Archive for the ‘842-902’ tag
Feel free to correct.
- The obvious first question involves the common LSA types and their function. Can you list them?
Type-1 – Router – Lists each router their connected IP addresses
Type-2 – Network – Lists all the transit, or multiaccess, networks
Type-3 – Net Summary – Defines a host route for interarea routes; this is from the ABR
Type-4 – ASBR Summary – Defines a host route for an external (to OSPF) route; this is from an ASBR
Type-5 – AS External – Lists the networks advertised into OSPF from external sources (redistribution)
Type-7 – NSSA External – External routes injected into a not-so-stubby area
- What information about the OSPF area does a router’s OSPF database contain?
Everything, basically. The database includes all the routers in the area, the IPs of every OSPF-enable interface, all the networks, and the costs of each hop. The SPF algorithm uses all this information to figure out the best path to all networks advertised.
- Define the reference bandwidth in OSPF. What’s the default value, and how do you change it?
The reference bandwidth is the bandwidth that is divided by an interface bandwidth to get a the cost for that link. The default value is 100Mbps, which can be changed with the auto-cost reference-bandwidth OSPF subcommand.
- How can you override the cost on an interface?
You can change the reference bandwidth, but that will change the costs of all interfaces. The ip ospf cost X command on an interface will do the trick. You can also give it the old bandwidth change (the one that tends to break or influence other things).
- What is the formula for calculating the cost of an interface?
reference bandwidth / interface bandwidth
- What are the five OSPF message types, and what do they do?
Hello – Establishes neighbor relationships
Database Description (DBD) – Send summaries of the LSAs a router has
Link State Requests (LSR) – Sent to a router to ask for more details on an LSA
Link State Update (LSU) – Reply to an LSR that includes the details of the requested LSA
Link State Acknowledgment (LSAck) – An acknowledgement of the DBD
- How often does a router send its full OSPF database to its neighbors?
It doesn’t. It does, however, send any self-originated LSAs (LSAs that it generated) every 30 minutes (1800 seconds).
- R1 is an ABR to area 1 with an area 0 route to a network with a cost of 100. R2, also the same ABR setup, advertises the same route to area 1 (and, thus, R1) with a cost of 10. Which route does R1 take?
ABRs always take an intra-area route over and interarea route, so the path with a cost of 100 will be chosen.
- You see a type-2 LSA in a router’s database. Without knowing what the details of the LSA are, list some things you can assume have happened.
Type-2 LSAs mean a transit network (multiaccess network) is turned up somewhere.
This transit network has two or more OSPF routers on it.
An election for DR and BDR has taken place.
The DR has started acting as a pseudonode for the transit network.
All other area routers have been told about that transit network.
[There are many others, I'm sure.]
- What configuration is required to enable unequal-cost load balancing in OSPF?
This isn’t EIGRP; you can’t do that.
- It seems that your OSPF database has 95 equal-cost paths to the same network. By default, how many show up in the routing table?
Four. You can change this with the maximum-paths directive under OSPF.
- Your 700 series router has been elected the DR on a transit network. How do you make sure your 12000 series is elected instead?
On the 12000′s interface, set the priority higher with the ip ospf priority x command.
What Command Was That
…shows all the type-1 (router) LSAs that a router has seen?
show ip ospf database router
…shows all the type-2 (network) LSAs that a router has seen?
show ip ospf database network
…shows all the type-3 (network summary) LSAs that a router has seen?
show ip ospf database summary
…shows the maximum paths OSPF will send to the routing table?
show ip protocols
…shows what transit networks exist in the area?
show ip ospf database network
…shows all the routers in the area?
show ip ospf database router
…shows what router advertise a particular transit network?
show ip ospf database
…shows the DR and BDR for a transit network?
show ip ospf interface
…shows the reference bandwidth?
show ip protocols
…shows how many times the SPF algorithm has been run in an area?
show ip ospf
- OR -
show ip ospf statistics
…shows how many of each message type a router has sent?
show ip ospf traffic
- How do you keep EIGRP from killing your WAN?
You can use the ip bandwidth-percent eigrp AS X command to limit the amount of bandwidth that EIGRP uses to update neighbors.
- How does EIGRP calculate how much bandwidth it can use for each frame relay PVC?
