Aaron's Worthless Words

As always, corrections are requested.

Study Questions

• I’ve got IGRP and EIGRP both configured with the same AS number.  What’s special about this configuration?

If both use the same AS number, then they automatically redistribute their routes into each other without using the redistribute command.

• When redistributing one IGP into another, where’s a good place to filter routes?

There’s no one good place, but at the router(s) that’s doing the redistribution is a good start.  There’s no need to send an IGP a bunch of routes it doesn’t need.

• When redistributing one IGP into another, where’s a good place to summarize routes?

There’s no one good place, but that may be best done at the router just inside the redistributing router.  If the redistributing router only sees the summary route, that’s what it will pass to the other IGP.

• What’s the default metric of RIP?

That’s infinity, so it’s unreachable with an explicit metric.

• I’ve redistributed OSPF into RIP, but I don’t see my subnets there.  What gives?

RIP automatically summarized routes, so look for summaries instead of specific subnets.

• How can you limit the number of routes redistributed into EIGRP or OSPF?

Use the redistribute maximum-prefix X directive under the routing protocol, where X is the maximum number of routes.

• What are the metrics of connected routes when redistributed into EIGRP?

Those routes take the metric of the associated interface instead of using the metric you gave to the redistribution.  [This seems fishy at best.  Can anyone help clarify, please?]

• I have 845734928 interfaces on my router, but I only want to use 3 of them for EIGRP and only want to configure a single network statement.  What’s the easiest way to do that?

Set all the interfaces as passive with the passive-interface default router subcommand.  Next, make all your interesting interfaces non-passive with the no passive-interface X subcommand.  Now you can configure network 0.0.0.0 255.255.255.255 to match all the interfaces, but only the interesting interfaces will participate.

• What is the term for the rank of trustworthiness a routing protocol provides?

Administrative distance

• How can I change the AD of external EIGRP routes to 201 while keeping the default AD for internal EIGRP routes?

Router1(config-router)#distance eigrp 90 201
You have to set both, so you’ll have to remember that EIGRP has an AD of 90 for internal routes by default.

• How can I change the AD of OSPF routes to 192.168.0.0/24 to 202?

Router1(config)#access-list 88 permit 192.168.0.0 0.0.0.255
Router1(config)#router ospf X
Router1(config)#distance 202 0.0.0.0 255.255.255.255 88

• Is it possible to set the AD of different OSPF routes types like intra-area and interarea?

Yes.  You can give it the old distance ospf inter-area X to change the AD.  It also works for intra-area and external routes.

• Is it possible to set the AD of an external OSPF route to 192.168.100.0/24 to 202 without changing the others?

I would have though you could use a route-map for that, but I can’t find a proper set command in a route-map.  [A little help, please.]

I didn’t do so well on IGP redistribution the last time out, so here’s some more stuff to study.  As always, feel free to correct.

Study Questions

• What three things are needed to be able to redistribute one routing protocol into another?

1. One or more links into each routing protocol
2. A proper, working config for each protocol
3. The addition of the redistribute command to one or more of the protocols

• You just configured OSPF to redistribute EIGRP routes, but EIGRP, with the network statement of network 0.0.0.0 0.0.0.0, is configured with a passive interface.  Does this interface’s connected network get redistributed?

Yes, it does.  Even if it’s not participating in routing, it’s still an interface that EIGRP is configured to use, so it goes along for a ride on the redistribution train.

• Name three ways to set the metric of redistributed routes in EIGRP.

1.  default-metric …
2.  redistribute X metric …
3.  redistribute X route-map …

• How can I set the metric for all OSPF routes redistributed into EIGRP?

Use the redistribute ospf X metric command.

• You are redistributing OSPF into EIGRP and want to set the metric of one particular route to another set of metric values (BW, delay, etc.).  How do you do that?

Use a route-map to match the single route and to set the new values.

• Routes from what routing protocol need a metric set when redistributing into EIGRP?  Routes from what protocols don’t?

Routes from another EIGRP instance have their metrics copied over; all others need to have it set.

• What’s the default metric of a BGP route when redistributed into OSPF?  EIGRP?

BGP has a metric of 1 in OSPF.  There is not default metric in EIGRP without some configuration.

• You left out the subnet keyword when redistributing EIGRP into OSPF.  What is the result?

