OSPF/EIGRP Tune & TSHOOT Flashcards
A dynamic routing protocol is enabled on an interface. What two actions will the routing protocol immediately try to perform?
Attempt to find suitable neighbor on connected subnet, attempt to advertise the subnet connected to that interface
Describe a router’s behavior when passive-interface is configured on an interface
Router will not attempt to find neighbors off that interface (stop sending/processing hellos), but will continue to advertise that interface’s subnet.
What command will list interfaces on which EIGRP is enabled? Will the output of that command list or identify passive interfaces?
show ip eigrp interfaces — no
What command will list interfaces on which OSPFv2 is enabled? Will the output of that command list or identify passive interfaces?
show ip ospf interface brief – passive interfaces are listed, but not explicitly identified as passive
What command implicitly reveals configuration of a dynamic routing protocols, including passive interfaces
show ip protocols
From an L1,L2,L3 perspective, what must be true for EIGRP or OSPf to successfully form neighbor relationships?
Interfaces must be up/up, must be on same subnet, ACLs must not be filtering RP messages
What are the neighborship requirements for EIGRP?
Must be on same subnet
Must use same ASN
Must use same k-values
Must pass authentication, if configured
Two EIGRP routers are expected to form a neighbor relationship. You suspect they might not be on the same subnet. What commands should be used to verify this?
show interfaces, show ip interfaces
Two EIGRP routers are expected to form a neighbor relationship. You suspect they might not be configured for the same ASN. What commands should be used to verify this?
show ip eigrp interfaces, show ip protocols
Two EIGRP routers are expected to form a neighbor relationship. You suspect they might not be using the same K-values. What commands should be used to verify this?
show ip protocols
Two EIGRP routers are expected to form a neighbor relationship. You suspect that authentication issues may be present. What commands should be used to verify this?
debug eigrp packets
What re the neighborship requirements for OSPF?
Must be on same subnet Must pass authentication, if configured RIDs must be unique Must be in same area Hello-interval and dead-interval must match
Two OSPF routers are expected to form a neighbor relationship. You suspect they may not be in the same subnet. What commands should be used to verify this?
show interfaces, debug ip ospf hello
Two OSPF routers are expected to form a neighbor relationship. You suspect their RIDs might not be unique. What commands should be used to verify this?
show ip ospf interface, debug ip ospf adj
Two OSPF routers are expected to form a neighbor relationship. You suspect they may not be in the same area. What commands should be used to verify this?
show ip ospf interface brief, debug ip ospf adj
Two OSPF routers are expected to form a neighbor relationship. You suspect a hold-interval or dead-interval mismatch. What commands should be used to verify this?
show ip ospf interface, debug ip ospf hello
Two OSPF routers. have different process IDs. Can they become neighbors?
Yes. Process IDs do not have to match for OSPF
Two EIGRP routers have different hello-intervals or hold-times. Can they become neighbors?
Yes. Timers do not have to match for EIGRP
OSPF is configured on a router as follows:
network 1.0.0.0 0.0.0.255 area 1
network 1.1.1.1 0.0.0.0 area 0
Which network will OSPF advertise?
OSPF will advertise network 1.0.0.0/24 as area 1. OSPF uses the first network statement that can be matched.
OSPF is configured on a router with a network statement matching an interface in area 0. That same interface includes a configuration statement:
ip ospf 1 area 2
Which configuration will end up being used?
The interface configuration takes precedence
An EIGRP or OSPF router shows evidence of full and correct configuration, but is not forming neighbor relationships. What might this indicate?
Indicates the routing process may have been configured with the shutdown command
Two OSPF routers are not forming the expected neighbor relationship. Inspection reveals that one interface has the following configuration:
ip mtu 1492
The other router’s interface on that subnet has no similar configuration. Explain how to determine what that interface’s MTU might be, and why this causes a neighborship problem
show ip interfaces int-id
If MTU mismatch, both routers will attempt to form a neighbor relationship, but will fail to exchange LSDBs. Eventually, the neighbor relationship fails.
