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07-28-2019 06:57 AM
I understand that in order for a router to advertise networks to another router over eBGP you need a network command for the subnet and mask. In addition, it has to be an exact match in the routing table as well. However, I set up a lab that has a router advertising a subnet to an eBGP peer without a network statement but I think I know why but would like someone's else's thoughts on this as well. The edge router "a" peers with router "b" over eBGP. Edge router "a" also peers with router "c" over iBGP. Router "c" is advertising 10.250.3.0/24 to edge router "a" over iBGP. So I see 10.250.3.0/24 on edge router "a" but I do not have a network statement to advertise it to router "b". Is the reason that router "b" is receiving that route is because anytime a router learns about a route from an iBGP neighbor it automatically advertises it to its' peers? I ask because when I create a loopback interface directly on router "a" and no network command to advertise it, router "b" does not see that route as expected.
Thank you
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07-28-2019 10:01 AM

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07-28-2019 11:24 AM
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07-29-2019 12:46 AM - edited 07-29-2019 12:53 AM
Hello Vi2,
synchronization is an old concept coming from the early days of BGP when BGP was running only on edge / border routers and internal routers were running only an IGP.
This was before introduction of route reflector servers and BGP confederations that solve the full mesh iBGP issue.
In that scenario border routers were expected to redistribute eBGP routes into IGP.
This is not feasible anymore given the very big size of a full internet BGP table ( 750,000 routes !!!) no IGP can support so many routes.
The modern approach is to run IGP in parallel with iBGP. The IGP provides connectivity between loopback addresses. And iBGP is used to propagate all service related prefixes.
Now, the new default value is no sync with sync disabled. This is why at the beginning the iBGP route was accepted and advertised.
The reason for this change is that most of the users had to disable sync and auto-summary on each device
no sync
no auto-summary
Both commands are now new defaults for BGP.
>> I also saw the synchronization should be enabled when an AS is passing traffic from one AS to another acting as a transit service
This is not true anymore for the reasons explained above and because for example with the introduction of MPLS internal P routers do not need to perform IP routing lookup but only perform MPLS label swap.
Hope to help
Giuseppe
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07-28-2019 08:07 AM
Not always. It's how you get prefixes from the route table into BGP.
For more information see:
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/19345-bgp-noad.html#intro

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07-28-2019 08:50 AM
Thank you
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07-28-2019 08:51 AM - edited 07-28-2019 08:57 AM
Yes, and it works the other way too. note: iBGP will not advertise eBGP-learned routes to other iBGPs.
there are several other ways to advertise networks/prefixes into BGP:
1. network x mask z
2. network x without mask will work if u have auto-summary enabled; also,
3. redistribution others or connected is my favorite.
4. aggregate-summary is tricky but works

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07-28-2019 09:00 AM
Thank you

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07-28-2019 09:22 AM

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07-28-2019 10:00 AM
1) Edge router "a" knows how to reach iBGP learned route via OSPF. Synchronization is enabled and no network statement configured to advertise iBGP learned route under the BGP instance. Result - edge router does not announce the iBGP learned route to the eBGP peer. - Announces the route to EBGP peer once the network statement is configured.
2)Same scenario above except I disable synchronization then clear all BGP peerings on the edge router "a". Edge router "a" now announces the iBGP learned route to the eBGP peer.
3)Did the same test without OSPF configured because the iBGP peer was directly connected and router "a" still knew about the iBGP learned route.

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07-28-2019 10:01 AM

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07-28-2019 11:24 AM
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07-29-2019 12:46 AM - edited 07-29-2019 12:53 AM
Hello Vi2,
synchronization is an old concept coming from the early days of BGP when BGP was running only on edge / border routers and internal routers were running only an IGP.
This was before introduction of route reflector servers and BGP confederations that solve the full mesh iBGP issue.
In that scenario border routers were expected to redistribute eBGP routes into IGP.
This is not feasible anymore given the very big size of a full internet BGP table ( 750,000 routes !!!) no IGP can support so many routes.
The modern approach is to run IGP in parallel with iBGP. The IGP provides connectivity between loopback addresses. And iBGP is used to propagate all service related prefixes.
Now, the new default value is no sync with sync disabled. This is why at the beginning the iBGP route was accepted and advertised.
The reason for this change is that most of the users had to disable sync and auto-summary on each device
no sync
no auto-summary
Both commands are now new defaults for BGP.
>> I also saw the synchronization should be enabled when an AS is passing traffic from one AS to another acting as a transit service
This is not true anymore for the reasons explained above and because for example with the introduction of MPLS internal P routers do not need to perform IP routing lookup but only perform MPLS label swap.
Hope to help
Giuseppe

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07-29-2019 04:03 AM