Only classful routes will be redistributed and only if EIGRP has a classful route to redistribute.

• You left out the subnet keyword when redistributing OSPF into EIGRP .  What is the result?

There is no subnet keyword for redistribution under EIGRP.

• Routes from what routing protocol need a metric set when redistributing into OSPF?  Routes from what protocols don’t?

OSPF set metrics automatically.  If the route came from another OSPF process, the metric is copied over.  If the route came from BGP, the metric is set to 1; if it came from any other routing protocol, the metric is set to 20.

• What are three ways to manipulate the metric of redistributed routes in OSPF?

1.  default-metric …
2.  redistribute X metric …
3.  redistribute X route-map …

• My ASBR is advertising static routes into area 0, but I’m not seeing any type-5 LSAs in area 1.  What’s gives?

Assuming everything else is configured correctly and no filtering is done, area 1 is probably a stub area of some kind.

• My ASBR is advertising static routes into area 1, but I’m not seeing any type-5 LSAs in that area.  What gives?

Area 1 is probably an NSSA or totally NSSA area, so any external routes are flooded as type-7s – note type-5s.

• If I look at the OSPF database on my router, I see that a whole bunch of type-5 LSAs advertised from the router with the ID of 1.1.1.1.  What does that say about that router?

Among other things, that router is an ASBR and is redistributing external routes into that area.

• I see several routes in the OSPF database with a cost of 20.  What metric type are those routes?

More likely than not, they are type-2 routes (O E2).

• I have two type-5 LSAs for the same network through two different ABRs; both are of the type-2 metric.  How does the router decide which one to use?

Since both routes are E2, they will have a metric of 20 (unless manipulated somehow), so looking at the intra-area cost results in a tie.  The router will then look at the type-4 LSAs which contain the cost from the ABR to the ASBR.  Since each ABR floods these type-4s, the router knows which ABR is closer to the ASBR advertising the route.  The lower metric in the type-4 LSAs wins.

• I have two type-5 LSAs for the same network through two different ABRs; both are of the type-1 metric.  How does the router decide which one to use?

Since both routes are E1, the costs to the ABR are first compared since they may be different.  If tied, the type-4 LSA’s cost to the ASBR is compared.  If still tied, the external (type-5 LSAs) cost is compared.

• I have two type-5 LSAs for the same network through two different ABRs; one is type-1 and the other is type-2 ?  How does the router decide which one to use?

E1 routes are always preferred over E2 routes.

• Why can’t you redistribute static routes into a stubby network?  How can you make it work?

Stub networks do not flood type-5 LSAs, so the routes cannot be advertised into the area.  You can change it to a regular area to make it work.  You can also make it an NSSA or totally NSSA area.

• How do OSPF routes that come from type-7 LSAs appear in the routing table?

They appear as “O N1″ or “O N2″ depending on the metric type.

Study Questions

• Why would anyone develop a version of RIP that supports IPv6?

I have no idea.  Boredom, maybe.  Whatever the case, it works just like RIPv2, which is pretty scary.

• In EIGRP for IPv4, there are several requirements for two routers to neighbor up.  Which of those is not true for EIGRP for IPv6?

The two routers don’t need to be in the same subnet.  The concept of the link local address takes care of that need since neighbors always share a common medium like an Ethernet segment or a serial link.

• I configured EIGRP for IPv6 on my completely IPv6 router, but it’s not working.  Nothing happens.  What’s going on?

For one, you have to do a no shutdown as an EIGRP subcommand; by default, EIGRP for IPv6 is in a shutdown state.  Another reason could be that a router ID hasn’t been set; EIGRP for IPv6 still uses the IPv4 addresses to establish a router ID, so you may have to set one manually.

• I tried to configure EIGRP for IPv6 with the network statements, but it’s not taking the command.  What gives?

You actually configure EIGRP for IPv6 (and RIPng and OSPFv3) the “new way” by using the interfaces.  Try doing a ipv6 eigrp X as an interface subcommand.

• When redistributing one IPv6 IGP into another, what kinds of routes will and won’t be redistributed?

Only routes discovered via the original IGP will be redistributed.  Connected routes, even the ones configured in the original IGP, won’t be redistributed.  Link local addresses and local routes will also NOT be redistributed.

