Never ever do this…

… on a production environment. Specially when you’re in California on a late Friday afternoon and the router in play is in New York and there is nobody there to reboot the router for you.

How did I get myself in such a predicament? Well, I was on our NY router doing some “investigatory” work and doing various show commands to print out and later peruse for self-study. Well turns out that oh-so convenient shortcut “sh” was not a shortcut for “show“, but a shortcut for “shutdown“. It actually took me a little while before I realized that I indeed bone-headedly shut down the interface when after a few minutes the prompt wasn’t showing up. All the while I thought I mistakenly typed a show command without any parameters so it’s trying to gather up all the parameters for me to try out. Then it occured to me, “I’m in the freaking interface mode. My heart starts palpitating and my mouth suddenly got so dry after the realization that I just killed a production router. But… never to be defeated by wanton careless mistake, I quickly realized that even though I killed a virtual tunnel, the serial connection to the router should still be up. So I quickly emailed our admin and casually asked what the public serial IP address was.

“Why?”, he asks.

“Just ’cause”, I reply.

And he gave it to me. I log on, got back to the interface, and “no shut” that puppy! And all is good in the IT world.

• This table lists some common EIGRP debug commands:

The majority of the notes will focus on discussing each show and debug commands. The sample outputs will be based on the figure and configuration below below:

R1#show running-config
<output omitted>
interface FastEthernet0/0
ip address 172.16.1.1 255.255.255.0

!
<output omitted>
interface Serial1/0
bandwidth 64
ip address 192.168.1.101 255.255.255.224

!

<output omitted>
router eigrp 100
network 172.16.1.0 0.0.0.255
network 192.168.1.0

• The R1 configuration has EIGRP enabled in autonomous system 100.
• The network 172.16.1.0 0.0.0.255 command configures EIGRP on interface Fa0/0 and advertise this network. The wildcard mask specifies that only interfaces on the 172.16.1.0/24 subnet will be advertised by EIGRP.
  • NOTE: However, the full Class B network 172.16.0.0 will be advertised because by default, EIGRP has autosummarization enabled.
• The network 192.168.1.0 enables EIGRP on the serial 0/0/1 interface and allows R1 to advertise this network.

R2#show running-config
<output omitted>
interface FastEthernet0/0
ip address 172.17.2.2 255.255.255.0

!

<output omitted>
interface Serial1/0
bandwidth 64
ip address 192.168.1.102 255.255.255.224

!

<output omitted>
router eigrp 100
network 172.17.2.0 0.0.0.255
network 192.168.1.0

• R2 is configured with AS 100.
• The network 172.17.2.0 0.0.0.255 configures EIGRP on interface Fa0/0 and advertises this network. The wildcard mask specifies that only interfaces on the 172.17.2.0/24 subnet will be advertised.
• NOTE: Because EIGRP automatically autosummarizes, the full Class B network 172.17.0.0 will be advertised.
• The network 192.168.1.0 command starts EIGRP on the serial 0/0/1 interface and allows R2 to advertise this network.

show ip route

figure 1: R1 sh ip route

• The leftmost column identified with a “D” indicates that the route is an internal EIGRP.
• Following that is the network number (172.16.0.0)
• The field in brackets [90/450514560] shows the administrative distance and the EIGRP metric, respectively.
  • The first number 90 is the administrative distance. It is used to choose the best route between two or more routes to the exact same destination from different routing protocols.
    • For example EIGRP with administrative distance of 90 beats RIP that has an administrative distance of 120.
  • The EIGRP metric is used as the FD in the EIGRP topology table.
  • NOTE: Administrative distance will only be used when comparing routes to the same destination (address and mask). For example, a router will use a RIP route over an EIGRP route if the RIP route is a more specific route than the EIGRP route.
• The next value on the first line of the routing table 192.168.1.102 is the address of the next-hop router.
• The next value 00:00:01 is the length of time since EIGRP last advertised this network to the router. EIGRP does not refresh routes periodically; it resends the routing table only when neighbor adjacencies change.
• The last value on the first line, Serial1/0 refers to the interface on which packets sent to 172.17.0.0 goes out.

