If you don't go into some advanced stuff, then ROS will route through interfaces with appropriate IP address set. E.g. if you have bridge1 with sfp-sfpplus1,sfp-sfpplus2 and sfp-sfpplus3, bridge2 with sfp-sfpplus4,sfp-sfpplus5 and sfp-sfpplus6 and ether1 (for OOB management), then you will probably have configuration something like
/interface bridge
add admin-mac=<some localy administered MAC here> auto-mac=no name=bridge1
add admin-mac=<another localy administered MAC here> auto-mac=no name=bridge2
/interface bridge port
add bridge=bridge1 interface=sfpsfpplus1
add bridge=bridge1 interface=sfpsfpplus2
add bridge=bridge1 interface=sfpsfpplus3
add bridge=bridge2 interface=sfpsfpplus4
add bridge=bridge2 interface=sfpsfpplus5
add bridge=bridge2 interface=sfpsfpplus6
/ip address
add interface=bridge1 address=10.1.2.3/24
add interface=bridge2 address=10.2.3.4/24
add interface=ether1 address=10.13.2.1/24
/ip route
add gateway=<default router IP address here>
/ip dns
set servers=10.13.2.200,10.13.2.201
This informs router that it's got 10.1.2.0/24 subnet directly attached to brdige1 (which is composed of SFP+ ports 1-3 ... so it bridges traffic between these ports without considering L3), then it's got 10.2.3.0/24 subnet directly attached to bridge2 (composed of SFP+ ports 4-6) and it's got 10.13.2.0/24 subnet directly attached to ether1. When it receives an IP packet with destination address in one of these subnets, it'll use appropriate interface (bridge1, bridge2 or ether1) to transmit the packet. When the destination IP address is not in one of these subnets, it'll use appropriate gateway. In example above it's only a single gateway defined and without destination subnet set it's implicitly using 0.0.0.0/0 (a.k.a. default). If there were multiple gateways defined, it would use gateway with the longest subnet mask defined that matches destination address of a packet.
And this routing table will be "global" for entire router (i.e. all L3 interfaces).
The
/ip dns settings are unrelated: ROS has (a simple) relaying DNS server built in and the settings simply configure upstream DNS servers to use. The built-in DNS server is not a full-featured recursive resolver (which would be able to resolve query starting from top-level servers), it needs so called "forwarders". Whenever router needs to resolve something (or receives request from clients when
allow-remote-requests=yes), it will contact one of configured upstream DNS servers ... and will use the usual routing decision logic. So in case of CRS configuration, CRS will use one of 10.13.2.200, 10.13.2.201 DNS servers and will use default gateway as immediate next hop. Router at 10.13.2.1 will route IP packets according to its own routing configuration.
These settings don't affect packets being outed - e.g. if a LAN host tries to contact DNS server at 8.8.8.8, router will not redirect those packets just because it's got DNS server addresses set.