• Show me a simple RIPng config.

R1(config)#ipv6 router rip PROC-NAME
R1(config-rtr)#int f0/0
R1(config-if)#ipv6 rip PROC-NAME enable

• Show me a simple EIGRP for IPv6 config.

R1(config)#ipv6 router eigrp 8
R1(config-rtr)#router-id 1.1.1.1
R1(config-rtr)#no shutdown
R1(config-rtr)#int f0/0
R1(config-if)#ipv6 eigrp 8

• Show me a simple OSPFv3 config.

R1(config)#ipv6 router ospf 4
R1(config-rtr)#router-id 1.1.1.1
R1(config-rtr)#int f0/0
R1(config-if)#ipv6 ospf 4 area 0

• How do you include connected routes when redistributing one IGP into another in IPv6?

Use the include-connected directive in the redistribution command.

• In EIGRP for IPv6, what address shows up as the next hop in the routing table?

The link local address of the advertising router.

What Command Was That

What command is used to…

• …show all the IPv6 routes?

show ipv6 route

• …shows the status of OSPFv3 neighbors?

show ipv6 ospf neighbor

• …shows the status of RIPng neighbors?

There is none; RIPng doesn’t have neighbors.

• …shows a route to a specific prefix?

show ipv6 route prefix::/length

As always, feel free to correct.

Study Notes

• When a router redistributes from one routing protocol to another, where does the router get the list of routes to redistribute?

From the routing table.  Only IGP A’s routes (not topology or successors) are redistributed into IGP B’s domain.

• What are two methods of filtering redistributed routes?

Use a route-map in the redistribute line or a distribute-list.

• Of the two methods for filtering, which one has more options?

The route-map method has more options.  You can match on all sorts of stuff, including an ACL or interface, and filter based on that.

• How does using distribute-lists differ between OSPF and EIGRP?

In EIGRP, distribute-lists are used to keep a route from being propagated.  In OSPF, they’re used to keep routes from reaching the routing table.  The effect is basically the same, but the cause is very different.

• How do I redistribute an EIGRP into OSPF as an E1?

You can set that that in the redistribute command.  You can also match a route-map and set the metric-type there.

• What is a big pitfall of having two routes mutually redistribute the same two IGPs?

A router could redistribute IGP A’s routes into IGP B where the second router redistributes them back into IGP A.  Potentially, either router could choose very long routes to get to a destination based on the different ADs and metrics of the IGPs.

• How can I keep this domain loop from happening?

Set the metrics of the redistributed routes so that the originating IGP has the preferred path
Set the AD on the redistributed routes so that the  originating IGP has the preferred path
Manually filter routes so one IGP isn’t presented with its own routes
Use route-tags to mark redistributed routes to filter or manipulate later

• How do you change the metrics of the routes?

You can use the redistribute command to set the metrics.  You can also use route-maps to match routes or tags and set the metric.

• How do I change the AD of the routes?

You can use the distance subcommand to set the AD on the whole domain or from a specific originating (or redistributing) router.

• How do I change the AD for route from 1.1.1.1 to 201?

R1(config-router)#distance 201 1.1.1.1 0.0.0.0

• How do I set a route-tag?

Use a route-map to match the routes you want to tag, and use the set tag directive.

• How does using EIGRP as one of my IGPs help me with mutual redistribution on multiple routers?

EIGRP actually has two ADs – 90 for internal and 170 for external routes.  If a route is redistributed into EIGRP, it will have an AD of 170, so it will be less preferred than interal EIGRP, OSPF, or RIP routes.  Unless you’re using internal BGP or custom ADs, this will keep a looping route out of the routing table and, thus, from being redistributed.

• How does using OSPF as one of my IGPs help me with mutual redistribution on multiple routers?

You may be able to use the metric-types to do some filtering, but the cool AD thing is for EIGRP only.  Since external OSPF routes have an AD of 110 just like internal routes, you can’t rely on AD to keep the looping routes out like you can with EIGRP.

• How does using RIP as one of my IGPs help me with mutual redistribution on multiple routers?

RIP never helped anyone with anything.  Just convert your RIP routers to EIGRP and be much happier.

• What’s a quick way to have OSPF set all external routes to an AD of 201?

R1(config-router)#distance ospf external 201

• What happens if I have OSPF redistributing into EIGRP redistributing into RIP (all mutually)?

