Default route in MPLS

[vin.marco]

Hi everyone I have a problem in injecting the default route, let me explain.

I have a working MPLS network, with various VRFs.

The protocol used between PE-CE is OSPF.
CE-1, in its OSPF process injects the default route through the command default-information originate.
PE-1, receives the default route, redeploys in OSPF in BGP and vice versa
PE-2, in the VRF routing table, sees the default route through BGP, redistributes BGP to OSPF and vice versa.
CE-2 does not install the ddefault route.

What's wrong ?

PE-1
RP/0/RSP0/CPU0:PE-1#sh route vrf XXXXXXX 
Wed Sep 22 11:30:52.989 utc

Codes: C - connected, S - static, R - RIP, B - BGP, (>) - Diversion path
       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 - ISIS, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, su - IS-IS summary null, * - candidate default
       U - per-user static route, o - ODR, L - local, G  - DAGR, l - LISP
       A - access/subscriber, a - Application route
       M - mobile route, r - RPL, (!) - FRR Backup path

Gateway of last resort is 10.36.0.89 to network 0.0.0.0

O*E2 0.0.0.0/0 [110/1] via 10.36.0.89, 18w6d, TenGigE0/0/1/3

PE-2
PE-2#sh ip route vrf XXXXXXX 

Routing Table: XXXXXXX
Codes: L - local, 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, H - NHRP, l - LISP
       a - application route
       + - replicated route, % - next hop override, p - overrides from PfR

Gateway of last resort is 10.86.1.1 to network 0.0.0.0

B*    0.0.0.0/0 [200/1] via 10.86.1.1, 21:10:56

CE-2

CE-2#sh ip route 0.0.0.0
% Network not in table
CE-2#

[Peter Paluch · Accepted solution]

Hello,

In Cisco IOS-based OSPF implementation, you cannot redistribute a default route into OSPF. All other routes can be redistributed but a default route must be injected with default-information originate. So if PE-2 does not have this command configured for the OSPF process in the correct VRF, it won't retake the default route from BGP and won't have it advertised toward CE-2.

Would this explain the behavior in your setup?

Best regards,
Peter

[vin.marco]

Hi Peter and thanks for the reply
the command default-information originate was inserted into the CE-1 OSPF process.

Should I include it in the OSPF process of PE-2 to CE-2?

[Peter Paluch]

Hello,

Should I include it in the OSPF process of PE-2 to CE-2?

Yes, precisely. Remove it from CE-2 and configure it on PE-2 instead. This command is supposed to be placed on the router that

• already has the default route learned by other means than OSPF
• and needs to propagate that default route in OSPF

Clearly, it's PE-2, not CE-2 : )

Best regards,
Peter

[vin.marco]

This is the configuration I did:

1. PE-1: in BGP I entered default-information originate
2. PE-2: in OSP I entered default-information originate
3. CE-2: I received the default route

But is this behavior normal?
If I redeploy i from a routing process in OSPF, shouldn't I see prefixes in Type-5?

[Peter Paluch · Accepted solution]

Hi Marco,

At least as far as the default route is concerned, this is an expected behavior.

Two things are important to remember:

1. You cannot redistribute the default route from another protocol into OSPF. The only way to make a router originate the default route into a regular OSPF area is to configure that router with the default-information originate command in the OSPF configuration. This is why you needed to configure PE-2 OSPF this way.
2. Even if an OSPF process is configured with the default-information originate command, it will only inject the default route into OSPF if there already is a default route in the router's routing table. It does not matter where the default route came from (whether it's a static default route, or from BGP, or from any other routing protocol) but it has to be there. In other words, the default-information originate command injects the default route into OSPF only if the router already has a default route from another routing source. You most likely met this requirement by configuring PE-1 BGP with the default-information originate command which caused the default route to propagate via BGP from PE-1 to PE-2.

Did you observe any other behavior? Do you have any doubts? Please feel free to ask further!

Best regards,
Peter

[vin.marco]

Thank you so much Peter

[Peter Paluch]

Hi Marco,

Very much welcome! : )

Best regards,
Peter