IB Router

An InfiniBand (IB) router enables communication between two or more IB subnets. This allows the network to scale beyond 40,000 end-ports, supports separation and fault resilience across subnet "islands," and facilitates connectivity between subnets with different topologies.

Forwarding between subnets is performed using Global Routing Header (GRH) lookup. The router's basic functionality includes:

• Removing the existing Layer 2 Local Routing Header (LRH)

• Performing routing

• Looking up the routing table using the GID extracted from the GRH

• Constructing a new LRH for the destination, based on the routing table

The DLID in the new LRH is derived using a simplified GID-to-LID mapping, where LID = the lower 16 bits of the GID. This approach eliminates the need for ARP queries or lookups.

Local Unicast GID Format

To enable this mechanism, the Subnet Manager (SM) assigns an alias GID to each host. The alias GID format is:

            

            
alias GID = {subnet_prefix[127:64], reserved[63:16], LID[15:0]}
        

Hosts must use these alias GIDs when transmitting traffic to peers located in remote subnets.

Host-to-Host IB Router Unicast Flow