By default, EIGRP takes 50% of the (sub)interface’s configured bandwidth (with the bandwidth command) to use for updates on NBMA (non-broadcast mutliaccess) networks like frame relay. This value is divided equally among all the PVC configured on that interface.
- Why should you use delay instead of bandwidth to manipulate EIGRP?
There are other mechanisms, like QoS, that use bandwidth, so changing that value would affect those mechanisms. Only EIGRP uses delay.
- What’s the difference between the feasible distance (FD) and the reported (advertised) distance (RD)?
Feasible distance is the EIGRP metric value after the router has added it’s own information like bandwidth and delay to the formula. The reported distance is what a router calculates before it has added it’s own values. Essentially, the FD of one router is the RD of the next.
- What is an offset list?
An offset list is a way to artificially increment the FD and RD of a route or set of routes.
- You can add the load k-value into the metric calculation in EIGRP, but it’s not generally a good idea. Why?
The load is constantly changing as traffic changes on an interface. This would cause a constant stream of updates as traffic flows change.
- How often does a router send its full EIGRP topology table?
When new neighbors come up, the neighbors exchange their full tables, but, from that point forward, only updates are sent.
- When we talk about bandwidth in EIGRP, what are we actually talking about?
The bandwidth is actually the bandwidth of the slowest link between a router and the destination network. This is what’s used in the calculations.
- Assuming we’re using k1 and k3, what is the formula for calculating the metric?
metric = 256 * ( 10^7 / bandwidth [in kbps] + cumulative delay )
- You’ve decided to use k2 in your metric calculations, so you add that to a router. What happens to all the neighbors?
The neighbors all drop and start generating a “K-value mismatch” error.
- What is a successor? Feasible successor?
A successor is the EIGRP route for a particular network entry with the lowest metric. This is the route that EIGRP submits to the routing table for inclusion. A feasible successor (FS) is another EIGRP whose RD is lower than the successor’s FD; feasible successors can be used as an alternate path to a network if the successor goes away somehow.
- Where would you run into split horizon issues with EIGRP?
Split horizon says that you don’t advertise a route over the interface on which it was received. If you have a multipoint WAN link of some kind, routes from one spoke won’t be passed to another spoke through the hub.
- In what unit is the delay directive?
Tens of microseconds (10 * usec). That means that delay 1000 is 10,000 usec, or 10 ms.
- How does EIGRP do unequal cost path load balancing?
You can set the maximum-paths value under EIGRP to set the maximum number of equal paths that can be used. You also set the variance command there with a multiplier integer. The variance is multiplied by the successor’s FD, and any feasible successor whose metric is less than this new number is considered equal cost.
- What is an EIGRP stub router?
A stub router only receives routes via EIGRP and does not send them to other EIGRP neighbors. Since all the other routers know a router is a stub, they won’t send query messages to the stub router if they’re looking for a route. This will cut down on time waiting in active state.
- What is “stuck in active”?
If a successor for a network becomes unavailable and there are no FSes, a router will query each of its neighbors for a new routes to that network. If that router does not have a route, it will then ask its own neighbors, etc. In the meantime, the original router is still waiting for everyone to answer (that is, the route is in the active state) and will wait patiently until that happens. This could take a long time and result in a several-second outage to the queried network.
- By default, which types of routes are sent to EIGRP neighbors from a stub router?
Connected and summary
What Command Was That?
- …disables split horizon on an interface?
R1(config-if)#no ip split-horizon eigrp 1
- …sets the delay of an interface to 10ms?
R1(config-if)#delay 1000 (remember the tens of usec unit)
- …restricts the amount of bandwidth that EIGRP uses to 20% of the total bandwidth on an interface?
R1(config-if)#ip bandwidth-percent eigrp 1 20
- …apply an offset list to interface F0/0?
R1(config-router)#offset-list ACL in OFFSET F0/0
- …show the metrics of all the EIGRP routes a router has recieved for a network?
show ip eigrp topology 192.168.0.0/24
- …shows the successors and feasible successors for a network?
show ip eigrp topology 192.168.0.0/24
- …shows if a neighbor is a stub?
show ip eigrp neighbor detail
- …shows the maximum-paths and variance values?
show ip protocols
Last time, we talked about a nifty little lab I set up for redistribution and how the OSPF ASBRs acted a little differently than I expected. This time, let’s look at how changing external OSPF routes to a metric-type of 1 (E1) affects the routing tables.