You may wind up with a VERY long path without some intervention.  You’ll need to do some filtering on the redistribution to keep routes short.

• Can’t I just use the redistribute command to set metrics coming in and out of all the IGPs to keep the looping routes out?

Yes, you can.  If you have a small network, that shouldn’t be a problem.  If you have a couple thousand routes, though, I’m sure people have better things to do than manage metrics.

• In what order do you configure the metrics when redistributing into EIGRP?

Bandwidth, delay, reliability, load, and MTU

• What’s weird about an extended ACL when using them with route filtering?

When matching routes, an extended ACL will use the ACL’s source field as the network of the route and the destination field as the subnet mask.  For example, access-list 101 permit ip host 172.16.0.0 host 255.255.255.0 matches 172.16.0.0/24.

• What can’t route-maps do other clean the dishes?

They’re used in many, many places on a Cisco router.  I imagine there’s a macro somewhere that will clean the dishes, though.

What Command Was That

What command…

• …shows the metric of the route you just redistributed into EIGRP?

show ip eigrp topology

• …shows the metric of the route you just redistributed into OSPF?

show ip ospf database external

• …shows the tag of a route?

show ip route

• …shows the admin distances you’ve messed up?

show ip protocols

• …shows the admin distance of a particular route?

show ip route x.x.x.x y.y.y.y

As always, feel free to correct.

Study Questions

• When you redistribute OSPF into EIGRP, what are you really redistributing?

Routes knows via OSPF
Networks of OSPF-enabled interfaces

• What’s the default cost of an EIGRP route redistributed into OSPF?

20

• What’s the default metric of an OSPF route redistributed into EIGRP?

There is none since EIGRP has all those nifty k-values that have to be processed.  Routes actually won’t redistribute without them.

• How do you set the metrics of a route redistributed into EIGRP?

Set the default metric with the default-metric subcommand
Set the metric in the redistribute … metric subcommand
Set the metric with a route-map in the redistribute … route-map subcommand

• If you have a default metric set under EIGRP and a metric set on a redistribution of OSPF, which does the router use?

The router uses the metric for the redistribution.

• What’s special about the metric when redistributing one EIGRP AS into another?

The metric is copied from one AS to the other.

• What is I redistribute one OSPF domain into another?

The same thing happens – the metric is copied from the originating domain.

• What’s the difference in AD between an EIGRP and an external EIGRP route?

EIGRP: 90
External EIGRP: 170  [Didn't I do a blog post about this last month?]

• What’s the difference between an external type 1 and  an external type 2 OSPF route?

External 2 routes, only the external cost is used; no router increments the cost.  For external 1 routes, the external cost is incremented by each route with the internal cost.

• Which of O E1 and O E2 routes is more preferred and why?

E1s are preferred because they’re considered more accurate.  [Didn't I blog on this last month, too?]

• I have redistribute eigrp 1 configured in my OSPF config, but 10.0.0.0/24 isn’t showing up in OSPF.  What gives?

OSPF only redistributes classful routes unless you add the subnets option to the redistribution command.

• What are the options in the redistribute directive when redistributing OSPF into EIGRP?

redistribute ospf process-id [ metric bandwidth delay reliabilityload  mtu ] [ match { internal | nssa-external | external 1 | external 2 } ] [ tag tag-value ] [ route-map route-map ]

• What are the options in the redistribute directive when redistributing EIGRP into OSPF?

redistribute ospf process-id [ metric metric ] [ metric-type metric-type ] [ match { internal |nssa-external | external 1 | external 2 } ] [ tag tag-value ] [ route-map route-map ] [ subnets ]

• What do type-4 LSAs do?

If an external route comes from another area, the ABR uses type-4 LSAs to advertise the cost of the route from the ABR to the ASBR.  Routes use this cost as a tie breaker if the internal cost is the same from two ABRs.

• What type of LSA are used to flood routing advertisements from an external NSSA area into area 0?

The NSSA ASBR uses type-7s to flood into the NSSA, but the ABR to area 0 converts those to type-5s.

