Doubling the core's routing table size— As was discussed when looking at dual homing distribution layer devices to the core, adding the link between Router A and Router B in Figure 3-9 doubles the size of the core routing table because Router D now has paths through both Router A and Router C to the 172.16.0.0/16 network.
Possible use of the redundant path for traffic transiting the core— If the link between Router D and Router C fails in Figure 3-9, it's possible that Router D could begin forwarding traffic to Router A, which is destined someplace beyond Router C, rather than forwarding the traffic to Router E. Router A and Router B can be effectively drawn into a core routing role. Preferring the redundant link to the core path— Distribution layer routers may end up preferring the redundant path through the distribution layer, rather than the path through the core. In Figure 3-9, it's possible that Router B would prefer the redundant link to the path through the core to reach the 172.16.0.0/16 network.
Routing information leaks— Routing information will leak between the distribution layer branches because the routers in one branch will need to be able to advertise the destinations in another branch as reachable through the redundant link. In Figure 3-9, this can result in instabilities occurring beyond Router A and spreading through all the distribution layer branches, rather than being contained. It can also slow convergence time because routing tables in the distribution layer routers become larger.
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