Here’s the network again.
The static routes are being redistributed into their respective IGPs, and EIGRP is being redistributed into OSPF. Let’s look at the routing table on R1.
Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks O 10.0.0.2/32 [110/11] via 192.168.0.102, 00:06:53, Ethernet0/0 O E2 10.0.0.3/32 [110/20] via 192.168.0.105, 00:06:53, Ethernet0/0 [110/20] via 192.168.0.102, 00:06:53, Ethernet0/0 S 10.10.10.0/24 is directly connected, Null0 C 10.0.0.1/32 is directly connected, Loopback0 O E2 10.0.0.4/32 [110/20] via 192.168.0.105, 00:06:53, Ethernet0/0 [110/20] via 192.168.0.102, 00:06:53, Ethernet0/0 O 10.0.0.5/32 [110/11] via 192.168.0.105, 00:06:53, Ethernet0/0 O E2 10.10.20.0/24 [110/20] via 192.168.0.105, 00:06:03, Ethernet0/0 C 192.168.0.0/24 is directly connected, Ethernet0/0 O E2 192.168.101.0/24 [110/20] via 192.168.0.105, 00:06:53, Ethernet0/0 [110/20] via 192.168.0.102, 00:06:53, Ethernet0/0
Notice that there are two routes to each of the networks discovered from EIGRP (the loopbacks of 10.0.0.3/32 and 10.0.0.4/32 as well as 192.168.101.0/24). There is nothing strange here; OSPF simply sees the exit paths through the ASBRs. How about if we change the metric-type on the routes from R2 and see what happens?
I know of at least two ways you can do it. First, you can set the metric-type in the redistribute command on the ASBR’s OSPF process.
redistribute eigrp 1 subnets metric-type 1
You can also use a route-map to set the metric-type and apply that to the redistribute command.
route-map TEST permit 10 set metric-type type-1 ! redistribute eigrp 1 route-map TEST subnets
Either way does the same thing. Now let’s check the route table on R1 again.
Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks O 10.0.0.2/32 [110/11] via 192.168.0.102, 00:18:30, Ethernet0/0 O E1 10.0.0.3/32 [110/30] via 192.168.0.102, 00:02:11, Ethernet0/0 S 10.10.10.0/24 is directly connected, Null0 C 10.0.0.1/32 is directly connected, Loopback0 O E1 10.0.0.4/32 [110/30] via 192.168.0.102, 00:02:11, Ethernet0/0 O 10.0.0.5/32 [110/11] via 192.168.0.105, 00:18:30, Ethernet0/0 O E2 10.10.20.0/24 [110/20] via 192.168.0.105, 00:17:40, Ethernet0/0 C 192.168.0.0/24 is directly connected, Ethernet0/0 O E1 192.168.101.0/24 [110/30] via 192.168.0.102, 00:02:11, Ethernet0/0
Only one route this time, and it’s the E1 route from R2. It seems that E1 routes are more preferred than E2 routes. Let’s look at the OSPF database for 192.168.101.0/24 on R1 to see if we can figure that out.
R1#sh ip ospf database external 192.168.101.0 OSPF Router with ID (10.0.0.1) (Process ID 1) Type-5 AS External Link States Routing Bit Set on this LSA LS age: 467 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 192.168.101.0 (External Network Number ) Advertising Router: 10.0.0.2 LS Seq Number: 80000004 Checksum: 0xEA58 Length: 36 Network Mask: /24 Metric Type: 1 (Comparable directly to link state metric) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0 Routing Bit Set on this LSA LS age: 1497 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 192.168.101.0 (External Network Number ) Advertising Router: 10.0.0.5 LS Seq Number: 80000001 Checksum: 0x6260 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0
You can see that everything is the same except for the metric-type field, which is exactly what we expect. By definition, if an external OSPF route is E1, the internal OSPF cost is added to the total cost of the route. This is reflected in the “Comparable directly to link state metric” text next to the Metric Type value. In contrast, an E2 route does not have the cost incremented; the cost is simply passed down the line as “Larger than any link state path”. This means that E1 routes are considered more accurate and should be more preferred than E2 routes.
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