What Command Was That

What command…

• …show all the EIGRP routes that originated from other routing protocols?

show ip route eigrp | inc ^D EX

• …shows all the OSPF routes that originated from other routing protocols?

show ip route ospf | incl ^O E[12]

• …show all the type-4 LSAs floating around in an OSPF area?

show ip ospf database asbr-summary

• …show the cost to get from a router to an ASBR?

show ip ospf border-routers

Feel free to correct.  I feel like I’m missing a big piece here, so please fill in a gap if you see one.  Thanks.  :)

Study Questions

• How many area 0s (zero) can you have in an OSPF implementation

Just one.

• If my company merges with another company, and we’re both running OSPF, how can we get our networks routing together properly?

The easiest thing to do is to connect your two area 0s together through some physical link.  If you can, you can use virtual links to connect an ABR to another ABR to extend the zones together.

• How do you configure virtual links?

R1(config-router)#area 1 virtual-link 1.1.1.1

• That IP in the virtual link command looks like a loopback.  What’s up with that?

It’s the RID of the router to which you want to connect.

• Why wouldn’t I just use a GRE tunnel between my two routes and put that in zone 0?

That’s a good question.  I would probably do that instead of virtual links if I had the choice because it eliminates any weird problems you may see with the virtual links.  [Someone pipe up on this one, please.]

• What types of authentication can you do with virtual links?

None
Clear text
MD5

• I’ve configured frame-relay map ip 1.2.3.4 101 on my s0/0/0.1, but I can’t get a neighbor to come up.  What gives?

A non-broadcast medium can’t detect neighbors dynamically.  You need static neighbors, or you can add the broadcast keyword to the end of your map statement.

• What is the big problem with partial mesh frame relay topologies when OSPF comes into play?

Not all routers are connected directly, so some routes won’t see all the neighbors.  When OSPF routes propagate on broadcast medium, the next-hop is the router what propagated it; you’ll wind up seeing routes to routers to which you don’t connect.

• How do you get over the partial mesh problem?

You can statically configure frame relay maps pointing the IPs of the unconnected routers to the DLCI of a router that is connected to them (like a central hub router).

• What network types use DRs and BDRs?

Anything multiaccess, so the NBMA and BMA.

• Which network types can dynamically discover neighbors?

Broadcast and point-to-multipoint

• Why do you have to configure static neighbors on NBMA and point-to-multipoint non-broadcast links?

Since OSPF uses multicast to talk to neighbors, a router treats the packets like a broadcast.  Since these network types don’t have a broadcast capability, the only way a neighbor will be established would be through static statements.

• You have a hub-and-spoke topology over a frame relay cloud.  One of your hubs sees routes for all the networks at the other other hub sites.  Is all as well as it seems?

No.  The routes to the other hub networks will have their next hop set to the frame relay IP of each hub router.  Since one hub router can’t get directly to others, the router won’t be able to pass traffic to those sites at all.  You’ll need to statically map those IPs to the DLCI of the hub site for traffic to flow as expected.

What Command Was That

Which command(s)…

• …define a virtual link using the MD5 has of the key “test”?

R1(config-router)#area 1 virtual-link 1.1.1.1 authentication message-digest
R1(config-router)#area 1 virtual-link 1.1.1.1 message-digest-key 1 md5 test

• …configure a static OSPF neighbor?

R1(config-router)#neighbor 1.2.3.4 [cost X] [priority Y]

• …shows the status of a virtual link?

show ip ospf virtual-links
show ip ospf neighbor

• …shows the authentication type and youngest key for a virtual link?

show ip ospf virtual-links

• …displays the network type for an interface?

show ip ospf interface

Feel free to correct all this stuff.  Additions are also welcome.

Study Questions

• How do I keep an area route from reaching a router in that area?

You don’t.  That defeats the whole purpose of having the topology database on every router.  If you filtered one route from a router, there’s no way that SPF could calculate routes correctly.

• Fine, then.  Where do I filter routes?

You filter routes on an ABR or ASBR.  Since routers only have the whole topology for their area, it’s safe to filter routes from another area or from a redistributed routing protocol.  On a more technical note, you’re filtering type-3 LSAs on an ABR and type-5 LSAs on an ASBR.

• Show me an example of keeping the area 1’s route of 192.168.0.0/24 from hitting area 0.

