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.]

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.

• 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

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

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

Study Questions

• Why would you ever want to summarize routes?

Summarizing routes minimizes the routes advertised to the network.  For example, instead of advertising 192.168.0.0/24, 192.168.1.0/24…192.168.n.0/24, a router can advertise a single route to 192.168.0.0/16.  Keeping routing tables small saves hardware resources, minimizes convergence times, helps avoid route flapping, and makes the routing table easier to read for humans.

• When will an EIGRP router auto-summarize a route?

If a router has interfaces that that are in different classes of network (Class A, B, C), then that router will auto-summarize those routes up to the classful boundary.  For example, if you have a 10.0.0.1/24 and a 192.168.100.1/30, the router will advertise 10.0.0.0/8 and 192.168.100.0/24.

• You have two routers advertising the same summarized route.  How do you make one preferred over the other?

Adjust the delay or bandwidth so one is favored.

• What is suboptimal forwarding in regards to summarization?

A summary route could be advertised from a router that’s not in the optimal path to the destination.  If that route is chosen by a downstream router, traffic is passed to that router instead of a more optimal path through another router.

• How do you avoid suboptimal forwarding in regards to summarization?

Disable summarization.  Advertising real networks will result in the optimal path being calculated.

• How do you manually summarize the route 192.168.100.0/22?

R1(config-if)#ip summary-address eigrp 1 192.168.100.0 255.255.252.0

• When will a summarized route stop being advertised by a router?

When the router no longer has any routes that fall inside the summary route, the summary is removed.  That is, if a router is advertising 192.168.100.0/22, the route will be removed if the router no longer has ANY routes that are in the 192.168.10[0123].0 networks.

• What’s the biggest route a router can summarize?

The default route.

• What are two ways to advertise a default route in EIGRP?

Advertise a static default route through redistribute static
Summarize 0/0 out of an interface

• How do you keep one part of the network from having a route to another part of the network?

This is done with route filtering.

• When configuring route filtering in EIGRP, what’s the big keyword?

distribute-list

• What are the three techniques for filtering routes?

ACLs
Prefix lists
Route-maps

• Do you mean to tell me that route filtering also uses ACLs and route-maps?

Yes.  Welcome to Cisco Systems.

• How do you use an ACL to filter out the route 192.168.0.0/24?

access-list 1 deny 192.168.0.0 0.0.0.255
access-list 1 permit any
router eigrp 1
 distribute-list 1 out

• How do you use a prefix list to filter out the route 192.168.0.0/24?

ip prefix-list PL1 deny 192.168.0.0/24
ip preffix-list PL1 permit 0.0.0.0/0 le 32
router eigrp 1
 distribute-list prefix PL1 out

• How do you use a route-map to filter out the route 192.168.0.0/24?

access-list 1 permit 192.168.0.0 0.0.0.255
route-map RM1 deny 10
 match ip address 1
route-map RM1 permit 999
router eigrp 1
 distribute-list route-map RM1 out

-OR-

ip prefix-list PL1 permit 192.168.0.0/24
route-map RM1 deny 10
 match ip address prefix-list PL1
route-map RM1 permit 999
router eigrp 1
 distribute-list route-map RM1 out

What Command Was That?

What command…

• …shows you the prefix lists configured?

show ip prefix-list

• …shows the summary route being advertised for a particular network?

show ip route 192.168.0.0 255.255.0.0 longer-prefixes

• …shows what networks are being summarizes on a router?

show ip protocols

• …shows what route a router considers to be a default candidate?

show ip route (look for the *)

• …shows the default network?

show ip route

• 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?

What command…

• …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

Study Questions

• What settings must match between two routers in order to become EIGRP neighbors?

Both routers must be in the same primary subnet
Both routers must be configured to use the same k-values
Both routers must in the same AS
Both routers must have the same authentication configuration (within reason)
The interfaces facing each other must not be passive

• What are the default hello and hold times in EIGRP?

On links with bandwidth > 1.544Mbps:
Hello:  5 sec
Hold:  15 sec

On links with bandwidth <= 1.544
Hello:  60 sec
Hold:  180 sec

• How do you change the hello and hold times?

You set these values at the interface.

R1(config-if)#ip hello-interval eigrp 1 X
R1(config-if)#ip hold-tim eigrp 1 X

• How do you keep an interface from being used for EIGRP discovery?

Don’t configure a network statement that includes that interface
Make the interface passive
Configure static neighbors for that interface

• Why might NTP be a good thing to use in regards to EIGRP?

EIGRP uses key chains for authentication.  Key chains can be configured with a range of valid dates and times.  If the time on two routers was off by even a few seconds, some keys would expire, causing neighbor relationships to drop.

• How do you configure EIGRP authentication?

In each interface participating in EIGRP, you configure the authentication mode and the key chain to use.

R1(config-if)#ip authentication mode eigrp 1 md5
R1(config-if)#ip authentication key-chain eigrp 1 KEYCHAIN1

• What are the k-values that EIGRP uses?

k1 = bandwidth
k2 = load
k3 = delay
k4 = reliability
k5 = MTU

• How does a router choose its router ID in EIGRP?

First, it looks for a configured router-id in the EIGRP configuration.  If none exists, it uses the highest (largest) IP address configured on a loopback interface.  If no loopbacks exist, it uses the highest IP configured on the other interfaces.

What Command Was That?

What command tells you…

• …whether a neighbor was discovered or statically configured?

show ip eigrp neighbor detail

• …what interfaces are involved in EIGRP?

show ip eigrp interfaces

• …what k-values your router is using?

show ip protocols

• …how long your router has been neighbored with another router?

show ip eigrp neighbors

• …what your router ID is?

show ip eigrp topology
show ip eigrp accounting

• …a summary of the configured network statements?

show ip protocols

• …the configured hello interval?

show ip eigrp interface detail

• …the configured hold time?

There’s isn’t a way to do it directly.  You have to check your neighbors several times over the course of a few seconds to see where the hold timers drop to before resetting.

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.

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.

Just another complexity of OSPF.  Thanks to @matthewnorwood, @jameskazin, @steve, @wannabeccie, @ciscotophat, and @lbsources for the insight into the route differences.

Send any twitter updates questions my way.

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