Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Authors: Anand V.,  Chakrabarty K.
 Published year: 2006
 Pages: 57-58/130
Buy this book on amazon.com >>

EIGRP Route Problem

In certain cases, not all EIGRP routes may be available in the routing table. This may be caused due to nonformation of neighbor relationships.

Look at the example depicted in Figure 8.5. Routers A1 and A2 are part of the EIGRP routing environment represented by a cloud.

click to expand
Figure 8.5: An EIGRP network depicting route problems.

Observation

In Figure 8.5, the route for network 10.10.1.0/24 is not available in the routing table of A1. This is checked using the show ip route 10.10.1.0/24 command.

Problem Isolation

The steps to isolate the route problem are:

  1. Check if any EIGRP routes are available in A1 by using the A1#show ip route eigrp command.

  2. Check the EIGRP topology table by using the A1#show ip eigrp topology command, if there are no routes available at A1.

  3. Check and verify the neighbor relationships of A1 if an empty topology table is viewed . The neighbor relationships can be checked as discussed in the previous section.

  4. Check and fix the distribute lists in both inbound and outbound directions if neighbor relationships are OK. The distribute lists are checked by using the show ip protocols command. The output of the show ip protocols command is shown in Listing 8.27.

    Listing 8.27 Output of show ip protocols Command at A1

    start example 
    A1#show ip protocols Routing Protocol is "eigrp 100" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 EIGRP maximum hopcount 100 EIGRP maximum metric variance 1 Redistributing: static, eigrp 300 Automatic network summarization is in effect Maximum

path

: 4 Routing for Networks: 10.10.1.0 Passive Interface(s): Ethernet0 Routing Information Sources: Gateway Distance Last Update 10.10.2.1 90 00:05:35 Distance: internal 90 external 170
    end example

    The output indicates whether any routing update filter is set or not.

  5. Check if the local router has the same Router ID (RID) as the originating router. In that case, RID needs to be changed accordingly , and the EIGRP process is restarted.



EIGRP Metric Problem

EIGRP uses a composite metric composed of bandwidth, delay, reliability, load, and MTU. The metric is calculated using the formula:

Metric = [(K1 x Bw) + (K2 x Bw) + (K3 x Delay)] + K5 (256 – Load)(Reliability + K4)

in which Bw refers to bandwidth.

In EIGRP, K2 = K4 = K5 = 0 and K1 = K3 = 1, by default. As a result, the formula reduces to:

Metric = Bw + Delay Bw = 10,000,000 x 1

.

Bandwidth in Kbps

Delay = 100 x (delay in ms)

The final metric used in EIGRP is:

Metric = 10,000,000 x 1 + 100 (delay in ms)

Bandwidth in Kbps

Bandwidth and delay are configurable parameters at the router interfaces with default values. For example, the default bandwidth value of a serial interface is that of a T1 link. Consider the example depicted in Figure 8.6 to detect and troubleshoot issues related to the EIGRP metric. In Figure 8.6, routers A2, A3, and A4 are directly connected to router A1.

click to expand
Figure 8.6: An EIGRP network depicting metric problems.

Observation

Data traffic moving between A1 and A4 is erratic, with some packets getting dropped from time to time. This erratic behavior was reported by users accessing the Web server 192.168.4.2. The Web server is directly connected to A4.

Problem Isolation

The steps to isolate the metric problem are:

  1. Check if there is any problem in the route availability in the routing table and the topology table. This is verified by using the show ip route 192.168.4.0 and show ip eigrp topology inc 192.168.4.0 commands. The output for the show ip route 192.168.4.0 command is shown in Listing 8.28.