• Notice in figure 1 that the routing table includes summary routes the forwards to null0.
  • null0 is a directly connected, software only interface.
  • null0 prevents the routing of a packet to other routers in search of a more precise, longer match.
  • For example, router R1 receives a packet to destination 172.16.3.5. This network does not match any known subnet. It then matches the summary route and subsequently gets forwarded to a null interface and dropped. This prevents the packet from being forwarded to a default route and possibly creating a loop.
  • The interfaces with the null0 routes are the connected interfaces of this router that are summarized. The only other summarized route is the route to network 172.17.0.0 on R2’s network. This was not summarized by R1. Rather, it was advertised by R2 as a summary.
  • Figure 2 shows the routing table with the no auto-summary configured on R1.

Figure 2: sh ip route on R1 with no auto-summary

show ip protocol

• The show ip protocol command provides information about any and all dynamic routing protocols running on the router
• It will display any route filtering occurring on EIGRP outbound or inbound updates.
• Identifies whether EIGRP is generating a default network or receiving a default network in EIGRP updates.
• Provides information about other settings (e.g. K values, hop count, variance).
  • NOTE: Because the routers must have identical K values for EIGRP to establish an adjacency, the show ip protocols command helps determine the current K-value setting before an adjacency is attempted

Figure 3: EIGRP show ip protocols

• Notice in the output that auto-summarization is enabled. This is the default setting
• Maximum Path: 4 - This line refers to the the maximum number of paths it can load-balance with.
  • By using the maximum-paths command, you can specify up to 16 paths.
• Routing for Networks: - This line shows the networks for which the router is routing.
  • Recall that in the network configuration, the 172.16.1.0 network specified a wildcard 0.0.0.255. Therefore in the routing information, the network address and prefix length is displayed.
  • Accordingly, the 192.168.1.0 network shows the major Class C network (without the prefix) because it was configured without a wildcard. Recall that if a wildcard is not used, the major network Class A, B, or C is displayed.
• Routing Information Sources: - Identifies all other routers that have an EIGRP neighbor relationship with this router.
  • show ip eigrp neighbors command provides a detailed display of EIGRP neigbors.
• Distance: - shows the administrative distances for EIGRP
  • Internal Networks - has an administrative distance of 90, which applies to networks from other routers inside the same ASN.
  • External Networks - has administrative distance of 170, which applies to networks introduced to EIGRP for this autonomous system through redistribution.

show ip eigrp interfaces

Figure 4: show ip eigrp interfaces Command Output

The following list describes the various elements of the output:

• Interface - the interface configured for EIGRP.
• Peers - how many EIGRP neighbors are connected to this router
• Xmit Queue Un/Reliable - number of packets remaining in the Unreliable and Reliable transmit queues.
• Mean SRRT - Mean SRTT interval, in milliseconds
• Pacing Time Un/Reliable - Pacing time used to determine when EIGRP packets should be sent out the interface.
• Multicast Flow Timer - Maximum number of seconds that the router will wait for an ACK packet after sending a multicast EIGRP packet, before switching from multicast to unicast.
• Pending Routes - Number of routes in the packets in the transmit queue waiting to be sent.

show ip eigrp topology

Figure 5: show ip eigrp topology Command Output

• This command displays the content of the EIGRP topology table.
• The command shows that the router has an ID of 192.168.1.101 and is in autonomous system 100.
  • The EIGRP ID is the highest IP address on an active interface for this router, unless loopback interfaces are configured, in which case it is the highest IP address assigned to a loopback interface.
• The command output also lists the networks known by this router through the EIGRP routing process. The codes associated with the networks are as follows:
  • Passive (P) - Means that the network is in a stable working condition. The network is available and installation can occur in the routing table.
  • Active (A) - This means that an outstanding query is in the process. The networks, therefore, is unavailable and the routing table cannot be updated at this time.
  • Update (U) - This network is being updated (placed in an update packet). This code also applies if the router is waiting for an ack for this update packet.
  • Query (Q) - There is an outstanding query packet for this network other than being in the active state. This code also applies if the router is waiting for an ack for a query packet.
  • Reply (R) - The router is generating a reply for this network or is waiting for an acknowledgment for the reply packet.
  • Stuck-in-Active (S) - There is an EIGRP convergence problem for this network.
• In each line:
  • Displays the number of successors available for a route. In the example above, each networks have 1 successor each.
  • After the mention of the successors, the FD is listed also telling us how the router was learned (eg. through the next-hop router, directly connected, or via summary).
  • The first number in brackets is the FD for that network through the next hop router. The second number is the AD form the next-hop-router to the destination network.

