Chapter 5 Introducing SDN Control in MPLS Networks - High ...

191This latter point can best be explained with Fig. 5.7. Assume packet-flows from R1 toR3 are routed via R5R6R2. When the TE tunnel is created between R5 and R2, theflows need to be re-directed through the tunnel, as the tunnel is now a single-hop pathbetween R5 and R2 (as opposed to a minimum of 2 hops for every other path † ). Note thatFig. 5.7a shows the tunnel ‘conceptually’ as a single-hop between R5 and R2. Butphysically the tunnel is routed over the IP links in the network. For example, Fig. 5.9bshows one possible path the LSP takes in the IP network. Despite the number of hops inthe physical tunnel path, from the perspective of routing packets in the IP network, thetunnel is still a single hop.(a)Fig. 5.7 Auto-Route: re-routing packets into a TE-LSP (tunnel). Tunnel is shown:(a) conceptually (b) physically routed over the IP links.However this perspective does not materialize directly in the map over which packetsare routed. The tunnels are not exposed to the distributed routing protocol i.e. routingadjacencies are not created across the tunnels. And so the map that the routing protocolcreates in each router does not include the tunnels. The primary reason for not exposingtunnels to OSPF or IS-IS is the same O(N 2 ) scaling issue discussed in the IP-over-ATMcontext in the previous chapter * . If the routing-protocol were exposed to the tunnels, thenthey would have to treat them as links and carry state for them, which is a scalingproblem – as the number of links increases, more state needs to be disseminated; andwhen failures happen lots of control messages are generated, which in-turn increasesrouter-CPU processing loads and could cause routing protocol instability.(b)† Assuming all links have the same IGP cost and the tunnel cost is that of a single-hop.* See Sec. 4.2.1 under SDN based Unified Control Plane