Calin Chiorean Mon, 08/25/2008 - 03:15
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Hello!


Yes it is possible with ebgp-multihop "x"

where x in your case should be 2 (number of the hops to the other peer side)


You have a small template above how I would do it:

--------------

R1-AS100


int lo0

ip address 1.1.1.1 255.255.255.255


router bgp 100

bgp log-neighbor-changes

neighbor 2.2.2.2 remote-as 300

neighbor 2.2.2.2 description R2

neighbor 2.2.2.2 ebgp-multihop 2

neighbor 2.2.2.2 update-source Loopback0


ip route 2.2.2.2 255.255.255.255 "next-hop-IP"

----------

R2-AS300


int lo0

ip address 2.2.2.2 255.255.255.255


router bgp 300

bgp log-neighbor-changes

neighbor 1.1.1.1 remote-as 100

neighbor 1.1.1.1 description R1

neighbor 1.1.1.1 ebgp-multihop 2

neighbor 1.1.1.1 update-source Loopback0


ip route 1.1.1.1 255.255.255.255 "next-hop-IP"


Hope it's ok for you. If you have any concern reply here.


Good luck!


Cheers,

Calin

Giuseppe Larosa Mon, 08/25/2008 - 12:10
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Hello Calin,

in this case there is another router in the middle so it needs:


neigh x.x.x.x ebgp-multihop 3 if using loopbacks



Hope to help

Giuseppe

Calin Chiorean Mon, 08/25/2008 - 12:24
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Hi Giuseppe,


I'm sure that is ebgp-multihop 2, I just tried :)

The idea, as I understand ebgp-multihop, is that this is counting the hops until destination. 1st hop is the router in the middle, the 2nd hope is bgp router's interface connected to the middle router and the 3 hop is actual the destination, and this is not count as hop.

If you want to talk more about this write to me: [email protected] . I don't want to transform this question into a flame here :)


Cheers,

Calin

Harold Ritter Mon, 08/25/2008 - 13:15
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Calin,


You are completely right that a TTL of 2 is sufficient to establish the session in this scenario. In principle, you should also be able to establish a session with a directly connected router using a TTL of 1, even if the session is establish using the loopback interface address. But in reality, establishing a session to the loopback interface address of a directly connected router requires to set the TTL to 2 on the ebgp-multihop command, unless you use the "neighbor disable-connected-check" command.


Regards

yandy_ramirez Mon, 08/25/2008 - 13:17
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you have to remember that if the router in the middle is not running BGP, unless you're doing some kind of mpls switching, most likely your packets will drop. Unless you have all the static routes possible that you need for that router in the middle. And running BGP i doubt it but i could be wrong. You can always create a GRE tunnel and do it that way.

Giuseppe Larosa Mon, 08/25/2008 - 13:29
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Hello Calin,

what works is correct.

I couldn't try but I had always used multihop 2 to establish an eBGP connection on loopbacks so I thought a value of 3 was needed.


I should have written it as a possible suggestion !


Thank you for your feedback and I've remarked your useful answer.


Best Regards

Giuseppe


paaljakobsen Tue, 08/26/2008 - 02:46
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thanks for your engagement!


This is the whole case, I have tried with the ebgp-multihop command, but still no luck.


new drawing :

http://home.netpower.no/paal/bgp-possible2.jpg


BGP sessions seems to be established:

Description: R1

BGP version 4, remote router ID 192.168.116.90

BGP state = Established, up for 00:17:39


I can not ping 193.214.20.211 from 192.168.234.163


There is no routing on R3 since the interfaces are DirectConnected


When I :

R2-AS300#ping 193.214.20.211 source fa0

there is no response


the following is my config:


R1-AS100

---------

!

interface FastEthernet0

ip address 192.168.116.90 255.255.255.0

!

interface FastEthernet1

ip address 172.28.10.9 255.255.255.252

!

router bgp 100

no synchronization

bgp log-neighbor-changes

redistribute static

neighbor 172.28.10.2 remote-as 300

neighbor 172.28.10.2 description R2

neighbor 172.28.10.2 ebgp-multihop 2

neighbor 172.28.10.2 update-source Fa1

no auto-summary

!

ip route 172.28.10.2 255.255.255.255 172.28.10.10

ip route 193.214.20.211 255.255.255.255 192.168.116.1



R2-AS300

---------

!

interface FastEthernet0

ip address 192.168.234.1 255.255.255.0

!

interface Vlan2

ip address 172.28.10.2 255.255.255.252

!

router bgp 300

no synchronization

!bgp router-id 172.28.10.2

bgp log-neighbor-changes

network 192.168.234.0

neighbor 172.28.10.9 remote-as 100

neighbor 172.28.10.9 description R1

neighbor 172.28.10.9 ebgp-multihop 2

neighbor 172.28.10.9 update-source vlan2

no auto-summary

!

ip route 172.28.10.9 255.255.255.255 172.28.10.1


Calin Chiorean Tue, 08/26/2008 - 02:53
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Hi there!


OK, so step by step:


1.

give us the output on both routers of

show ip bgp sum


2.

from R1-AS100 can you ping 172.28.10.2 ?

the same

from R2-AS300 can you ping 172.28.10.9


3. try this on both router bgp config


neighbor 172.28.10.2 ebgp-multihop 1


neighbor 172.28.10.9 ebgp-multihop 1


Actual you have only one hop to the destination of peer bgp.


Good luck!


Cheers,

Calin

paaljakobsen Tue, 08/26/2008 - 03:37
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I am able to ping

172.28.10.2 from R1

and

172.28.10.9 from R2



When I set

neighbor 172.28.10.9 ebgp-multihop 1

the BGP State = Idle


reset connection with :

R1-AS100#clear ip bgp 300


when I set

neighbor 172.28.10.9 ebgp-multihop 2

BGP state = Established, up for 00:00:07


I thinks that's because it is 2 hops ?


Output of 'show ip bgp sum' is in the new drawing with 'show ip route'


http://home.netpower.no/paal/bgp-possible2.jpg


additional info:


R2-AS300#traceroute 193.214.20.211 source vlan2


Type escape sequence to abort.

Tracing the route to 193.214.20.211


1 172.28.10.1 0 msec 0 msec 0 msec

2 172.28.10.1 !H * !H

R2-AS300#



R2-AS300#traceroute 193.214.20.211 source fa0


Type escape sequence to abort.

Tracing the route to 193.214.20.211


1 * * *

2 * * *

3 * * *



Many thanks,

Paal


Harold Ritter Tue, 08/26/2008 - 06:16
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Paal,


It will definitely not work with "ebgp-multihop 1". You need "ebgp-multihop 2".


As someone already mentioned, you need to make sure that the router in between will be able to route the traffic via static routes (might be cumbersome), MPLS, etc.


Regards,

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