{"id":2204,"date":"2019-07-26T20:23:49","date_gmt":"2019-07-26T20:23:49","guid":{"rendered":"http:\/\/wiki.thomasandsofia.com\/?p=2204"},"modified":"2020-06-23T19:59:47","modified_gmt":"2020-06-23T19:59:47","slug":"section-18-connectivity-troubleshooting","status":"publish","type":"post","link":"https:\/\/wiki.thomasandsofia.com\/?p=2204","title":{"rendered":"Section 18: Connectivity Troubleshooting"},"content":{"rendered":"<p><a href=\"http:\/\/wiki.thomasandsofia.com\/?p=2175\">&lt; Section 17<\/a> | <a href=\"\/course-introduction\/\">Home<\/a> | <a href=\"http:\/\/wiki.thomasandsofia.com\/?p=2222\">Section 19 &gt;<\/a><\/p>\n<p>57% Complete<\/p>\n<h1>116. Introduction<\/h1>\n<p><a href=\"https:\/\/www.udemy.com\/course\/cisco-icnd1\/learn\/lecture\/8605642#overview\" target=\"_blank\" rel=\"noopener\">https:\/\/www.udemy.com\/course\/cisco-icnd1\/learn\/lecture\/8605642#overview<\/a><\/p>\n<ul>\n<li>Ping<\/li>\n<li>Traceroute<\/li>\n<\/ul>\n<h1><\/h1>\n<h1>117. Basic Connectivity Troubleshooting<\/h1>\n<p><a href=\"https:\/\/www.udemy.com\/cisco-icnd1\/learn\/lecture\/8605640#overview\" target=\"_blank\" rel=\"noopener\">https:\/\/www.udemy.com\/cisco-icnd1\/learn\/lecture\/8605640#overview<\/a><\/p>\n<h2>Ping<\/h2>\n<ul>\n<li>Uses ICMP\n<ul>\n<li>Internet Control Message Protocol<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-8.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2207\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-8.png\" alt=\"\" width=\"651\" height=\"199\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-8.png 651w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-8-300x92.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-8-150x46.png 150w\" sizes=\"auto, (max-width: 651px) 100vw, 651px\" \/><\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-9.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2208\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-9.png\" alt=\"\" width=\"652\" height=\"201\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-9.png 652w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-9-300x92.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-9-150x46.png 150w\" sizes=\"auto, (max-width: 652px) 100vw, 652px\" \/><\/a><\/p>\n<h3>Message Replies<\/h3>\n<ul>\n<li>! (Exclamation Point): The Ping reached its destination and returned successfully.\n<ul>\n<li>It is not uncommon to miss the first 1 (or even 2) pings if a devices has not updated its ARP cache.<\/li>\n<\/ul>\n<\/li>\n<li>. (Period): Indicates the Ping has failed.\n<ul>\n<li>Router does not have a corresponding route<\/li>\n<li>Destination IP address is not responding<\/li>\n<\/ul>\n<\/li>\n<li>U (Unreachable): The packet has been discarded\n<ul>\n<li>Blocked by an Access Control List<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<h2>Extended Ping<\/h2>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-10.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2209\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-10.png\" alt=\"\" width=\"603\" height=\"309\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-10.png 603w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-10-300x154.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-10-150x77.png 150w\" sizes=\"auto, (max-width: 603px) 100vw, 603px\" \/><\/a><\/p>\n<pre>PC1&gt; ping 10.1.2.10\r\n10.1.2.10 icmp_seq=1 timeout\r\n10.1.2.10 icmp_seq=2 timeout\r\n...\r\n10.1.2.10 icmp_seq=5 timeout<\/pre>\n<pre>R1&gt; ping 10.1.2.10\r\nSending 5 100-byte ICMP Echos to 10.1.2.10, timeout is 2 seconds:\r\n!!!!!