IBGP source address

This concept of “update-source loopback “ is leaving me in the dark. Below is the excerpt from the BSCI cert guide. The scenario here is that the three routers are meshed and running IBGP and rip is running as well between all three. BGP states that if the peering is established between the routers using interface of the next connected router, the router is not reachable if the link between them goes down. Solution suggested is to establish peering using loopback instead. It’s also mentioned that the routers advertise loopbacks using IGP. Ok now here’s the problem if the routes are advertised using IGP then establishing peering through loopback is irrelevant since IGP is running between all three and it will automatically fall back to the alternate route regardless of the peering relationship.

Topology has three fully meshed routers running IGP and IBGP

I wish I could post a picture but this site has its limitations. The example is in the BSCI cert guide 4th edition page 418 under the heading ‘Source Address’

What am I overlooking here? please someone explain.
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Comments

marlon23

I didnt clearly understand your question, so please correct me if I dont answer what you asked for.

When you have peering on for example ethernet interface IP between directly connected peers, and interface is going down, then also route is withdrawn from routing table (becouse interface is down/down or up/down). Route is also not propagated through IGP anymore (Not reachable as well). So peering is going down.
When there is a switched path between (VLAN or something) then if switch path is broken but interfaces are up/up, IGP will fix this as you mentioned.

EdTheLad

singh8281 wrote:

This concept of “update-source loopback “ is leaving me in the dark. Below is the excerpt from the BSCI cert guide. The scenario here is that the three routers are meshed and running IBGP and rip is running as well between all three. BGP states that if the peering is established between the routers using interface of the next connected router, the router is not reachable if the link between them goes down. Solution suggested is to establish peering using loopback instead. It’s also mentioned that the routers advertise loopbacks using IGP. Ok now here’s the problem if the routes are advertised using IGP then establishing peering through loopback is irrelevant since IGP is running between all three and it will automatically fall back to the alternate route regardless of the peering relationship.

Topology has three fully meshed routers running IGP and IBGP

I wish I could post a picture but this site has its limitations. The example is in the BSCI cert guide 4th edition page 418 under the heading ‘Source Address’

What am I overlooking here? please someone explain.
[/img][/url]

Ok, so you have a mesh of 3 routers running rip & ibgp.Rip is used to exchange local routing information, ibgp is used to external route information.
RouterA has two physical links, one connected to RouterB and the other to RouterC.On RouterA i will define two ibgp neighbors, to do this i specify the farend interface ip address in the neighbor statements.
If the link between RTA and RTB fails, the ip associated with these links is no longer valid and the ibgp neighbor will fail, even though we have a backup path through RTC, it will not use it as the destination ip address is down.
If you peer to a loopback address and the physical link goes down, that loopback address is still available via RTC, so the ibgp neighborship will stay up.

singh8281

Thanks Marlon and ED.

ED, What you’re saying is there’s another router called D in a different AS and it is peered to A and when the link between A to B goes down, the traffic from B wouldn’t find its way to D via alternate route if next interfaces are defined in the neighbor command?.

ixg123

It's all quite simple really. If you don't define the update source then your router will advertise the interface closest to the destination. To form the BGP neighborship you need to include that address in your config and so it works perfectly if you've only a single path:

A 10.1.1.0/30 B

router bgp 123
neighbor 10.1.1.2 remote-as 234

However, if you've got multiple paths such as within your AS, or by physically having multiple interfaces, then you lose out as you can only maintain the neighborship to one ... and if that path fails then your BGP relationship also fails, even though the neighbor may still be reachable through another interface:

10.1.1.0/30
A B
10.2.2.0/30

Manually defining the update source as an interface that never goes down (such as a loopback) then the neighbor relationship will remain up for as long as the neighbours address is reachable via the IGP. Eg

interface loopback 0
ip address 1.1.1.1 255.255.255.255

ip route 2.2.2.2 255.255.255.255 10.1.1.2
ip route 2.2.2.2 255.255.255.255 10.2.2.2 240

router bgp 123
neighbor 2.2.2.2 remote-as 234
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 update-source loopback0

... obviously IBGP would be a lot prettier if you're advertising the loopback addresses within the IGP!

singh8281

ixg123 wrote:

However, if you've got multiple paths such as within your AS, or by physically having multiple interfaces, then you lose out as you can only maintain the neighborship to one ... and if that path fails then your BGP relationship also fails, even though the neighbor may still be reachable through another interface:

10.1.1.0/30
A B
10.2.2.0/30

I understand that but if IGP is running on all routers within the AS then wouldn't IGP detect the broken link and automatically routes the packets via alternate path (internal only).

networker050184

No, if the interface is down the route would not be advertised through the IGP therefore the other routers would not know about the route.

darkuser

simply stated

you would tie a service to a loopback interface when you
want a service
ie routing protocol , snmp , syslog
ebgp, ibgp

NOT tied to (a) physical interface.

http://www.routergod.com/?p=40

miller811

darkuser wrote:

simply stated

http://www.routergod.com/?p=40

Funny stuff. Thanks for the laugh.