R1(config)#ip prefix-list PL1 deny 192.168.0.0/24
R1(config)#ip prefix-list PL1 permit 0.0.0.0/0 le 32
R1(config)#router ospf 1
R1(config-router)#area 0 filter-list prefix PL1 in

• How about keeping a router from even learning about that same route from area 1?

R1(config)#router ospf 1
R1(config-router)#area 1 filter-list prefix PL1 out

• You know that that seems a little backwards, don’t you?

You have to think of filtering in terms of the area instead of in terms of the router.  You’re filtering into the area or out of the area…not into or out of the router.

• How do you keep the OSPF route to 192.168.0.0/24 from being submitted to the routing table?

I’ll use the same prefix list above.
R1(config)#router ospf 1
R1(config-router)#distribute-list prefix PL1 in

• Isn’t that almost the same syntax to filter EIGRP routes?

Almost.

• How do I send area 1 the summary route of 192.168.0.0/16 from area 0?  That would be a type-3 LSA.

On the ABR:  R1(config-router)#area 1 range 192.168.0.0 255.255.0.0

How do I do the same thing for external routes (type-5 LSAs)?

On the ASBR:  R1(config-router)#summary-address 192.168.0.0 255.255.0.0

• If you see “totally” in the stub area description, what does that mean?

Someone at Cisco is a surfer.  It also means that there are no type-3 LSAs in that area.

• Is the term “stubby” an insult?

No.  It’s a term for an OSPF area that has certain types of LSAs filtered.  Summary routes are usually involved.  This is not filtering that we discussed above, though.  This is keeping all instances of an LSA type from entering an area.

• What the heck is a type-7 LSA?

If an NSSA has an external route it needs to flood, it uses a type-7 instead of a type-5.  This allows a router in a NSSA to advertise external routes without being bombarded by type-5s from other areas.

• What are the four types of stubby areas?  What LSA types do they filter?  What LSA types do they allow?

Stub – filters type-5s – allows type-3s
Totally stubby – filters type-3s and type-5s
NSSA – Filters type-5s – allows type-3s and type-7s
Totally NSSA – Filters type-3s and type-5s – allows type-7s

• What area can never be a stubby?

Area 0, of course.

• If area 1 is a stub, what LSA types will area 0 see from it?

Type-3s.  The routes from area 1 are still advertised into area 0 as normal.

• How about if area 1 is a totally NSSA?

Type-3s and type-5s.  The routes from area 1 are still advertised into area 0 as normal, and the type-7s would be translated to type-5s.  [Someone check me on this one.]

• Where do you configure an area to be a stub?

On all the routers in the area.  The same goes for NSSA.

• Where do you configure an area to be a totally stubby?

The totally stubby part is configured on the ABR.  The other routers in the area should be configured as stub.  The same goes for totally NSSA.

• What route always shows up in a stubby or totally stubby area unless someone has done something weird?

0.0.0.0/0

• Speaking of the default route, how do you manually summarize the default route in OSPF?

You can use the area 1 range 0.0.0.0 0.0.0.0. You can also use the default-information originate command in OSPF.

• What would you see on the internal routers if you had an ABSR that only had full BGP tables from your ISP configured with default-information originate?

You would see nothing.  You need to have a default route somewhere for the router to advertise into OSPF.  Since BGP full routes don’t contain a default, it won’t advertise.

What Command Was That

What command…

• …shows what type of stubby area an area is configured to be?

show ip ospf

Feel free to correct.

Study Questions

• 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

What command…

…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

Feel free to correct.

Study Questions

• What are the definitions of the hello and dead intervals?

The hello intervals is how often a router sends hello messages.  The dead interval is how long to wait before considering a neighbor dead from lack of hello messages; this is 4x the hello interval by default.

• How do you keep OSPF from trying to detect neighbors on an interface?

Don’t configure a network statement for that interface
Make that interface passive

• What type of routers connect to multiple areas?

Area border router (ABR)

• What fields in the hello packet need to match in order for a router to neighbor with another via OSPF?

Subnet
Area
Hello and dead intervals
IP MTU
Authentication method
Authentication key

• What’s another way to put an interface into an OSPF area other than the old network statement?

R1(config-if)#ip ospf 1 area 0

• Don’t the process IDs have to match?

No.

• How is the router ID calculated?

The router ID is discovered just as EIGRP does it.  First, it looks for a route-id command.  If one does not exist, the highest IP of on a up/up loopback interface is used.  If one does not exist, the highest IP of the rest of the up/up interfaces is used.