\r\nSuccess rate is 100% (5\/5)<\/pre>\n<h3>Scenario:<\/h3>\n<ul>\n<li>PC1 complains he cannot access services on PC3<\/li>\n<li>R4 does not have a route to 10.0.1.0\/24<\/li>\n<li>Traffic which originates from a router always uses the IP address on the outgoing interface as the source address.<\/li>\n<li>A ping from R1 to 10.1.2.10 will succeed because R4 has a route to 10.0.0.1<\/li>\n<\/ul>\n<h3>Solution:<\/h3>\n<ul>\n<li>Force the PING to source from the same subnet the PC is on (10.0.1.0\/24)\n<ul>\n<li>You should always do this when checking connectivity from a subnet!<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<pre>R1#ping\r\nProtocol [ip]\r\nTarget IP address: 10.1.2.10\r\nRepeat count [5]:\r\nDatagram size [100]:\r\nTimeout in seconds [2]:\r\nExtended commands [n]: y\r\nSource address or interface: 10.0.1.1\r\n... (Default the remaining options)\r\nSending 5 100-byte ICMP Echos to 10.1.2.10, timeout is 2 seconds:\r\nPacket sent with a source address of 10.0.1.1\r\n.....\r\nSuccess rate is 100% (5\/5)<\/pre>\n<h3>Extended Ping option descriptions<\/h3>\n<ul>\n<li>Repeat count: Set this to a higher value if you suspect a connection is going up and down<\/li>\n<li>Datagram size [100]: Good to troubleshoot a suspected MTU (Maximum Transmission Unit) issue.\n<ul>\n<li>This allows you to set the packet to different sizes to check this.<\/li>\n<\/ul>\n<\/li>\n<li>Timeout in seconds [2]: Possibly increase this if running over very slow connections.<\/li>\n<li>Extended commands [n]: Required &#8216;y&#8217; if you want to manually set a source address or interface.\n<ul>\n<li>Source address or interface: Use the IP address of the interface that corresponds to the subnet experiencing the issue.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<h2>Traceroute<\/h2>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-11.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2210\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-11.png\" alt=\"\" width=\"749\" height=\"256\" \/><\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-12.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2213\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-12.png\" alt=\"\" width=\"746\" height=\"252\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-12.png 746w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-12-300x101.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-12-150x51.png 150w\" sizes=\"auto, (max-width: 746px) 100vw, 746px\" \/><\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-13.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2214\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-13.png\" alt=\"\" width=\"745\" height=\"253\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-13.png 745w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-13-300x102.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-13-150x51.png 150w\" sizes=\"auto, (max-width: 745px) 100vw, 745px\" \/><\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-14.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2215\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-14.png\" alt=\"\" width=\"745\" height=\"242\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-14.png 745w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-14-300x97.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/a1-14-150x49.png 150w\" sizes=\"auto, (max-width: 745px) 100vw, 745px\" \/><\/a><\/p>\n<ul>\n<li>Very similar to PING.<\/li>\n<li>Great for checking connectivity issues between 2 points.<\/li>\n<li>1st. attempt is set with a TTL (time to live, although really means &#8216;maximum hop count&#8217;) of 1\n<ul>\n<li>TTL is actually used as a route loop prevention mechanism.