Archive for the ‘redistribution’ tag
Redistribution Notes – AD Manipulation
- Manipulating administrative distance (AD) is another way to help with a mutual redistribution scenario.
- EIGRPs has different ADs for internal and external (redistributed) routes
- OSPF and RIP have the same AD no matter where the route orginated.
- This means that routes redistributed into OSPF may be used instead of a local RIP route.
- AD 110 (OSPF) beats 120 (RIP) every time.
- The distance subcommand allows you to change the AD on specific routes from specific neighbors.
- This example changes the AD of the route to 10.0.0.0/16 advertised from 1.1.1.1 to 121.
- This will make this router prefer a RIP route to the same destination.
ip access-list standard RIP-ROUTES permit 10.0.0.0 0.255.255.0 ! router ospf 1 distance 121 1.1.1.1 0.0.0.0 RIP-ROUTES
–
Corrections are encouraged.
Redistribution Notes – Tagging
- Tagging provides a way to mark common or similar routes to manipulate later.
- In redistribution scenarios with mutual redistribution on two different routers, any routes that gets redistributed from one route process to another are tagged.
- When the other router sees those tags on the route, that route to keep from adding non-optimal routes to its routing table.
- Tags can also be used to do other manipulation such as setting higher metrics or changing ADs.
OSPF
R102#show run ... router ospf 1 log-adjacency-changes redistribute connected subnets route-map SETTAG network 192.0.2.0 0.0.0.255 area 0 ! route-map SETTAG permit 100 set tag 55555 ... R101#sh ip route 10.0.0.2 Routing entry for 10.0.0.0/24 Known via "ospf 1", distance 110, metric 20 Tag 55555, type extern 2, forward metric 10 Last update from 192.0.2.102 on Ethernet0/0, 00:00:13 ago Routing Descriptor Blocks: * 192.0.2.102, from 192.0.2.102, 00:00:13 ago, via Ethernet0/0 Route metric is 20, traffic share count is 1 Route tag 55555
EIGRP
R102#sh run
...
router eigrp 1
network 192.0.2.0
redistribute connected route-map SETTAG
no auto-summary
!
route-map SETTAG permit 100
set tag 55555
...
R101#sh ip route 10.0.0.2
Routing entry for 10.0.0.0/24
Known via "eigrp 1", distance 170, metric 409600
Tag 55555, type external
Redistributing via eigrp 1
Last update from 192.0.2.102 on Ethernet0/0, 00:00:14 ago
Routing Descriptor Blocks:
* 192.0.2.102, from 192.0.2.102, 00:00:14 ago, via Ethernet0/0
Route metric is 409600, traffic share count is 1
Total delay is 6000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
Route tag 55555
–
Corrections are encouraged.
Tagging External Routes in EIGRP
EIGRP allows you to tag external routes. That is, any route redistributed into EIGRP can be tagged with a numeric descriptor from 0 to 4294967295. Read the rest of this entry »
ROUTE Notes – Further IGP Redistribution
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.]
ROUTE Notes – Even More IGP Redistribution
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.
ROUTE Notes – Routing IPv6
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
ROUTE Notes – More IGP Redistribution
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
ROUTE Notes – IGP Redistribution
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
ROUTE – Redistribution Nuance #2 – OSPF External Metric Types
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.
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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ROUTE – Redistribution Nuance #1 – Admin Distance FTW
I just got back from Global Knowledge’s ROUTE class, and I must say that it was a great class. John Barnes puts on quite the show and is the best instructor I’ve ever had. I digress, though.
One of the topics we covered was route redistribution, so I went back to the hotel one night and fired off this network in GNS3 to study a bit.
The object was to see how redistributing statics into OSPF and into EIGRP differ. It was also an opportunity to see how EIGRP redistributes into OSPF (and OSPF into EIGRP, but I didn’t make it that far). To do that, I redistributed 10.10.10.0/24 from R1 into OSPF and 10.10.20.0/24 from R4 into EIGRP. I then had R2 and R5 redistribute all EIGRP routes into OSPF. It’s a nice mix, but I saw some weirdness in the paths to 10.10.20.0/24.
Here’s the simple config from R4 that shows the redistribution. I’m just shoving the static routes into EIGRP without touching the K-values.
ip route 10.10.20.0 255.255.255.0 Null0 <SNIP> router eigrp 1 redistribute static network 10.0.0.4 0.0.0.0 network 192.168.101.0 no auto-summary
Nothing out of the ordinary going on there. I can see the route on R3 and R2 as a “D EX”.
D EX 10.10.20.0/24 [170/281600] via 192.168.101.104, 00:29:33, Ethernet0/0
It’s not the case for R5, though. Check this out.
O E2 10.10.20.0/24 [110/20] via 192.168.0.102, 00:34:52, Ethernet0/0
That, my friend, is an external OSPF route (Type-5 LSA for those scoring at home) going through R2. Why the heck does that happen? The physical path is shorter through R4, but it really comes down to administrative distance. EIGRP has an AD of 90, but external EIGRP routes (D EX) have an AD of 170. Since OSPF routes have an AD of 110, the OSPF route wins. Even though you may have a much “better” route through one protocol, simply having a route in another protocol with a lower AD will be enough to win.
I found another nuance with R1, as well, but that will have to wait until next time.
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So, why didn’t R2 see the route via OSPF through R5? I experimented a bit by reloading each router to see what would happen. When I reload R2, R5 quickly picks up the EIGRP route, but, from there, things seem a little random to me. Sometimes, R5 picks the OSPF route through R2 again. Sometimes, R2 picks R5 as the best path and leaves R5 to route via EIGRP. The same happens when I reload R5; sometimes the route goes one way and sometimes it goes the other. Can one of you higher-ups shed any light on why this happens like it does?