Question

Consider 5 sequentially connected switches labeled switch 1, 2, 3, 4 and 5, and each switch has 4
hosts connected to it (total of 20 hosts). If a host on switch #1 made a virtual circuit connection to
every other host on the network, how many rows would be in each switch's virtual circuit table?

Answer 1

There will be 2 rows for each switch's virtual circuit table.

There are several things to note about virtual circuit switching:

• Since host 4 has to wait for the connection request to reach the far side of the network and return before it can send its first data packet, there is at least one round-trip time (RTT) of delay before data is sent.
• While the connection request contains the full address for host 2(which might be quite large, being a global identifier on the network), each data packet contains only a small identifier, which is only unique on one link. Thus, the per-packet overhead caused by the header is reduced relative to the datagram model. More importantly, the lookup is fast because the virtual circuit number can be treated as an index into a table rather than as a key that has to be looked up.
• If a switch or a link in a connection fails, the connection is broken and a new one will need to be established. Also, the old one needs to be torn down to free up table storage space in the switches.
• The issue of how a switch decides which link to forward the connection request on has been glossed over. In essence, this is the same problem as building up the forwarding table for datagram forwarding, which requires some sort of routing algorithm. Routing is described in a later section, and the algorithms described there are generally applicable to routing setup requests as well as datagrams.