Hi everybody.
Please consider the following BGP scenario with the default settings i.e AS path will be used to choose the best path.
Customer Network(AS1)---link1----ISP1(AS2)----------ISP3(AS3)--------ISP4(AS4)--PC
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| |
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|---link2----ISp5(AS)---------------------------------------------------
ISP1 has been assigned the block 12.12.0.0/16 by IANA. Later ISP1 has assigned the 12.12.12/24 to its customer.
and creates static route 12.12.12.0 pointig to customer network over link 1 i.e no bgp peering between customer and isp1. Customer has created a static default route pointing to ISP2 over link 1.
Customer connected to isp2 over linlk 2 for redundancy. This time Customer is peering with ISP2 using BGP. Customer is advertising 12.12.12.0/24 to ISP5.
Please focus on the traffic flowing to customer's network.
ISP4's router peering with isp3 and isp 5, will have two bgp prefixes in its routing table ( know this is not how routing table looks like but just for simplicity)
12.12.0.0/16 via ISP3 ( AS 1,2,3)
12.12.12.0/24 via ISP5( AS 1,5)
Let assume we issue a ping on PC attached to ISP4 as:
ping 12.12.12.1
ISP4's router checks its routing table and will match the longest match i.e 12.12.12.0/24 sends the packet over to isp5 which delivers the packet to host on 12.12.12.0/24 network ( customer's network).
The only time ISP2 will direct traffic to 12.12.12.0/24 if the customer link to isp5 goes down.
Is my understanding correct ?
Even If ISP1 starts suppressing summary 12.12.0.0/16 and starts advertising 12.12.12.0/24, traffic from PC going to customer network still uses isp5 because of shorter ' AS path".
Is my understanding correct?
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PLease Consider the same topology but this time Customer is not peering with ISP5 either. ISP5 has created a static route pointing to customer network over link 2. Customer has created a static default route pointing to isp5 over link 2. ISP5 is advertsising 12.12.12.0/24 to ISP4. and ISP2 is advetising summarised prefix 12.12.0.0/16 only.
I expect traffic from PC connected to ISP4 to Customer network should travel the same path i.e via isp5 to customer's network because ISP4 matches the uses the more specific prefix i.e 12.12.12.0/24 in its routing table even though it contains longer AS path as compared to 12.12.0.016.
Is my understanding correct?