show ip eigrp traffic

• This command displays the number of various EIGRP packets sent and received.
• The figure below goes through the output of the command:

Figure 6: show ip eigrp traffic Command Output

• This example shows that Router 1 has sent 26296 hello messages and received 13151 hello messages.

debug eigrp packets

• This command can be used to verify EIGRP connectivity.
• Also displays the types of EIGRP packets sent and received by the other router that this command is executed on.

Figure 7: debug eigrp packets Command Output When a Neighbor’s Interface Comes Up.

• In the example Hello packets are sent reliably, therefore the sequence (Seq) number does not increment.
• In this example, when R2 receives an update from R1, values appear in the sequence number field.
  • Seq 18/18 indicates that 192.168.1.101 is sending this packet as sequence number 18 (the first number indicated) to R2 and the sequence number 18 (the second number) has been received from R2 by neighbor 192.168.1.101. 192.168.1.101 is expecting to receive sequence number 19 in the next reliable packet from R2.
• R2 returns an ACK packet with Seq 0/18. Ack packets are sent as unreliable, but notice that the unreliable/reliable flag (un/rel 1/0) is set. This means that the acknowledgment was sent in response to a reliable packet.
• serno 17-17 (at the end of the last highlighted line) refers to the Serial number. This indicates the number of changes that the two neighbors register in their EIGRP topology tables.

Sequence Number vs Serial Number

• Whenever a query, update, or reply is sent, the sequence number increments.
• The serial number increments each time the topology table changes.
  • The topology table can have more than 100 changes, and yet the sequence number can only increase by 1.

Figure 7: debug eigrp packets Command Output When a Neighbor’s Interface is Shut Down.

• An interface on R1 is shutdown.
• R1 sends a query packet to R2 to find out if R2 has a path to the lost network
• Because a query is a reliable packet, R2 responds with an ACK packet to acknowledge the receipt of the query packet from R1.
• At the same time, R2 sends a reply packet to R1.
• The serial number 18-20 represents the number of changes to the topology table since the start of the neighbor relationship between R1 and R2.

debug ip eigrp

• This command can be used to verify EIGRP operation.
• It displays EIGRP packets sent and received by the router.

Figure 8: debug ip eigrp Command Output of R2 When a Neighbor’s Interface Comes Up

• In the above example, and internal route (indicated by Int) for 172.16.0.0/16 is advertised to R2.
• To calculate the FD to reach network 172.16.0.0/16, the bandwidth and delay from “M” information seen on the highlighted output above.
  • M 40514560 - 40000000 514560
    • which means, the FD is 40514560. It is derived by adding the 40000000 (the bandwidth) + 514560 (the delay)
• To calculate the AD from R1 for network 172.16.0.0, the bandwidth and delay from the Source Metric information (SM) is added. In the highlighted text:
  • SM 28160 - 25600 2560 means: the AD is 28160. It is derived by adding 25600 (the bandwidth) + 2560 (the delay)

Figure 9: debug ip eigrp Command Output of R2 when an interface in R1 that leads to network 172.16.0.0/16 is shut down.

• R2 discovers that its neighbor no longer knows how to get to network 172.16.0.0/16.
  • The metric value (4,294,967,295) is the highest possible value using 32-bit numbers (or 2³²); it indicates that the route is unreachable.
  • R2 removes this entry form the EIGRP table and looks for alternative EIGRP routes.
• The debug output indicates that the routing table is not updated; this means that EIGRP did not find an alternative route to the network.
• The next statment verifies that the EIGRP process has removed the old route and that the route is not in the IP routing table
• R2 then informs the neighbor that it does not have a path to this network either.

Figure 10: Experimental IOS 12.5 command:

Oops! I guess not!

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.