link-to-path-aggregation.body

Distribution of all links available in the global Internet is

certainly possible; however, it not desirable from a scaling and

privacy point of view. Therefore, an implementation may support a

link to path aggregation. Rather than advertising all specific links

of a domain, an ASBR may advertise an "aggregate link" between a non-

adjacent pair of nodes. The "aggregate link" represents the

aggregated set of link properties between a pair of non-adjacent

nodes. The actual methods to compute the path properties (of

bandwidth, metric, etc.) are outside the scope of this document. The

decision whether to advertise all specific links or aggregated links

is an operator's policy choice. To highlight the varying levels of

exposure, the following deployment examples are discussed.

Consider Figure 33. Both AS1 and AS2 operators want to protect their

inter-AS {R1, R3}, {R2, R4} links using RSVP-FRR LSPs. If R1 wants

to compute its link-protection LSP to R3, it needs to "see" an

alternate path to R3. Therefore, the AS2 operator exposes its

topology. All BGP-TE-enabled routers in AS1 "see" the full topology

of AS2 and therefore can compute a backup path. Note that the

computing router decides if the direct link between {R3, R4} or the

{R4, R5, R3} path is used.

            |   :   | \
            |   :   |  R5
            |   :   | /

         Figure 33: No Link Aggregation

The brief difference between the "no-link aggregation" example and

this example is that no specific link gets exposed. Consider

Figure 34. The only link that gets advertised by AS2 is an

"aggregate" link between R3 and R4. This is enough to tell AS1 that

there is a backup path. However, the actual links being used are

hidden from the topology.

            |   :   |
            |   :   |
            |   :   |

         Figure 34: ASBR Link Aggregation

Service providers in control of multiple ASes may even decide to not

expose their internal inter-AS links. Consider Figure 35. AS3 is

modeled as a single node that connects to the border routers of the

aggregated domain.

            |   :         : \
            |   :         :   vR0
            |   :         : /

         Figure 35: Multi-AS Aggregation