Posts tagged ‘ospf’

May 27, 2011

OSPF on CE-PE links

by costiser

And here it comes a prettly long post. For those without too much free time (I also include myself here), you can take a fast look at the drawing above. If you liked it and want to find out more details, then go ahead and read this till the end (other cool drawings will come as well below:-) ).

Let’s start with some basic comments/features:

  • MP-BGP is used to transport the routing info accross the MPLS backbone
  • all internal routes (LSA 1, LSA 2, LSA 3) are carried as inter-area summary Type-3 LSAs
  • PE routers appear as ABR (OSPF Area Border Routers) for the devices in customer OSPF domains
  • there are NO OSPF ADJACENCIES or flooding across the MPLS cloud (except when sham-links are used)
  • MP-BGP cloud can be seen as a “superbackbone” / “super area 0” that gives the following advantages:
    •     we may have non-zero areas at different VPN sites without the need of an area 0
    •     we may have area 0 at different sites together with non-zero areas attached to the superbackbone
  • OSPF information is carried across the MPLS VPN cloud using BGP extended communities

The BGP extended communities used to carry OSPF information are:

  • router-id = router ID of the PE in the relevant VRF instance of OSPF
  • domain-id
    • by default, equal to the OSPF process number
    • manually configured via command “domain-id”
    • if the domain ID of the route does not match the domain ID on the receiving PE the route is translated to the external OSPF route (LSA Type 5) with metric-type E2
  • OSPF route-type
  • MED (metric)
    •   routes sent across the MP-BGP cloud do not increment their metric
    •   MED can be manually manipulated to influence path selection

Loop Prevention Mechanisms

The addition of the super-backbone area also introduces the possibility of routing loops. In order to prevent them, several basic loop prevention rules apply:

1) “DOWN” bit set (to prevent loops in multi-homed sites)
= Automatically set in all summary LSA 3 (not in LSA 5)  when routes are redistributed from MP-BGP into OSPF.
When a prefix with DOWN bit set is received on an interface which is configured with VRF, that LSA is dropped.
(correct sentence is: that LSA is not considered during the SPF calculations)

This feature has undesirable effect when using VRF-lite in the customer cloud:

The solutions to this problem:
A) is to configure the command “capability vrf-lite” on the customer router(s), but this is not supported on all IOS:

router ospf 1 vrf VPN_A
capability vrf-lite

B) configure different domain-IDs as this will force all redistributed routes to become external (LSA 5, thus bypassing the DN bit check)

*  *  *

2) Route Tagging
The DN bit helps preventing routing loops when summary Type-3 LSAs are redistributed, but not when external routes are announced (the DN bit is not set on LSA Type 5/7)
= All routes redistributed via a particular PE will carry the OSPF route tag which by default is the BGP AS number.
This tag is preserved when the external route is propagated across the entire OSPF domain (including redistribution into another OSPF domains). When another PE receives this route and it sees that the local AS number matches the AS number in the tag, it will ignore this LSA.

router ospf 1 vrf VPN_A
 domain-tag 777


router ospf 1 vrf VPN_A
 redistribute bgp 100 subnets tag 777


Situation/problem description: when there is a backup link between sites (backdoor), this link will be always used for inter-site traffic because intra-area routes (LSAs received via backdoor) are preferred to the inter-area (LSA 3 received from PE).
The solution to this problem is the usage of OSPF sham-links = special tunnel similar to virtual-links between PE routers and configured in the same area as the PEs.

Sham-links have the following characteristics:

  • OSPF adjacency established via MPLS cloud
  • routes in the OSPF database are now seen as intra-area (even though they are received via the super-backbone)
  • the information across the sham-link is ONLY used for SPF calculations – the actual forwarding is being done based on the info learned via MP-BGP

One last note about sham-links: sham endpoints IP addresses should be advertised into the VRF by means other than OSPF (commonly via BGP) – known before creating the sham-links.

I hope you did not fell asleep reading this long post. At least the drawings helped you do a faster “reading”.