How does ecmp work

It could cause the packets to arrive out of order if there's a longer delay on one path than the other. It can also cause some applications to fail. Thus, a single flow always goes over the same path. Observe the updates being sent and received. What kind of traffic is used unicast, broadcast or multicast?

The updates are being sent on the broadcast address All hosts on the subnet must process the packets. R1 RIP: sending v1 update to The updates are being sent on the RIPv2 multicast address Only RIPv2 routers will process the packets beyond layer 3. Check that RIP routes have been added to R1 and it has a route to every subnet in the lab.

Why are there two routes to the Due to less congestion it would also help in reducing the time taken in data transfer. Also with use of this algorithm network performance of shuffle phase data transfers in Hadoop MapReduce can be enhanced. Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Context Data center networks every so often use compactly interconnected topologies to deliver high bandwidth for internal data exchange.

This blog explains hash based ecmp load balancing algorithm. Multiple best path selection algorithm When the first packet of a flow arrives on source switch it forwards the packet to SDN controller as switch does not have rules that match with it. The Controller looks up in this dictionary to get the destination switch If destination MAC address entry is updated : It gets the respective switch and the port on which destination is connected.

If source switch and destination switch both are same then controller simply installs the rule to forward the packet on out port as there is only one possible path to reach the destination.

If source switch and destination switch both are different we try to get all possible shortest paths to reach the destination using Dijkstra algorithm. We did slight modification in Dijkstra algorithm to get multiple shortest paths between source and destination instead of a single path. Topology information is represented as non directional graph and stored as an adjacency list.

All edges have equal cost. This algorithm computes the weight of the path where weight of the path is sum of weight of constituent edges and returns paths with minimum weightage. This gives us all possible shortest path to reach the destination. Here is the algorithm to compute all shortest path from source to destination. We populate dictionary with this entry so that next time when packet for that destination arrives instead of flooding all ports we can just send to the appropriate port.

This is based on backward learning concept. Next packet in the flow will follow the above mentioned logic and ECMP forwarding rule will be installed for the flow. However, receiving packets out of order makes TCP think there was packet loss, so it retransmits packets and slows down its transmission rate.

To avoid such issues, routers and switches work hard to make sure that all packets that belong to the same TCP session or UDP flow are transmitted over the same link. A more granular way to perform load balancing is based on the 3-tuple: the protocol number in the IP header i.

This works better than just using the MAC addresses to determine which packets should be transmitted over the same link. Routers and switches implementing ECMP calculate a hash function over these fields and then use part of the resulting hash value to select the link to transmit the packet over. See RFC As the fields in the 5-tuple are the same for all packets belonging to the same session and thus the hash is the same, all packets belonging to the same session end up using the same link.

This works well, but in practice, it can still take as many as a thousand TCP sessions before all the links are utilized equally. However, we have been getting ahead of ourselves. Before the ECMP algorithm can distribute packets over parallel links, routing protocols such as BGP must first be convinced to use multiple links in parallel.

It has been available since the s. In , the IEEE standardized a very similar mechanism under the catchy name As far as the IP layer is concerned, the bundled ports are a single interface with a single IP address. An alternative is to configure each port with its own IP subnet, but then still run a single BGP session over the collection of ports.

That works as follows:.