Summary

In large networks, sometimes there is a need to split the network into multiple domains, for reasons such as:

• Better scalability
• Isolating network segments from local IGP instability
• Connecting different IGP protocols from various acquired networks

Typically such designs include BGP-LU to connect different IGP domains. Another option is route redistribution between domains, but it is rarely used because it is prone to errors and also defeats the purpose of IGP domain isolation.

Multi-domain Traffic Engineering

Consider the topology:

In this example, Traffic Dictator has visibility of all 3 IGP domains. This way it can compute an end-to-end SR-TE policy, including link affinity and bandwidth constraints.

Note the label stack: Traffic Dictator doesn’t know whether there is route redistribution or not (in most designs there will be no redistribution), so it will use SID of every border router in the segment list.

Anycast SID

In this design, anycast SID works great as it provides load balancing, redundancy, saves TCAM space on routers and further isolates routing from local IGP instability.

Consider the same topology, bur ABR1 and  ABR2 share an anycast SID, and so do ABR3 and ABR4.

Traffic Dictator will try to use anycast SID in SR-TE policies whenever possible, so traffic will be load balanced across both ABR pairs.

Anycast SID is a preferred design method with Segment Routing and is a significant advantage over RSVP-TE.

BGP-only links and Inter-AS policies

Consider the following topology:

Unlike the previous example, now there are BGP-only links between different IGP domains. Traffic Dictator leverages EPE functionality to build paths in such topologies. It will use the EPE label of the link between ASBR1 and ASBR3 and then calculate the path within the IGP topology.

Such policies can also use affinity and bandwidth constraints.

Other Large Scale designs

While Seamless MPLS and Inter-AS VPN are the most common network designs with different IGP domains, there are more possibilities to introduce hierarchy in routing.

For example, IS-IS area proxy allows to hide L1 “islands” within a large L2 area, so that each L1 area will be represented as just one node.

Large Scale Interconnect allows to scale the network to millions of routers, by relying on a controller to provide even basic network connectivity, not just traffic engineering.

Further information

Check out Traffic Dictator White Paper on the Products page or email info@vegvisir.ie with any questions.