• What is the protocol, source, and destination in an OSPF hello packet?

OSPF used protocol 89 (not TCP or UDP) sourced from the interface sending the packet to 224.0.0.5.

• What happens after neighbors are brought up successfully?

Topology databases are exchanged.  Each router sends all the information it has about the area to its new neighbor.

• What would happen if the MTUs of two potential neighbors was different?

The routers will become neighbors, but the topology exchange will not occur successfully.  [I think I’m missing a piece here.  This seems very prescriptive in a situation where I would expect more chaos.]

• What types of authentication can be configured with OSPF?

None (type 0)
Cleartext (type 1)
MD5 hash (type 2)

• How do you configure a router to use MD5 authentication with OSPF?

R1(config-router)#area 0 authentication message-digest

• That didn’t work.  What are we missing?

You have to set the key, dummy.  Do that on the interface.

R1(config-if)#ip ospf message-digest-key 1 md5 KEY

• What is a DR?  How about a BDR?

A designated router (DR) is a router that has been elected to advertise all the router for a multi-access segment (like a LAN segment).  Every time a change occurs in the network, the router that detected it will notify the DR, who will then relay the information to the other routers on that segment.  The backup designated router (BDR) takes over that functionality if the DR fails.

• What are the default hello intervals for broadcast, point-to-point, NBMA, and point-to-multipoint interfaces?

Broadcast:  10 sec
P-P:  10 sec
NBMA:  30
P-M:  30

• How do you change the default hello timer?

R1(config-if)#ip ospf hello-interval X

• What types of segments use DRs and BDRs?

Only broadcast and NBMA segments use DRs and BDRs.

• What are some important states of an OSPF neighbor?  [Yes, there are others.]

INIT:   A router has received a hello from another router, but its own hello has not been acknowledged.
2WAY:  A router has received a hello and a acknowledgement from another router
EXSTART:  The DRs and BDRs are elected, and exchanges are beginning with them
FULL:  The router has successfully exchanged topology data with the neighbor

What Command Was That?

What command…

• …shows all the OSPF neighbors?

show ip ospf neighbor

• …shows the number of OSPF neighbors on an interface?

show ip ospf interface
- OR -
show ip ospf interface brief

• …shows the DR and BDR on an interface?

show ip ospf interface

• …shows the authentication method used?

show ip ospf interface

• …shows the hello and dead intervals configured on the router?

show ip ospf interface

• …shows the OSPF network type for an interface?

show ip ospf interface

• …shows the state of a neighbor?

show ip ospf neighbor
- OR -
show ip ospf neighbor detail

I’ve seen and used the command before, but I’ve never really seen any use of the show ip protocols command until tonight while reading up for my ROUTE test.  There’s a lot of good information in the output, and, from the way the book is reading, this is a great candidate for use in a lab question.

To check it out a bit, I set up a small network with four routers connected only to a single Ethernet segment.  I set up one router to run EIGRP, OSPF, and BGP to each one of the other routers just so I could see the output for the different routing protocols.  Here’s what puked out after struggling with GNS for a few minutes.

R1#sh ip protocols
Routing Protocol is "eigrp 1"
  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 1
  EIGRP NSF-aware route hold timer is 240s
  Automatic network summarization is in effect
  Maximum path: 4
  Routing for Networks:
    192.168.0.0
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: internal 90 external 170

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 192.168.0.101
  Number of areas in this router is 1. 1 normal 0 stub 0 nssa
  Maximum path: 4
  Routing for Networks:
    192.168.0.0 0.0.0.255 area 0
 Reference bandwidth unit is 100 mbps
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: (default is 110)

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
    192.168.0.104
  Maximum path: 1
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: external 20 internal 200 local 200

The EIGRP section shows some important details, including what k-values are used, networks configured, and administrative distance (AD) of the various route types (internal and external).  The OSPF section shows the router ID, number of areas on the router, and number of area types (normal, stub, NSSA), as well as the networks configured and the AD.  The section regarding BGP shows summarization status, neighbors (along with any filter lists, distribution lists, local weights, and route-maps if they were configured), and the ADs again.

That’s good stuff to know.  I’ll have to put that command in usual repertoire.