<\/li>\n<li>This prevents a packet from getting stuck in eternal loops.<\/li>\n<li>Each time a packet is passed, it decrements the TTL by one.<\/li>\n<li>Once the TTL reaches 0:\n<ul>\n<li>The router drops the packet<\/li>\n<li>It replies to the sender with a &#8216;time exceeded&#8217; message<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<li>Each successive attempt adds one more to the TTL.\n<ul>\n<li>2nd attempt = 2 hops<\/li>\n<li>3rd attempt = 3 hops<\/li>\n<\/ul>\n<\/li>\n<li>Each &#8216;hop&#8217; replies with the IP address it reached before returning and total time taken to run reach and return.<\/li>\n<\/ul>\n<p>Successful Traceroute:<\/p>\n<pre>R1#traceroute 10.1.2.1\r\nType escape sequence to abort.\r\nTracing the route to 10.1.2.1\r\nVRF info: (vrf in name\/id, vrf out name\/id)\r\n  1 10.0.0.2 20 msec 16 msec 16 msec\r\n  2 10.1.0.1 36 msec 40 msec 40 msec\r\n  3 10.1.1.1 60 msec 54 msec 60 msec<\/pre>\n<h3>Traceroute Responses<\/h3>\n<ul>\n<li>First run Ping to verify the connection is failing<\/li>\n<li>Then run Traceroute to find out where the failure starts.<\/li>\n<li>In this example, the packet gets as far as 10.1.0.1.\u00a0 Start troubleshooting there.<\/li>\n<li>Press &lt;<span class=\"kbtn\">Ctrl&gt;&lt;<\/span><span class=\"kbtn\">Shift&gt;&lt;<\/span><span class=\"kbtn\">6&gt;<\/span> to abort.<\/li>\n<\/ul>\n<pre>R1#traceroute 10.1.2.1\r\nType escape sequence to abort.\r\nTracing the route to 10.1.2.1\r\nVRF info: (vrf in name\/id, vrf out name\/id)\r\n  1 10.0.0.2 28 msec 16 msec 16 msec\r\n  2 10.1.0.1 36 msec 36 msec 40 msec\r\n  3  *  *  *\r\n  4  *  *  *<\/pre>\n<h2>Other basic tools<\/h2>\n<ul>\n<li>Layer 1\n<ul>\n<li>show ip interface brief<\/li>\n<li>show interface<\/li>\n<\/ul>\n<\/li>\n<li>Layer 2\n<ul>\n<li>show arp<\/li>\n<li>show mac address-table<\/li>\n<\/ul>\n<\/li>\n<li>Layer 4\n<ul>\n<li>telnet &lt;destination IP&gt; &lt;destination Port&gt;<\/li>\n<\/ul>\n<\/li>\n<li>DNS\n<ul>\n<li>nslookup (like dig)<\/li>\n<li>Ping by FQDN<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<h1>118. Connectivity Troubleshooting &#8211; Lab Exercises<\/h1>\n<p><a href=\"https:\/\/www.udemy.com\/course\/cisco-icnd1\/learn\/lecture\/8631424#overview\" target=\"_blank\" rel=\"noopener\">https:\/\/www.udemy.com\/course\/cisco-icnd1\/learn\/lecture\/8631424#overview<\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-2216\" src=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting.png\" alt=\"\" width=\"949\" height=\"508\" srcset=\"https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting.png 949w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting-300x161.png 300w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting-768x411.png 768w, https:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting-150x80.png 150w\" sizes=\"auto, (max-width: 949px) 100vw, 949px\" \/><\/a><\/p>\n<p><a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting-Lab-Exercise.pdf\">18 Connectivity Troubleshooting Lab Exercise\u00a0<\/a><br \/>\n<a href=\"http:\/\/wiki.thomasandsofia.com\/wp-content\/uploads\/2019\/07\/18-Connectivity-Troubleshooting-Answer-Key.pdf\">18 Connectivity Troubleshooting Answer Key<\/a><\/p>\n<p><strong>1. Use ping to test connectivity from PC1 to PC3.<\/strong><\/p>\n<pre>PC1&gt; ping 10.1.2.10\r\n*10.1.0.1 icmp_seq=1 ttl=253 time=108.341 ms (ICMP type:3, code:1, Destination host unreachable)\r\n*10.1.0.1 icmp_seq=2 ttl=253 time=82.847 ms (ICMP type:3, code:1, Destination host unreachable)\r\n*10.1.0.1 icmp_seq=3 ttl=253 time=98.612 ms (ICMP type:3, code:1, Destination host unreachable)<\/pre>\n<p>&nbsp;<\/p>\n<p><strong>2. Use traceroute to determine where the problem is likely to be.