306 Chapter 7 MPLS VPNs and OSPF

Summary

In this chapter you learned about OSPF and how it works when implemented in an MPLS VPN. First of all, OSPF is not the best dynamic routing protocol to use between the PE and CE routers. OSPF is an intensive protocol, and having many instances of it running on a single router can slow the router down. In addition, OSPF can quickly use up the maximum number of routing processes (32) on a PE router.

However, many customers run OSPF, and the service provider needs to support it. To this end, there have been many extensions added to how OSPF works to ensure its proper operation in an MPLS VPN.

To start with, the hierarchy has changed with the introduction of the super-backbone. The super-backbone, or the service provider MP-BGP backbone, replaces the requirement for all OSPF areas to be connected to the OSPF backbone (Area 0). A new extended BGP community is used to preserve OSPF information such as the LSA type. In addition, standard BGP rules still work, such as preserving the OSPF cost in the MED attribute. When a PE router receives an LSA Type 1 or Type 2 from a CE and redistributes it into MP-BGP, a downstream OSPF customer learns this route as an inter-area (O IA) router. When an LSA Type 3 route is learned by the PE, the route is propagated through MP-BGP and learned by a downstream OSPF customer router as an inter-area (O IA) router. OSPF LSA Type 5 external routes keep their external attributes and are listed as external routes on a downstream OSPF customer router’s global routing table.

To make sure that routing loops do not occur, the down bit is used. One PE redistributes a route from MP-BGP into OSPF, and the down bit is set. When the same route is learned by another PE, upon observing the down bit, the route is not redistributed back into MP-BGP. The down bit can be lost, and therefore the tag field (set to the originating BGP AS number) is used. When a PE receives a route, with the tag field set to its own AS number, the route will not be redistributed.

To ensure proper path selection, any route learned with the down bit set results in the routing bit being set on the PE router. Any route with the routing bit set does not show up in the VRF routing table on the PE, even if it is the best path according to OSPF.

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Exam Essentials 307

To configure global OSPF, you use the router ospf process_id command. To configure an OSPF routing context, you use the router ospf process_id vrf vpn_name command.

Exam Essentials

Be able to describe how OSPF operates in a VPN. OSPF operates normally in an MPLS VPN. Customer routers do not need an IOS upgrade to have an MPLS VPN that uses OSPF as the CE-to-PE routing protocol.

What is new is how the service provider handles these routes. To prevent routing loops, the down bit is set when routes are redistributed from MP-BGP into OSPF. The down bit prevents routing loops because when a PE router sees the down bit set, it does not redistribute the route back into MP-BGP. If the down bit is removed as it travels through the customer’s network, the tag field, containing the originating BGP AS number, is used to prevent loops. To ensure proper routing, a learned route with the down bit results in the routing bit (only on the PE router) being set. With the routing bit set, the PE router does not use the route, even if it is the best path as dictated by OSPF.

Be able to describe the enhanced OSPF hierarchical model. The standard OSPF rule is that all areas must connect to the backbone area (Area 0). Now, with MPLS, a new super-backbone is available. The super-backbone replaces the old OSPF backbone (an Area 0 requirement). Service provider routers appear as ABRs to customers.

Understand the interaction between OSPF and MP-BGP. When routes are learned by a PE router, from a CE router, the OSPF type is preserved in the new extended BGP community when redistributed into MP-BGP. When an OSPF LSA Type 1 or Type 2 is redistributed into MP-BGP, its attribute is preserved. When an OSPF LSA Type 3 is redistributed into MP-BGP, its attribute is preserved. When an OSPF LSA Type 5 is redistributed into MP-BGP, its attribute is preserved. When these routes are redistributed back into OSPF, an OSPF LSA Type 1 or Type 2 becomes an OSPF LSA Type 3, an OSPF LSA Type 3 remains an OSPF LSA Type 3, and an OSPF Type 5 remains an OSPF Type 5.

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308 Chapter 7 MPLS VPNs and OSPF

Key Terms

Before you take the exam, be certain you are familiar with the following terms:

area border router (ABR)	link state advertisements (LSAs)
autonomous system boundary	OSPF domain
router (ASBR)
backbone area	OSPF super-backbone
backbone router	routing bit
down bit	tag field
internal router

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Review Questions 309

Review Questions

1.How many routing processes are supported on a PE router?

A.16

B.32

C.48

D.64

2.With standard OSPF, all areas must connect to Area ___________.

A.0

B.1

C.Super-backbone

D.None of the above

3.Without the OSPF super-backbone, PE routers are viewed as

___________ routers.

A.ABR

B.ASBR

C.Internal

D.External

4.Which of the following commands is used to configure OSPF for a VPN?

A.router ospf process

B.router ospf process address-family ipv4 vrf vpn_name

C.router ospf process_id vrf vpn_name

D.None of the above

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310Chapter 7 MPLS VPNs and OSPF

5.Intra-area routes within a customer network are displayed as

___________ in a customer router’s global routing table.

A.O

B.O IA

C.O E2

D.None of the above

6.The OSPF cost is carried in which of the following?

A.New extended BGP community

B.MED

C.Cost field

D.None of the above

7.Intra-area routes are redistributed into MP-BGP by a PE router. When the route is ultimately learned by a downstream OSPF customer router, the route is displayed as ___________ in a customer router’s global routing table.

A.O

B.O IA

C.O E2

D.None of the above

8.Inter-area routes are redistributed into MP-BGP by a PE router. When the route is ultimately learned by a downstream OSPF customer router, the route is displayed as ___________ in a customer router’s global routing table.

A.O

B.O IA

C.O E2

D.None of the above

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Review Questions 311

9.External routes are redistributed into MP-BGP by a PE router. When the route is ultimately learned by a downstream OSPF customer router, the route is displayed as ___________ in a customer router’s global routing table.

A.O

B.O IA

C.O E2

D.None of the above

10.With the OSPF super-backbone, PE routers are viewed as

___________ routers.

A.ABR

B.ASBR

C.Internal

D.External

11.The BGP AS number is mapped to which of the following?

A.Down bit

B.Tag field

C.Routing bit

D.None of the above

12.Which of the following is not sent to customer OSPF routers?

A.Down bit

B.Tag field

C.Routing bit

D.None of the above

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312Chapter 7 MPLS VPNs and OSPF

13.An inter-area OSPF route is an LSA Type ___________.

A.1 or 2

B.3

C.4

D.5

14.An external OSPF route is an LSA Type ___________.

A.1 or 2

B.3

C.4

D.5

15.An intra-area OSPF route is an LSA Type ___________.

A.1 or 2

B.3

C.4

D.5

16.Which of the following OSPF LSA types is flooded throughout the OSPF domain?

A.1 or 2

B.3

C.4

D.5

17.Which of the following OSPF router types generate LSA Type 5 routes?

A.ABR

B.ASBR

C.Internal

D.Backbone

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Review Questions 313

18.Which of the following OSPF router types generate LSA Type 3 routes?

A.ABR

B.ASBR

C.Internal

D.Backbone

19.Which of the following are used to prevent routing loops? (Choose all that apply.)

A.Down bit

B.Tag field

C.Routing bit

D.MED

20.To ensure optimal path selection, the ___________ is used.

A.Down bit

B.Tag field

C.Routing bit

D.None of the above

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