Let's consider a typical star topology, there is a head office R1 and
branches R2 R3 R4 are connected to it via one WAN link. in this case OSPF
fully provides the required functionality.
but now many companies already require increased fault tolerance and
connect 2 or more WAN links to each branch as in the figure. how does
OSPF work in this case? by default, it loads both channels equally if
they are equal. or if the channels are different then OSPF will use one
channel with a higher bandwidth and not load the second channel, the
second channel will be used only if the first one fails.
some companies stop there. or they buy two identical channels from two
different providers or use one but pay for both.
the option to buy, for example, 10 Mbit MPLS and 50 Mbit Internet
disappears.
solution can be PBR, SD-WAN, EIGRP and BGP
PBR can be an output, but the configuration becomes more complicated in
the case of multiple interfaces on the LAN as shown.
PBR will have to be applyed all LAN interfaces
about BGP, people usually immediately say a learned phrase from a
textbook - it's EGP and has poor convergence.
but look again at the picture. for example two links between R1 and R2
in this case, poor BGP convergence does not matter.
if one link fails, then both routers R1 and R2 instantly redirect traffic
to the remaining link.
yes, there is poor convergence, and compared to OSPF and the R4 router
will receive the update with some delay, but this will not play a
fundamental role since there is a second working link there.
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