Thursday, June 29, 2006

 

Redistribution. (Problem #75)



Consider the network shown above. Router R2 is redistributing between the EIGRP and OSPF protocols. Here is the config on router R2

router eigrp 100
redistribute ospf 100
network 12.1.1.0 0.0.0.255
default-metric 1000 10 255 1 1500
auto-summary
!
router ospf 100
log-adjacency-changes
redistribute static subnets
redistribute eigrp 100 subnets
network 2.2.2.2 0.0.0.0 area 0
network 23.1.1.0 0.0.0.255 area 0
!
ip route 4.4.4.4 255.255.255.255 24.1.1.2


Router R4 has a static default route back to router R2. The network admin has configured a static route to 4.4.4.4/32 on R2 and is redistributing that static route into OSPF. His reasoning is that the static route will make it into OSPF and then since OSPF is being redistributed into EIGRP, the route to 4.4.4.4 will also make it into the EIGRP domain and hence router R1 will have a route to 4.4.4.4/32.

With this in mind, he issues "show ip route" on R1 fully expecting a route to 4.4.4.4/32 to be present there.

R1#sh ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 1.1.1.1/32 is directly connected, Loopback0
D 1.0.0.0/8 is a summary, 1d22h, Null0
2.0.0.0/32 is subnetted, 1 subnets
D EX 2.2.2.2 [170/2588160] via 12.1.1.2, 1d22h, Ethernet0/0
3.0.0.0/32 is subnetted, 1 subnets
D EX 3.3.3.3 [170/2588160] via 12.1.1.2, 1d22h, Ethernet0/0
23.0.0.0/24 is subnetted, 1 subnets
D EX 23.1.1.0 [170/2588160] via 12.1.1.2, 1d22h, Ethernet0/0
12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 12.1.1.0/24 is directly connected, Ethernet0/0
D 12.0.0.0/8 is a summary, 1d22h, Null0


However, there is no route to 4.4.4.4/32 on R1.

What is the flaw in the network admin's reasoning? What is the reason a route to 4.4.4.4/32 is not being redistributed from the OSPF domain to the EIGRP domain as he expects? How can he get a route to 4.4.4.4/32 to appear in R1's routing table?

Tags: ospf, eigrp, static route, redistribution, ccie, ccnp

Comments:
Add the following commands ?

router ospf 100
network 24.1.1.0 0.0.0.255 area 0
 
No, the underlying assumption was that you can't change the routing domains like that. Explain why the redistribution of the static route from the OSPF domain to the EIGRP domain did not happen as the admin expected and come up with some redistribution related config to make it happen.
 
It will not be accepted into the EIGRP routing domain since the route 4.4.4.4 is already installed in the Routing Information Base of R2 (defined as a static route).

I guess the only possibility would be to also redistribute static under router eigrp.
 
Yes, the route is not making it into the EIGRP routing domain because R3 cannot advertise it back to R2 because of split-horizon. Redistributing the static route into the eigrp process at R2 will do the trick as you correctly suggested.
 
I wouldn't say "split horizon" (we're talking about a link-state protocol here -OSPF-, and split horizon is characteristic for distance-vector protocols).

More like "the route received via OSPF will not be installed in R2's routing table, since it already has a static route configured towards that destination (and the AD for a static route is lower than the AD for an OSPF route)". And since the entry in the routing table is a static one and EIGRP is not configured to redistribute static entries, no redistribution of the route takes place.
 
the networks shown in R1 must be sourced by static routes or ospf, as stated by redistribution command. since the network 4.4.4.4 is neither sourced by static nor ospf, it will not appear on R1 Routing table.

there are two possible solutions :
1- adding redistribute directly connected on R2

2- add static route to R3 pinting to 4.4.4.4
 
This comment has been removed by the author.
 
Reason:
the static route 4.4.4.4/32 never appear in ospf because the AD of OSPF is 110 and the AD of static route is 1 so this route is known on R1 static routing table but not in ospf routing table on R1.

To do best:
1st) Delete the static route 4.4.4.4/32 on R2
2nd) Enable OSPF between R4 and R2

3rd) configure redistribute connected subnet under router ospf 100 on R4 router.

Adjété
 
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