IP SLA

Answered Question
Aug 17th, 2010

Hi All,

we are using T1 point to point with DSL backup. if T1 is down, VPN is established over DSL connection by IP SLA. everything works as expected. now we are migrating to MPLS with BGP. in BGP configuration we don't have static route in order to monitor the link, there is only neighbor command. how can I setup IP SLA on this situation? which means if MPLS is down, then uses DSL to have VPN connection to HQ.

thanks a lot

Alex

I have this problem too.
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Correct Answer by Peter Paluch about 6 years 3 months ago

Hi Alex,

Okay, according to this template, you will advertise all your directly connected and statically defined routes to your ISP. This is done so that the ISP and also the HQ are both aware of the networks on your location and can route packets to them via the MPLS cloud. Unfortunately, this does not explain whether your ISP sends you a default route via BGP or not which would slightly change the behavior. Do you know at this moment if the ISP sends you a default route via BGP?

If the ISP does send you the default route then the BGP-learned default route will have the same administrative distance as all other BGP routes, which is probably 20, as you have an external BGP peering with your ISP (your and his autonomous system numbers differ). The static default route would then need an administrative distance higher than 20 but less than 255 (the 255 means that the route is completely untrustworthy and won't be installed in the routing table).

In any case, if the ISP does not send you a default route, a different approach would need to be selected because in that case, you would always have the static default route in your routing table and by means of the redistribute static command, you would advertise the default route back to ISP which could have very unpleasant consequences

Best regards,

Peter

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Peter Paluch Tue, 08/17/2010 - 12:55

Hi Alex,

What routes will the BGP advertise from you and to you? What is going to be the purpose of the BGP in your case?

I believe that the IP SLA is not actually necessary... Assuming that losing the MPLS connectivity results in loss of the BGP-discovered routes, you can simply have static routing entries with a higher administrative distance than the BGP (i.e. floating static routes) that will point towards the DSL connection.

Would this work for you?

Best regards,

Peter

alex goshtaei Tue, 08/17/2010 - 13:58

Hi Peter,

thanks for your reply. I have this template from SP:

interface Serial0/0.100 point-to-point
ip address 172.1.1.2 255.255.255.252
frame-relay interface-dlci 100 ietf
ip routing
ip classless
!
router bgp 65057
no auto-summary
neighbor 172.1.1.1 remote-as 65000
redistribute connected
redistribute static

then there is a DSL connection for backup with static route:

ip router 0.0.0.0 0.0.0.0.0  74.1.1.1  255

so with this configuration, I don't need IP SLA? I don't know what the administrative distance is for default route advertised by BGP. do I need to ask SP?

thanks again

Alex

Correct Answer
Peter Paluch Tue, 08/17/2010 - 14:25

Hi Alex,

Okay, according to this template, you will advertise all your directly connected and statically defined routes to your ISP. This is done so that the ISP and also the HQ are both aware of the networks on your location and can route packets to them via the MPLS cloud. Unfortunately, this does not explain whether your ISP sends you a default route via BGP or not which would slightly change the behavior. Do you know at this moment if the ISP sends you a default route via BGP?

If the ISP does send you the default route then the BGP-learned default route will have the same administrative distance as all other BGP routes, which is probably 20, as you have an external BGP peering with your ISP (your and his autonomous system numbers differ). The static default route would then need an administrative distance higher than 20 but less than 255 (the 255 means that the route is completely untrustworthy and won't be installed in the routing table).

In any case, if the ISP does not send you a default route, a different approach would need to be selected because in that case, you would always have the static default route in your routing table and by means of the redistribute static command, you would advertise the default route back to ISP which could have very unpleasant consequences

Best regards,

Peter

alex goshtaei Tue, 08/17/2010 - 14:29

Peter,

very appreciate. it's 100% make sense. I'll get a hold on SP to see what they send.

thanks

Alex

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