|------------1 same cost----|
Router 1|                           | Router 2
        |------------2 Same cost----|

If we have a ecmp link between two routers using which interface a TCP connection will be formed, say be are trying to use loopback addresses as source and destination in both routers ?

Generally TCP runs over interface level or system level ?

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2 Answers 2


TCP does not choose an interface to use. It is handled by IP layer while TCP is transport layer. TCP simply tosses packets to IP layer, and IP layer selects which route and consequently which interface it will use for the session.

In general, ECMP is applied to a flow or a session level, in a given session, packets are always forwarded to the same path unless the link is broken.

A session is normally described as 5-tuple - src ip/dst ip/src mac/dst mac/tcp or udp port.


ECMP could be applied at session level or packet level it depends on functionality of Layer 3 device. Like example Palo Alto 3020 does ECMP at session level not at packet level. Means once TCP/UDP session established after the handshake the data will flow for the same destination on that ISP only, even though other link is available. Only for next session traffic will flow by other link.

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