IF EIGRP is configured with auto-summart, where does EIGRP perform the summarization?
at classful boundaries
What command is used to determine whether EIGRP is performing auto-summarization?
show ip protocols
If EIGRP is performing auto-summarization, one what interfaces will summarized updates be sent
Only via interfaces on different major classful networks
R1 is using EIGRP to advertise a summary route of 172.16.0.0/16, but is actually only connected to 172.16.1.0/24, 172.16.2.0/24, and 172.16.4.0/24. How would EIGRP respond to incoming packets for 172.16.3.0/24?
EIGRP would create a summary route to null0 for 172.16.0.0/16. Packets to 172.16.3.0/24 would be routed to null0 (and thus dropped).
Under what conditions does EIGRP include a null0 summary route?
Auto-summary is enabled
At least one subnet was learned by EIGRP
At least 2 network EIGRP commands are configured
What is the purpose of a null0 summary route
loop prevention
Under EIGRP, what command is used to propagate a default route?
redistribute static router subcommand
What is the administrative distance of an external EIGRP route, such as a propagated default route?
170
By default, what percentage of an interface’s bandwidth will EIGRP attempt to use for for EIGRP information?
50 percent
On an interface with low bandwidth, you suspect that EIGRP overhead is interfering with throughput. What command should be used to throttle EIGRP traffic on that interface so that only 30 percent of the interface’s bandwidth is used? Assume the AS number for the eigrp process is 209
ip bandwidth-percent eigrp 209 30
How often will EIGRP Hello messages be sent by default on links low-speed NBMA networks?
every 60 seconds
By default, how long will an EIGRP router wait to receive hello messages before declaring it’s neighbor router dead on low-speed NBMA networks
180 seconds (3x hello-interval)
How often will EIGRP Hello messages be sent by default on links with bandwidth > 1.544 Mbps?
Every 5 seconds
By default, how long will an EIGRP router wait to receive hello messages before declaring it’s neighbor router dead on > 1.544 Mbps
15 seconds (3x hello-interval)
Two EIGRP routers are configured with different hello-intervals and hold-times. Will they form an adjacency?
Yes, those intervals do not have to match, but they should, as the link will probably flap
What command is used on an interface to set the frequency of hello message transmission on a router running EIGRP?
ip hello-interval eigrp $asn $seconds
What command is used on an interface to tell a neighbor router how long to wait for hello message on a router running EIGRP?
ip hold-time eigrp $asn $seconds
When considering equal-cost load balancing with EIGRP, how many equal-cost paths will EIGRP use by default. How would this be adjusted? How would this change be verified?
4 by default. Adjust with maximum-paths router subcommand. Verify via show ip protocols (lists max-paths)
When considering unequal cost load balancing with EIGRP, describe when it might be used. How would it be accomplished?
used when metrics are different, but close. Accomplished with variance $multiplier router subcommand.
What are some key considerations about the variance command?
Variance is multiplied by feasible distance
Assuming max-paths allows more routes, any FS routes with metrics < variance product are added to the routing table as balanced
Routes that are not successors or FS can never be added to the routing table, regardless of the variance
When troubleshooting EIGRP neighbor issues, what are conditions that should be checked?
Are interfaces operational? Are interfaces enabled for EIGRP? Same ASN? Same K-values? Passive interfaces?
When troubleshooting EIGRP routing table issues, what are conditions that should be checked?
Passive-interfaces?
Auto-Summary?
Missing network statements?
What formula is used to calculate the number of potential adjancnies is an OSPF network
[n(n-1)]/2
How many adjacencies are potentially possible in an OSPF network with 10 routers?
45
What is the purpose of the OSPF DR?
Collection and distribution point for LSAs
What address do DROTHER routers send their LSAs to when using OSPFv2?
224.0.0.6 (all DR)
What address to DR routers send their LSAs to when using OSPFv2?
224.0.0.5 (all OSPF)
What is the role of the BDR?
Listen passively for LSAs. Promote self to DR if DR stops responding to hellos
To what routers will DROTHER routers form full adjacencies?
