[vc_row][vc_column][vc_column_text]Connecting to one ISP over multiple independent circuits offers protection against interrupted cables, and to some degree, against failing equipment. When communication moved to fiber, technologies such as SONET/SDH and FDDI allowed for fiber rings with built-in “protection” mechanisms.

Under normal circumstances, all data flows over the primary ring in one direction. When there is a cable cut, the stations on both sides of the cut reroute over the backup ring so all stations remain reachable. The downside of these fiber protection systems is that the capacity of the second ring remains unused. More modern systems, such Resilient Packet Rings (IEEE 802.17) allow for the full use of the available bandwidth.

However, today it’s much more common to use Ethernet, both within a datacenter and over longer distances.[/vc_column_text][vc_single_image image=”6297″ img_size=”449×525″ alignment=”center” css=”.vc_custom_1449152907588{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_single_image image=”6298″ img_size=”449×525″ alignment=”center” css=”.vc_custom_1449152917273{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_column_text]

Figure 1: A fiber ring.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text][wc_divider style=”solid” line=”single” margin_top=”” margin_bottom=””][/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]Figure 2 shows the simplest way to use two connections towards one ISP: simply have them both connect to the same router. This protects against cable failures, but the single router on the customer side is still a single point of failure.[/vc_column_text][vc_single_image image=”6305″ img_size=”full” css=”.vc_custom_1449154057580{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_column_text]

Figure 2: Two connections terminating on one router.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text][wc_divider style=”solid” line=”single” margin_top=”” margin_bottom=””][/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]Even worse is the situation in Figure 3 with a switch between the two connections and the router (perhaps because the router doesn’t have enough high speed ports) there are now two single points of failure: the router and the switch. If either of those fails, both connections go down.[/vc_column_text][vc_single_image image=”6307″ img_size=”full” css=”.vc_custom_1449154163658{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_column_text]

Figure 3: Two connections terminating on one router through a switch.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text][wc_divider style=”solid” line=”single” margin_top=”” margin_bottom=””][/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]In Figure 4, there is no longer a single point of failure: there are two routers on the ISP side as well as two routers on the customer side, with separate circuits connecting them.[/vc_column_text][vc_single_image image=”6309″ img_size=”full” css=”.vc_custom_1449154398736{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_column_text]

Figure 4: Two connections terminating on two routers.

[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text][wc_divider style=”solid” line=”single” margin_top=”” margin_bottom=””][/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]In the setup in Figure 5 switches are put in front of the routers. Through the switch, each customer router can talk to both of the ISP routers. In this setup, there is again no single point of failure. The reason some networks use this setup is that it also provides protection against the situation where router 1t on the ISP side and router 2 on the customer side both fail at the same time. In the situation in Figure 4, this would take both connections down. But in the situation in Figure 5, communication is then still possible from ISP router 2 to switch 2 to customer router 1.[/vc_column_text][vc_single_image image=”6312″ img_size=”full” css=”.vc_custom_1449154582706{padding-top: 30px !important;padding-right: 30px !important;padding-bottom: 30px !important;padding-left: 30px !important;}”][vc_column_text]

Figure 5: Two connections terminating on two routers through switches.

[/vc_column_text][vc_column_text]However, the downside of the Figure 5 setup is that it isolates the customer routers from the connections. So if the circuits go down, the routers don’t detect this and they will continue to send packets until the routing protocol that’s used (usually BGP) determines that the connection is down. This takes much longer than simply observing a link down event on a physical circuit, and all this time packets disappear into a black hole.[/vc_column_text][/vc_column][/vc_row][vc_row][vc_column][vc_column_text]

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