gigabit v/s serial

[ncnaveen_arasu]

                   Dear team, whenever we terminate the wan connectivity to serial links when link is down we can able to see protocol down in sh int se0/0

command so if back up configured it will failover.  but in case if we terminate to fastethetnet or gigabit interface it will not go down even there is a link failure so untill it goes down it will not switch over to secondary path. Why it will happen like this. please clarify.

Thanks in advance.

Regards,

Naveen

[ncnaveen_arasu]

HI friendz,

Why no response for my request, Please reply and clarify the same.

[shillings]

E1 circuits have their own end-to-end keepalive mechanism. Ethernet doesn't have this once you start spanning multiple devices, so you have to rely upon your own method of end-to-end failure detection. Options include tracking learned routes via a routing protocol, such as BGP, or IP SLA echo.

[ncnaveen_arasu]

How to track through BGP, i have little idea about IP echo. Can you share how to track through BGP.

Regards,

Naveen

[shillings]

Hi Naveen - There are many variations for Internet access circuits, but typically your ISP will advertise a default route to you and you will advertise your ISP allocated public IPs back to them.

If you're using two ISPs with Provider Independent IPs, then each ISP will also send you their own routes. No point routing via ISP A to reach a direct customer of ISP B.

Best to apply inbound and outbound route filtering to ensure you don't learn any routes you shouldn't and don't advertise any routes you shouldn't.

If a link fails, then the routes advertised on that link are withdrawn. This will stop both inbound and outbound routing to the failed circuit, although there will be a brief 'blackhole' period whilst BGP is updating.

There is more to it, including the ability to allocate a preference level to received routes, but that is some of the basics.

[ncnaveen_arasu]

HI Shillings,

Thnaks for the very useful information.

Do you have any link which breifly explains the keepalive mechanisms of ether and serial/E1 links.

Regards,

Naveen

[shillings]

Been a few years since I worked at an ISP, so things may have changed a bit. Haven't had much time to look, but Frame Relay uses LMI: http://www.ciscopress.com/articles/article.asp?p=170741&seqNum=3

If the ISP is just using HDLC, then this might help: http://en.wikipedia.org/wiki/Cisco_HDLC#SLARP_Keep-Alive_frame_structure