When considering system design using ESRP, we recommend using a direct link.
A direct link can provide a more direct routed path, if the ESRP switches are routing and supporting multiple VLANs where the master/slave configuration is split such that one switch is master for some VLANs and a second switch is master for other VLANs. The direct link can contain a unique router-to-router VLAN/subnet, so that the most direct routed path between two VLANs with different master switches uses a direct link, instead of forwarding traffic through another set of connected routers.
A direct link can be used as a highly reliable method to exchange ESRP hello messages, so that the possibility of having multiple masters for the same VLAN is lessened if all downstream Layer 2 switches fail.
A direct link is necessary for the ESRP host attach option. The direct link is used to provide Layer 2 forwarding services through an ESRP slave switch.
Direct links may contain a router-to-router VLAN, along with other VLANs participating in an ESRP domain. If multiple VLANs are used on the direct links, use 802.1Q tagging. The direct links may be aggregated into a load-shared group, if desired. If multiple ESRP domains share a host port, each VLAN must be in a different ESRP group.