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2 links on single router

Let say I have one single router which connects to internet via two leased lines, with 2 default routes being defined as follows:

ip route s0

ip route s1

Under normal circumstances, router will perform per-destination load balancing between the 2 lines. If the leased line which attaches to s0 fails, will router send all the traffic to s1 or still load balancing between the 2 lines, causing first destination unreachable, second destination reachable, third destination unreachable......??

Thanks a lot.


Re: 2 links on single router

If the link failure is detected (such as on a point-to-point T1 with keepalives) the dead route will be removed from the routing table.

If the link failure is not detected (common problem on frame relay unless end-to-end keepalives are used) then you will indeed be sending packets into a black hole.

Note that if you are using frame relay or any other link protocol which cannot reliably detect end-to-end link failures, you can use BGP to do the failure detection. When used this way with multiple links to a single ISP, there is minimal overhead and no need to pay for a globally unique ASN. It also ensures that the ISP will send all packets down good links. Keep in mind that the link failure must be detected by BOTH ends to keep the data flowing.

Good luck and have fun!

Vincent C Jones

Cisco Employee

Re: 2 links on single router

In case s0 fails, the route to s0 is removed and all traffic is forwarded to s1.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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