    Listing 8.28 Output for the show ip route 192.168.4.0 Command at A1

    start example 
    A1#show ip route 192.168.4.0 Routing entry for 192.168.4.0/24 Known via "eigrp 300", distance 90, metric 1787392, type internal Redistributing via eigrp 100 Last update from 10.10.3.1 on Serial0, 1d06h ago Routing Descriptor Blocks: * 10.10.3.1, from 10.10.3.1, 1d06h ago, via Serial0 Route metric is 1787392, traffic share count is 1 Total delay is 21000 microseconds, minimum bandwidth is 2048 Kbit Reliability 254/255, minimum MTU 1500 bytes Loading 1/255, Hops 1
    end example

    The output for the show ip eigrp topology inc 192.168.4.0 command at A1 is: 

    A1#show ip eigrp topology  inc 192.168.4.0 P 192.168.4.0/24, 1 successors, FD is 1787392

  2. Make sure there is no neighbor relationship problem by using the A1#show ip eigrp neighbors command.

  3. Make sure the transmission link connecting the rest of the EIGRP network with A4, which is terminated at interface Serial 0, has no errors. This is verified by using the show interface Serial 0 command. Listing 8.29 shows the output for the show interface Serial 0 command for A1.

    Listing 8.29 Output of the show interface Serial 0 Command for A1

    start example 
    A1#show interface Serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Description: WAN link for Router A4 Internet address is 10.10.3.2/30 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation HDLC, loopback not set Keepalive set (10 sec) Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/3134350/0 (size/max/drops/flushes); Total output drops: 11 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (

size

/max total/threshold/

drops

) Conversations 0/11/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) 5 minute input rate 13000 bits/sec, 29 packets/sec 5 minute output rate 2000 bits/sec, 3 packets/sec 115001192 packets input, 4155732628 bytes, 272 no buffer Received 425130 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0

overrun

, 0 ignored, 0 abort 21650119 packets output, 3170761599 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out 0 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
    end example

    The command output shows errors in the link. EIGRP routing updates are exchanged without any problem, but the data traffic gets hampered.

  4. Check the configured bandwidth parameter at the interface by using the show interface Serial 0 command. The output for the show interface Serial 0 command for A1 is shown in Listing 8.30.

    Listing 8.30 Output for the show interface Serial 0 Command for A1

    start example 
    A1#show interface Serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Description: WAN link for Router A4 Internet address is 10.10.3.2/30 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation HDLC, loopback not set Keepalive set (10 sec) Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/3134350/0 (size/max/drops/flushes); Total output drops: 11 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/11/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) 5 minute input rate 13000 bits/sec, 29 packets/sec 5 minute output rate 2000 bits/sec, 3 packets/sec 115001192 packets input, 4155732628 bytes, 272 no buffer Received 425130 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 21650119 packets output, 3170761599 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out 0 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
    end example

In Listing 8.30, the bandwidth configured for the interface is 1544 KB. There can be two conditions of a faulty bandwidth configuration in EIGRP. They are:

  • Bandwidth is not configured. EIGRP uses a maximum of 50% of the link bandwidth for the routing packets, by default. The default of a serial link is a T1 with a bandwidth of 1.5 mbps. In this case, the link is a fractional T1 with a bandwidth of 768 Kbps. The configured bandwidth being 1.5 mbps, EIGRP reserves 750 Kbps for itself, which leaves less than 50 Kbps for data traffic causing dropping of traffic. Bandwidth is configured correctly using the commands 

    A1(config)#interface S0 A1(config-if)#bandwidth 768

  • Bandwidth is incorrectly configured. Sometimes, a value of bandwidth other than the actual value is configured in serial links to manipulate the metric value to influence route selection in routing protocols. In Figure 8.6, both serial links are 56 Kbps bandwidth. To choose the path via A2 over the path via A4 to network 192.168.1.0/24, the bandwidth parameter of the link to A4 is set at a higher value of 224 Kbps. Out of this, 50% (that is, 112 Kbps) may be used for

  • EIGRP traffic, which is more than the actual bandwidth of the link. This condition causes congestion of the link. To overcome this condition, without affecting the desired effect on path selection process, you can use the commands 

    A1(config)#interface Serial0 A1(config-interface)#ip bandwidth-percent eigrp 100 12

These commands restrict the maximum percentage of the configured value of bandwidth to be used in EIGRP to 12% (27 Kbps), which is around 50% of the actual bandwidth.


Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Authors: Anand V.,  Chakrabarty K.
 Published year: 2006
 Pages: 57-58/130
Buy this book on amazon.com >>

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