Routing fundamentals
Topic 7 of 26. 45 questions: 23 pick one, 5 pick all, 6 type it, 3 put in order, 8 interview
What this topic covers
- routing table and how a router chooses a path
- longest prefix match
- administrative distance values
- metrics
- connected and local routes
- static default and floating static routes
- recursive lookup
- ECMP
- RIB vs FIB
- CEF and the adjacency table
- distance vector vs link state vs path vector
- route redistribution and loops
- route maps
- prefix lists vs ACLs
- policy-based routing
- VRF and VRF-lite
- summarization tradeoffs
- null0 routes
- convergence
Study it in depth on Octet.
- 1R1 receives a packet for 10.20.30.100. Which entry does it use to forward it?
- 2R2 has the routes below. A packet arrives for 172.16.6.10. Type the next-hop address R2 forwards it to.
- 3R1 holds all five of these routes, and every one of them covers 10.1.1.70. Put them in the order R1 prefers them for that destination, first choice first.
- 4A Cisco router with default settings learns 10.5.0.0/24 from OSPF with cost 2, from EIGRP as an internal route with metric 3072, and from RIP with 1 hop. Which route is installed in the routing table?
- 5Put these route sources in order of their default administrative distance on Cisco IOS, lowest first.
- 6On Cisco IOS, ip route 192.0.2.0 255.255.255.0 Serial0/1/0 appears in show ip route as directly connected. What is its administrative distance?
- 7Which of these pair a routing protocol with its default metric correctly? Choose all that apply.
- 8What does the L entry 10.0.12.1/32 represent?
- 9A router has no default route, and a packet arrives for a destination that matches nothing in its routing table. What does the router do?
- 10Type the Cisco IOS global configuration command that adds a static default route with next hop 203.0.113.1.
- 11A Catalyst switch is managed on VLAN 99 and uses ip default-gateway 10.10.99.1. An engineer enables ip routing on it, and the switch can no longer be reached from other subnets. Why?
- 12A branch learns its only route to the data center, 10.0.0.0/8, from OSPF over MPLS. You add a static route for 10.0.0.0/8 over an LTE backup that must be used only if OSPF loses the route. Which administrative distance do you give the static route?
- 13ISP1 has an outage deep inside its own network. R1's Ethernet link to the ISP1 router stays up, users lose internet access, and the LTE backup route never takes over. Why?
- 14This is R1's dual-path default routing. How would you make it fail over to LTE when ISP1 breaks upstream but the Ethernet link stays up?
- 15Walk me through how a router decides where to send a packet.
- 16A packet for 192.0.2.25 arrives on R1. Type the address of the neighbor R1 actually hands the packet to.
- 17R1 has ip route 192.0.2.0 255.255.255.0 10.255.0.3, where 10.255.0.3 is R3's loopback learned through OSPF. R1 also has a static default route toward the internet edge. If R3's loopback disappears from OSPF, what can happen to the 192.0.2.0/24 route on Cisco IOS?
- 18R1 connects to its ISP on GigabitEthernet0/0/0 with address 198.51.100.2/30, and the ISP router is 198.51.100.1. The ARP table on that interface looks like this. What is the most likely cause?
- 19On Cisco IOS, how many equal-cost paths does OSPF install for one prefix by default, before you change maximum-paths?
- 20Cisco CEF shares traffic across equal-cost paths per flow by default, not per packet. Why?
- 21Which statement correctly describes the FIB on a Cisco router?
- 22R1 and R2 were just connected, and R1 has never sent anything to 10.0.12.2. Why is the first ping lost?
- 23What does the 14-byte string in this CEF adjacency contain?
- 24Explain how CEF forwards a packet. What are the FIB and the adjacency table?
- 25A core router and the distribution routers below it all hash the same packet fields in the same way to pick among equal-cost paths. Each distribution router ends up using only one of its two uplinks. What is this, and what is the usual fix on IOS?
- 26Which of these statements about routing protocol families are correct? Choose all that apply.
- 27Compare distance vector, link state and path vector routing protocols.
- 28Put the stages of routed network convergence after a link failure in order, first to last.
- 29Two routers connect through a provider's layer 2 switch. When the far router fails, the near router's interface stays up and OSPF takes about 40 seconds to notice. What is the usual way to cut detection to well under a second?
- 30What determines how fast a routed network converges after a failure, and how would you make it faster?
- 31What is the smallest single summary that covers exactly 172.16.8.0/24 through 172.16.15.0/24? Give it as prefix/length.
- 32What are the benefits and the costs of route summarization?
- 33CORE1 sends all of 10.20.0.0/16 to DSW1 with a static route. A user pings 10.20.99.5, an address that is in no VLAN. What happens to the packet, and what is the standard fix?
- 34Which of these are common, valid reasons to install a static route to Null0? Choose all that apply.
- 35R2 runs OSPF and classic EIGRP 100. Under router eigrp 100 you enter redistribute ospf 1 with no metric, and there is no default-metric. What do R2's EIGRP neighbors receive?
- 36R2 runs RIP, OSPF and EIGRP. It redistributes RIP into OSPF and OSPF into EIGRP, and nothing else. Do the RIP routes reach the EIGRP domain?
- 37R2 and R3 both redistribute between OSPF and EIGRP with this configuration. A route that started in EIGRP is redistributed into OSPF at R2, and R3 installs the OSPF copy of it. What happens when R3 redistributes OSPF into EIGRP?
- 38You redistribute both ways between OSPF and EIGRP on two routers. What can go wrong, and how do you prevent it?
- 39The connected subnet 10.10.99.0/24 is redistributed into OSPF with this configuration. What happens to it?
- 40Which of these routes does this prefix list permit? Choose all that apply.
- 41Why do engineers use a prefix list rather than a standard ACL to filter routes?
- 42Traffic from the guest subnet 10.50.0.0/24 must leave through a second ISP regardless of the routing table. Where do you apply the PBR route-map on Cisco IOS?
- 43A guest host at 10.50.0.10 reaches the internet through R1 without problems, but ping 10.50.0.10 typed on R1 always fails. What is wrong?
- 44What is a VRF, what is VRF-lite, and how do you configure it?
- 45show ip route on R1 has a valid route for 10.5.1.0/24, yet traffic to 10.5.1.20 that enters R1 is dropped. Which of these could explain it? Choose all that apply.