DR and BDR
You want to quickly verify which routers are acting as DR and BDR in a working OSPFv2 network. What command will immediately reveal this information?
show ip ospf int $int-id on a working OSPF interface will reveal this routers role, as well as the DR (by RID and interface address) and BDR (by RID and interface address)
What determines the outcome of an OSPF DR/DBR election
- Highest interface priority
2. Highest RID
Two OSPF routers have equal interface priorities, but different RIDs. Which becomes DR?
The router with the highest RID
Multiple routers are operating in an OSPF network. All are configured with the same interface priority. A new router is introduced with a higher interface priority. Will that trigger an election?
No. Elections are not preemptive
What will trigger an OSPF election?
DR failure
OSPF process restart
Multi-access interface goes down
What happens during an OSPF election?
BDR immediately becomes DR. Other routers on the shared link will attempt to become new BDR. Router with the highest interface priority will become BDR. If all interface priorities are equal, router with the highest RID will become BDR.
Is it better to influence the DR election process by interface priority or RID?
Interface priority preferred
What command is used to configure OSPF interface priority on a given interface?
ip ospf priority $value
$value=0-255
What is the default OSPF interface priority?
1
An interface is configured with ip ospf priority 0. What does that imply?
The router will never attempt to become DR or BDR
What is the default interface priority on a serial link?
- NBMA networks don’t require a DBR or BDR
An OSPF network has been working correctly for some time. A decision is made to change interface priority on certain devices to influence future DR/BDR elections. After the priorities are changed, what action should be taken immediately?
OSPF process on all routers should be restarted to force a new election
How is a default route propagated when using OSPF
default-information originate router subcommand
You notice a default route on an OSPF router listed as O*E1. What does that imply?
Cost to reach default route includes both external and internal cost.
You notice a default route on an OSPF router listed as O*E2. What does that imply?
Cost to reach external route only includes external cost
When two OSPF routers are attempting to become neighbors, is it a requirement that their hello-interval and dead-intervals match?
Yes
What is the default hello interval for OSPF
10 sec
What is the default dead interval for OSPF?
40 sec (4x hello)
How would one verify the current hello or dead intervals on a given interface?
show ip ospf interface $int-id
How would one change the frequency of hellos sent on a given OSPF-enabled interface?
per interface:
ip ospf hello-interval $sec
How would one change how long an OSPF router should wait to receive hellos before declaring its neighbor dead?
ip ospf dead-interval $sec
What OSPF states are related to adjacency formation?
Down/Init/2-Way
What OSPF states are related to database synchronization
ExStart/Exchange/Loading/Full
Describe the OSPF Down state
No Hellos received yet
Router sends hello
Transition to INIT
Describe the OSPF Init state
Hello rec’d
Sending router’s RID noticed
Transition to 2-WAY
Describe the OSPF 2-Way state
DR/BDR election on multiaccess networks
Transition to EXSTART
Describe the OSPF ExStart state
Master/slave negotiation
DBD sequence determination
Master initiates EXCHANGE
Describe the OSPF Exchange state
Routers exchange DBD packets
*decsion:
if more information about routes needed, transition to LOADING. Otherwise transition to FULL
Describe the OSPF LOADING state
Exchange of LSRs and LSUs
Possible routes processed with SPF
Routes added to routing table
Transition to FULL
Describe the OSPF Full state
routers have reached convergence
When troubleshooting OSPF neighbor issues, what conditions should be checked?
Interfaces operational? Same subnet? Interfaces enable fo OSPF? Unique RIDs? OSPF areas match? Passive interfaces?
When troubleshooting OSPF neighbor issues, what conditions should be checked?
Are networks advertised?
ACL blocking?
Another RP with lower AD?
All areas connected to area 0?
When troubleshooting OSPF path selection issues, what conditions should be checked?
verify interface cost
verify ospf reference bandwidth
What is contained in the OSPF interface table
list of enabled interfaces
type 1 LSAs, including subnets
Describe the OSPF neighbor table
Maintains adjacencies via hello/dead timers
Entries added/refreshed when hellos received
Entires removed when dead timer expires
Describe the purpose of the OSPF LSDB
Stores full topology information
Describe how OSPF routes are added to the routing table.
After SPF calculation, best routes are offered to the routing table.
