Things sure can get spooky in the Troubleshooting section of the new Version 4 Blueprint! Cisco can present a pretty vague issue; give you a very lame diagram; and then really press you for time to solve the Trouble Ticket. In this blog post, I will walk you through this graveyard and attempt to provide some ideas on an efficient and effective approach. For much more detail and practice, our premier products for this exam section are the Volume 4 workbook (currently being edited and improved upon), and the brand new 5-Day Troubleshooting Bootcamp.
Are you ready to find prefixes that go bump in the night? Here is the sample Trouble Ticket we will attack, and the appropriate portion of the Cisco diagram. You will want to have some scratch paper handy (just like in the actual exam). Diagramming can prove to be more important here then in any exam section. You should practice a diagram now based on the show output that follows.
Trouble Ticket 1
Vampires located in VLAN 666 (behind Amityville) are unable to access any resources located behind Transylvania. Correct this issue without static routing, additional routing protocols, or redistribution of any kind.
The first order of business is to “expand” upon their diagram and determine what protocols are in use and where. A quick show ip protocols on each of the devices in the transit path should do the trick.
I decide to start as close the destination as possible and examine the relevant configurations.
Sure enough, here is our first BGP configuration issue. We need to advertise a component prefix of the aggregate. The little gremlin that attempted this configuration tried to accomplish this with the network statement, but they got it wrong. Subnetting is being done here of the 10.x.x.x space, so you need to reconfigure the network statement as follows:
Are you ready to find prefixes that go bump in the night? Here is the sample Trouble Ticket we will attack, and the appropriate portion of the Cisco diagram. You will want to have some scratch paper handy (just like in the actual exam). Diagramming can prove to be more important here then in any exam section. You should practice a diagram now based on the show output that follows.
Trouble Ticket 1
Vampires located in VLAN 666 (behind Amityville) are unable to access any resources located behind Transylvania. Correct this issue without static routing, additional routing protocols, or redistribution of any kind.
The first order of business is to “expand” upon their diagram and determine what protocols are in use and where. A quick show ip protocols on each of the devices in the transit path should do the trick.
Transylvania#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: eigrp 100 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is not in effect Maximum path: 4 Routing for Networks: 53.234.10.23/32 Routing Information Sources: Gateway Distance Last Update Distance: internal 90 external 170 Routing Protocol is "bgp 65001" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Unicast Aggregate Generation: 10.10.0.0/16 summary-only Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 53.234.10.44 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200 HalloweenTown#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: eigrp 100 EIGRP NSF-aware route hold timer is 240s Automatic network summarization is not in effect Maximum path: 4 Routing for Networks: 53.234.10.44/32 Routing Information Sources: Gateway Distance Last Update Distance: internal 90 external 170 Routing Protocol is "bgp 65001" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 23.10.24.190 53.234.10.23 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200 Salem#show ip protocols Routing Protocol is "ospf 1" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Router ID 66.150.201.12 Number of areas in this router is 1. 1 normal 0 stub 0 nssa Maximum path: 4 Routing for Networks: Routing on Interfaces Configured Explicitly (Area 0): FastEthernet0/0 Reference bandwidth unit is 100 mbps Routing Information Sources: Gateway Distance Last Update 102.34.102.12 110 00:30:55 Distance: (default is 110) Routing Protocol is "bgp 65000" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 23.10.24.200 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200 CrystalLake#show ip protocols Routing Protocol is "ospf 1" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Router ID 102.34.102.1 Number of areas in this router is 1. 1 normal 0 stub 0 nssa Maximum path: 4 Routing for Networks: Routing on Interfaces Configured Explicitly (Area 0): FastEthernet0/1 FastEthernet0/0 Reference bandwidth unit is 100 mbps Routing Information Sources: Gateway Distance Last Update Distance: (default is 110) Routing Protocol is "bgp 65000" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 66.150.201.12 Address FiltIn FiltOut DistIn DistOut Weight RouteMap 102.34.102.12 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200 Amityville#show ip protocols Routing Protocol is "ospf 1" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Router ID 102.34.102.12 Number of areas in this router is 1. 1 normal 0 stub 0 nssa Maximum path: 4 Routing for Networks: Routing on Interfaces Configured Explicitly (Area 0): FastEthernet0/0 Reference bandwidth unit is 100 mbps Routing Information Sources: Gateway Distance Last Update 102.34.102.1 110 00:33:23 Distance: (default is 110) Routing Protocol is "bgp 65000" Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set IGP synchronization is disabled Automatic route summarization is disabled Neighbor(s): Address FiltIn FiltOut DistIn DistOut Weight RouteMap 102.34.102.1 Maximum path: 1 Routing Information Sources: Gateway Distance Last Update Distance: external 20 internal 200 local 200As I parse the results, I create a more detailed diagram to provide a better picture of what is going on. Looking at the diagram, I immediately contemplate some design issues that could come into play. The iBGP split-horizon rule, next-hop reachability, and synchronization all could come into play here given the protocol configuration.
I decide to start as close the destination as possible and examine the relevant configurations.
Transylvania#show run | section bgp router bgp 65001 no synchronization bgp log-neighbor-changes network 10.0.0.0 aggregate-address 10.10.0.0 255.255.0.0 summary-only neighbor 53.234.10.44 remote-as 65001 no auto-summaryIt appears the intent is to advertise prefixes (the Loopback interfaces) beyond Transylvania using BGP prefix aggregation. Let me ensure that configuration is done correctly.
Sure enough, here is our first BGP configuration issue. We need to advertise a component prefix of the aggregate. The little gremlin that attempted this configuration tried to accomplish this with the network statement, but they got it wrong. Subnetting is being done here of the 10.x.x.x space, so you need to reconfigure the network statement as follows:
Transylvania#show ip int brief Interface IP-Address OK? Method Status Protocol FastEthernet0/0 53.234.10.23 YES manual up up FastEthernet0/1 unassigned YES unset administratively down down Loopback1 10.10.10.1 YES manual up up Loopback2 10.10.11.1 YES manual up up Loopback3 10.10.12.1 YES manual up up Transylvania#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Transylvania(config)#router bgp 65001 Transylvania(config-router)#network 10.10.10.0 mask 255.255.255.0After making this change, I check Salem to see if it is receiving the aggregate from Transylvania and we have reachability:
Salem#show ip bgp BGP table version is 2, local router ID is 66.150.201.12 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.10.0.0/16 23.10.24.200 0 65001 i Salem#show ip route bgp 10.0.0.0/16 is subnetted, 1 subnets B 10.10.0.0 [20/0] via 23.10.24.200, 00:06:26 Salem#ping 10.10.10.1 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/33/40 msWe are on our way to solving this Trouble Ticket…we hope you will follow this blog and continue this (and more) troubleshooting scenarios!
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