Cisco – Why have all the lab routers set their OSPF priority to 0

OSPF Backbone

Why is area 0 the backbone area in OSPF? Why must all other areas connect to it?

This is explained very well in RFC 3509, Section 1.2¹:

1.2 Motivation

In OSPF domains the area topology is restricted so that there must be a backbone area (area 0) and all other areas must have either physical or virtual connections to the backbone. The reason for this star-like topology is that OSPF inter-area routing uses the distance-vector approach and a strict area hierarchy permits avoidance of the "counting to infinity" problem. OSPF prevents inter-area routing loops by implementing a split-horizon mechanism, allowing ABRs to inject into the backbone only Summary-LSAs derived from the intra-area routes, and limiting ABRs' SPF calculation to consider only Summary-LSAs in the backbone area's link-state database.

OSPF is usually considered a link-state protocol. What some people miss is that OSPF uses both link-state protocol and distance-vector protocol algorithms.

• Routes within the backbone, or a non-backbone area are computed as a link-state protocol does (ref Dijkstra's algorithm).
• When OSPF must carry non-backbone routes through the backbone, it uses some distance-vector behavior (i.e. parts of the Bellman Ford algorithm) to propagate Type3 LSA metrics into non-backbone areas.

Simple example of OSPF's distance-vector behavior:

<-- Area 5 --><-- Area 0 --><--           Area 4           -->

R5-----------R1-----------R2------------R3---------------------R4
     Cost 3      Cost 5        Cost 7            Cost 12

               LSA-->          LSA-->
               Type3 LSA       Type3 LSA
               {From R1}       {From R2}
               R5 cost is 3    R5 cost is 8

Consider what happens to a /32 Loopback route for R5.

1. R5 sends a Type1 LSA containing the /32 Loopback
2. R1 (Area 5 ABR), is connected to Area 0; it translates the Type1 LSA into a Type3 LSA with a cost of 3.
3. R2 (Area 4 ABR) receives R1's Type3 LSA (metric 3) and changes the metric to R5's Loopback, based on R2's cost to R1. Now R2's Type3 LSA for R5 has a cost of 8. This is the distance-vector behavior I mentioned above.

Requiring all non-backbone routes to go through the backbone is a loop-prevention mechanism.

Connecting non-backbone OSPF areas at an ABR

If 2 areas aren't connected through area 0 (discontiguous), how does OSPF behaving as a link state protocol increase the possibility of routing loops?

As we saw above, OSPF uses distance-vector behavior to send routes through the Area 0 backbone. Distance-vector protocols have well-known limits, such as the count-to-infinity problem. OSPF would be vulnerable to the same issues, if we didn't have boundaries on its behavior.

¹_{RFC 3509 describes Cisco IOS's ABR behavior}

but what I don't see are labels for the 192.168.100.1 that is going to go over the P routers. the BGP points to 8.1.1.1 and that has a label - If you have configured PE1 as 8.1.1.1 with next-hop-self then that is how the routes should look.

I suggesst you read more about BGP behaviour for MPBGP sessions and sending labeled updates for IPV4 unicast and VPNV4 etc.