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Superbackbone OSPF DomainIn MPLS

What is the interaction between the routers in Area 2 and the super-backbone?

A. The OSPF super-backbone is completely transparent to OSPF Area 2.

B. The super-backbone appears as a BGP domain to the routers in OSPF Area 2.

C. The super-backbone appears as another OSPF area to the routers in OSPF Area 2.

D. The super-backbone appears as another OSPF domain to the routers in OSPF Area 2.

4 REPLIES
Cisco Employee

Re: Superbackbone OSPF DomainIn MPLS

The answer is that the superbackbone (being your MPLS core) is completly transparent to OSPF area 2. Routes from other areas will be seen as InterArea routes in area 2 as expected.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Bronze

Re: Superbackbone OSPF DomainIn MPLS

OSPF route is redistributed into BGP

MP-BGP route is propagated to other PE routers

MP-BGP route is redistributed into OSPF

OSPF route is propagated as external route into other sites

Local subnet is announced to the PE router as type 1 or type 2 LSA

From the customer perspective, an MPLS VPN-based network has a BGP backbone with Interior

Gateway Protocol (IGP) running at customer sites.

Redistribution between IGP and BGP is performed to propagate customer routes across the MPLS VPN

backbone.

In the case of MPLS-VPN Backbone as Area0

The OSPF superbackbone behaves exactly like Area 0 in regular OSPF:

PE routers are advertised as ABRs.

Routes redistributed from BGP into OSPF appear as interarea summary routes or as external routes

(based on their original LSA type) in other areas.

It is important to note that the MPLS/VPN backbone is not a real Ospf area 0 backbone.No adjacencies are formed between PE routers,and all ospf routes are translated into VPN-IPv4 routes.This means that the redist of routes into BGP does not cause these routes to become external ospf routes when advt to other member sites of the same VPN.

Whenever a PE router recieves an MP-BGP update that contains a prefix learned through OSPF by the originating PE router, it must be capable of identifying what type of ospf route is contained

within the update.This is necessary to allow the PE router to generate an appropriate LSA towards

the VPN cust CE router based on the ospf route type.To support this,when the PE router propogates

OSPF routes into MP-BGP thro redist,the BGP extended community attrib is used to preserve and

convey the ospf attrib of the route.

EXT Comm 0x8000

-4 bytes-Ospf area number

-1 byte-Ospf route type (1 thro 7)

-1 byte-Option (used for external metric type.

Here Ext comm shows the prefix is an inter-area with route-type 3.

OSPF RT:0:3:0

Router#show ip bgp vpnv4 vrf Customer_A 10.0.1.0

BGP routing table entry for 1:10:10.0.1.0/24, version 64

Paths: (1 available, best #1, table Customer_A)

Advertised to non peer-group peers:

10.2.3.6

Local

10.2.3.4 from 0.0.0.0 (10.2.3.4)

Origin incomplete, metric 2, localpref 100, weight 32768, valid, sourced, best

Extended Community: RT:100:27 OSPF RT:0:3:0

New Member

Re: Superbackbone OSPF DomainIn MPLS

Yes ,Very well explained thanks to you Aditya.What I feel every time the the routes receiving PE (remote PE)has to give update to it's attached CE as ONLY TYPE3 irrespective of the type at the orignating end.This means the OSPF super-backbone is completely transparent to customer OSPF Area's in the given VPN.

Cisco Employee

Re: Superbackbone OSPF DomainIn MPLS

You can also receive type 1 and 2 LSAs through the core by using a sham-link between the PEs. For more information on sham-links, refer to the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120limit/120st/120st21/shamlink.htm

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
5241
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