ospf question to E2 and IA routes

[rabeder]

hi, i have a router which has connectivity to a net 200.200.200.0/24.

this router hast two ways to this network:

the first way is:

linde_pe1#sh ip rou

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

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

O IA 200.200.200.0/24 [110/148] via 192.168.1.1, 00:02:38, Serial0/0

100.0.0.0/24 is subnetted, 1 subnets

C 100.100.100.0 is directly connected, Loopback0

192.168.1.0/30 is subnetted, 1 subnets

C 192.168.1.0 is directly connected, Serial0/0

192.168.2.0/30 is subnetted, 1 subnets

O IA 192.168.2.0 [110/148] via 192.168.1.1, 00:02:38, Serial0/0

C 192.168.100.0/24 is directly connected, FastEthernet0/0

150.150.0.0/32 is subnetted, 1 subnets

O E1 150.150.150.2 [110/148] via 192.168.1.1, 00:02:29, Serial0/0

linde_pe1#

the second way is:

linde_pe1#sh ip rou

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

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

O E2 200.200.200.0/24 [110/10000] via 192.168.100.2, 00:01:00, FastEthernet0/0

100.0.0.0/24 is subnetted, 1 subnets

C 100.100.100.0 is directly connected, Loopback0

192.168.1.0/30 is subnetted, 1 subnets

C 192.168.1.0 is directly connected, Serial0/0

C 192.168.100.0/24 is directly connected, FastEthernet0/0

as u can see one way is over a inter-area

connection - one is over external (redistribution)

my problem ist that i want the IA route to be preffered over E2 routes.

as soon as the router has switched to the connection over the E2 path (e.g. failure of connection) - it does not switch back to the IA connection which is the preffered path.

any idea ???

[stephtchoko]

Hi,

Ospf use cost to built short path algorithm,these cost depend on bandwith ( cost=100 000 000/bandwith). The E2 use Ethernetwhich has more bandwith than IA with serial, the cost via E2 is less than cost via IA.

if you want IA to be preferred than E2, you change cost of serial interface with command ip ospf cost.

http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017d028.html#wp1038322

make show ip ospf interface to see the cost:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094056.shtml

Please rate this post.

Regards.

[rabeder]

hi,

thx for answer, BUT:

as u can see the E2 path (over eth) has a larger metric(cost) as the IA path.

(i have set the cost with "bandw 256" on ethernet)

E2 connection:

O E2 200.200.200.0/24 [110/10000] via 192.168.100.2, 00:01:00, FastEthernet0/0

IA connection:

O IA 200.200.200.0/24 [110/148] via 192.168.1.1, 00:02:38, Serial0/0

u can see the metric in the square brackets

110 ... admin dist

148 ... cost / metric

[Harold Ritter]

This is not an OSPF issue as the IA would normally be selected over the E2.

This is in an MPLS VPN context, right? And I suppose the IA is being received over BGP via the MPLS core, right? The issue is probably that you are redistributing the E2 in BGP then.

As soon as the IA dissapears, the E2 is selected and then redistributed into BGP. Even when the IA comes back (via BGP) BGP will select the locally originated route (the redistributed E2) and will ignore the route coming from the other side of the cloud.

One way to prevent this is by filtering when you redistribute from OSPF to BGP on this PE.

Hope this helps,

[rabeder]

hi,

thanks for answer.

the szenario is exactly what u mean.

the problem is, that we need the E1 path (from backuplink) on our pe router because of other ce sites.

what did u mean with filtering?

should we filter all e1 path when we redistr. from ospf to bgp?

perhaps the only possibility is a "sham-link" ??

[Harold Ritter]

Kurt,

What I meant by filtering was to prevent the E2 to be redistributed in BGP although it could be present in the VRF RIB.

The sham-link approach would definitely work too.

Hope this helps,

[rabeder]

hi harold,

the problem is that we have several pe-sites with connectivity to ce-sites in this vrf.

so i must redistr. the e2 learned route into bgp that the pe gives this information to the other pe´s.

so i think the only possibility is the sham-link, which works - i have tested this

thx for your help - greetings from austria

[Harold Ritter]

Kurt,

I completely concur that if you need to redistribute this E2 in BGP then the sham-link is the way to go.

You are very welcome,