What if you need to traffic engineer how detour tunnels are established in your topology? Since fast reroute provides a one-to-one protection mechanism 1, turns out you can use affinity (link-coloring) to achieve that. Let’s analyze a particular use case where specifying affinity constraints in the detour LSP might be useful for your MPLS network design.
In this topology, composed of DATACOM Ethernet Switches 2, there’s one LSP from
SW207_TO_SW194_1. As you can see in Figure 1, the primary path of this LSP goes through
DM4004_196, which is a PLR, since fast-reroute protection was requested by the ingress PE (
DM4001_207). However, note that there are two potential alternate paths that
DM4004_196's detour could be established downstream. For instance, these two paths are represented in the topology
SW196's detour option #1 (red dashed line) and
SW196's detour option #2 (orange dashed line).
Let’s assume you explicitly want to enforce that the detour LSP of
DM4004_196 is established as represented by
option #2. In this case, you can simply color the undesired link as red
affinity 2 and exclude it when requesting fast-reroute protection.
When configuring affinity (link-coloring) there are always multiple approaches, you can either include or exclude links with logical operators (include-any, include-all, exclude-any). Depending on what you’re trying to accomplish, you have to figure out which approach is more suitable to meet your requirement.
I’ll focus on the main configuration parts related to fast-reroute.
DM4001_207 has a explicit-path to enforce the LSP primary path as illustrated in Figure 1. Plus, the command
tunnel mpls traffic-eng fast-reroute one-to-one affinity 0 0 2 request fast-reroute protection and excludes any link that has the affinity value 2.
rsvp signalling link attributes 2 sets the affinity as 2 on this RSVP-enabled VLAN.
DM4004_196, affinity value 2 is set on this VLAN.
Let’s first verify how detour is established without requesting any affinity constraints in the detour.
As you can see bellow,
DM4004_196's (PLR - Point of Local Repair) detour got established over the shortest path, since no additional constraints were specified.
Now, by explicitly requesting that any downstream detour excludes affinity value 2.
As shown bellow, note that the outgoing VLAN of
SW207_TO_SW194_1's detour on
DM4004_196 (PLR) is
VLAN4091 as opposed to
VLAN 4084 when compared to the previous example.
RSVP is a swiss-army knife when it comes to MPLS traffic engineering. If you need to traffic engineer how detour tunnels are established, affinity (link-coloring) is the way to go.