<\/strong><\/p>\n<pre>PC1&gt; trace 10.1.2.10\r\ntrace to 10.1.2.10, 8 hops max, press Ctrl+C to stop\r\n1 10.0.1.1 33.801 ms 16.563 ms 26.429 ms\r\n2 10.0.0.2 50.240 ms 98.177 ms 75.777 ms\r\n3 10.1.0.1 121.613 ms 121.023 ms 109.946 ms\r\n4 *10.1.0.1 106.448 ms (ICMP type:3, code:1, Destination host unreachable<\/pre>\n<p>Problem between R3 and R4<\/p>\n<p><strong>3. Determine the issue and fix it to restore connectivity between PC1 and PC3<\/strong><\/p>\n<p>Check R3 Interfaces Up &#8211; OK<\/p>\n<pre>R3#sh ip int bri\r\nInterface IP-Address OK? Method Status Protocol\r\nFastEthernet0\/0 10.1.1.2 YES NVRAM up up\r\nFastEthernet1\/0 10.1.0.1 YES NVRAM up up\r\nFastEthernet2\/0 unassigned YES NVRAM administratively down down\r\nFastEthernet3\/0 unassigned YES NVRAM administratively down down<\/pre>\n<p>Check R3 for routes to 10.1.2.0\/24 &#8211; FAIL<\/p>\n<pre>R3#show ip route\r\n10.0.0.0\/8 is variably subnetted, 9 subnets, 2 masks\r\nS 10.0.0.0\/24 [1\/0] via 10.1.0.2\r\nS 10.0.1.0\/24 [1\/0] via 10.1.0.2\r\nS 10.0.2.0\/24 [1\/0] via 10.1.0.2\r\nS 10.0.3.0\/24 [1\/0] via 10.1.0.2\r\nC 10.1.0.0\/24 is directly connected, FastEthernet1\/0\r\nL 10.1.0.1\/32 is directly connected, FastEthernet1\/0\r\nC 10.1.1.0\/24 is directly connected, FastEthernet0\/0\r\nL 10.1.1.2\/32 is directly connected, FastEthernet0\/0\r\nS 10.1.3.0\/24 [1\/0] via 10.1.1.1<\/pre>\n<p>Add static route to 10.1.2.0\/24 via 10.1.1.1<\/p>\n<pre>R3#conf t\r\nR3(config)#ip route 10.1.2.0 255.255.255.0 10.1.1.1<\/pre>\n<p>Verify ping from PC1<\/p>\n<pre>PC1&gt; ping 10.1.2.10\r\n84 bytes from 10.1.2.10 icmp_seq=1 ttl=60 time=170.952 ms\r\n84 bytes from 10.1.2.10 icmp_seq=2 ttl=60 time=122.586 ms\r\n84 bytes from 10.1.2.10 icmp_seq=3 ttl=60 time=93.745 ms\r\n84 bytes from 10.1.2.10 icmp_seq=4 ttl=60 time=146.945 ms\r\n84 bytes from 10.1.2.10 icmp_seq=5 ttl=60 time=141.337 ms<\/pre>\n<p>Done!<\/p>\n","protected":false},"excerpt":{"rendered":"<p>&lt; Section 17 | Home | Section 19 &gt; 57% Complete 116. Introduction https:\/\/www.udemy.com\/course\/cisco-icnd1\/learn\/lecture\/8605642#overview Ping Traceroute 117. Basic Connectivity Troubleshooting https:\/\/www.udemy.com\/cisco-icnd1\/learn\/lecture\/8605640#overview Ping Uses ICMP Internet Control Message Protocol Message Replies ! (Exclamation Point): The Ping reached its destination and returned successfully. It is not uncommon to miss the first 1 (or even 2) pings if ..<\/p>\n<div class=\"clear-fix\"><\/div>\n<p><a href=\"https:\/\/wiki.thomasandsofia.com\/?p=2204\" title=\"read more...\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[45],"tags":[],"class_list":["post-2204","post","type-post","status-publish","format-standard","hentry","category-icnd1-ccent"],"_links":{"self":[{"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/posts\/2204","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2204"}],"version-history":[{"count":12,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/posts\/2204\/revisions"}],"predecessor-version":[{"id":2206,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=\/wp\/v2\/posts\/2204\/revisions\/2206"}],"wp:attachment":[{"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2204"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2204"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wiki.thomasandsofia.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2204"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}