two routes are different.
By default, EIGRP load balances over routes with equal-cost metrics (that is, equal
feasible distances); however, the two routes present in the EIGRP topology table have
different metrics.
Examine the two metrics (that is, 2585600 and 3950848), and notice that the metrics differ
by less than a factor of two. Specifically, if you took the smallest metric of 2585600
and multiplied it by two, the result would be 5171200, which is greater than the largest
metric of 3950848.
Because the metrics for the two routes vary by less than a factor of two, EIGRP’s variance
feature could be configured to specify a variance of two, as shown in Example 6-20.
Specifically, this configuration tells EIGRP on router R2 to not only inject the best EIGRP
route into the IP routing table, but rather inject the route with the best metric in addition
to any route whose metric is within a factor of two of the best metric (that is, in the range
2585600–5171200). This allows the route with a metric of 3950848 to also be injected
into the IP routing table.
Example 6-20 Enabling the Variance Feature on Router R2
R2# conf term
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)# router eigrp 1
R2(config-router)# variance 2
To confirm that router R2 can now load balance across routers BB1 and BB2 to reach
the 10.1.2.0/24 network, examine the output of the show ip route command seen in
Key
Topic
R2# show run | begin router
router eigrp 1
network 10.2.2.2 0.0.0.0
network 172.16.1.0 0.0.0.3
network 172.16.2.0 0.0.0.3
network 192.168.0.0
auto-summary
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Example 6-21. This output confirms that router R2 can now load balance over two unequal-
cost paths to reach the 10.1.2.0/24 network.
Example 6-21 Examining Router R2’s IP Routing Table After Enabling the Variance
Feature
R2# show ip route
Codes: 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
Gateway of last resort is not set
172.16.0.0/30 is subnetted, 2 subnets
C 172.16.1.0 is directly connected, Serial1/0.1
C 172.16.2.0 is directly connected, Serial1/0.2
10.0.0.0/8 is variably subnetted, 6 subnets, 3 masks
C 10.2.2.2/32 is directly connected, Loopback0
D 10.1.3.0/30 [90/3072000] via 172.16.2.2, 00:00:03, Serial1/0.2
[90/4437248] via 172.16.1.1, 00:00:03, Serial1/0.1
D 10.3.3.3/32 [90/2713600] via 172.16.2.2, 00:00:03, Serial1/0.2
[90/4053248] via 172.16.1.1, 00:00:03, Serial1/0.1
D 10.1.2.0/24 [90/2585600] via 172.16.2.2, 00:00:03, Serial1/0.2
[90/3950848] via 172.16.1.1, 00:00:03, Serial1/0.1
D 10.1.1.1/32 [90/409600] via 192.168.0.11, 00:00:03, FastEthernet0/0
D 10.4.4.4/32 [90/2688000] via 172.16.2.2, 00:00:03, Serial1/0.2
[90/4078848] via 172.16.1.1, 00:00:03, Serial1/0.1
C 192.168.0.0/24 is directly connected, FastEthernet0/0
D 192.168.1.0/24 [90/284160] via 192.168.0.11, 00:00:04, FastEthernet0/0
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