Hello Deepika
Thanks for indicating this. The * indicates that it is a valid route and that BGP is able to use it. However, in this case the next-hop is not reachable, so it should not be valid. I will let @ReneMolenaar know about this…
Thanks again…
Laz
Rene,
When you run iBGP with multiple nodes we should use IGP protocols to advertise prefixes between the iBGP nodes since iBGP does not carry forward the prefixes to another peer when It was learned from another neighbor. please correct me if I’m wrong.
Hello Durga
An iBGP neighbour relationship can form between any BGP routers even if they are not directly connected. The only prerequisite is that there is a routed path between the IP addresses of the interfaces through which the relationship is formed.
Now the routed path can be established either via IGP or static routing. iBGP cannot be used because it requires the routing to form a neighbor relationship. It’s kind of like the chicken and the egg. iBGP cannot create a route for itself to create a neighbor relationship because it has no neighbor relationship to create it with.
I hope this has been helpful!
Laz
Does eBGP change the next hop ip address to itself? or not the same as iBGP does?
Hello Rey
In an external BGP (eBGP) session, by default, the router does indeed change the next hop attribute of a BGP route (to its own address) when the router sends out a route. This behaviour can be changed however using the BGP Next Hop Unchanged feature. This is a command that is issued when defining a BGP neighbour. An example of this implementation is:
Router(config-router-af)# neighbor 10.0.0.100 next-hop-unchanged
I hope this has been helpful!
Laz
Yeah, so to summarise:
Learned from eBGP -> Advertise to eBGP -> Next-hop-self automatic
Learned from eBGP -> Advertise to iBGP -> “Next-hop-self” required
Learned from iBGP -> Advertise to iBGP -> “Next-hop-self all” required
Why IBGP speaking router by default does not change next hop to its ibgp peer
Hello Anul
When routes are being exchanged between iBGP peers, the next hop IP address used by default is that of the router in an external AS. iBGP will not provide a next hop address of an iBGP peer. This is because within an autonomous system, it is assumed that there are routing protocols present that will allow for internal routing. The primary purpose of BGP in general is to find routes to other autonomous systems and not to route within an AS. So, a router will get a next hop IP address of the next eBGP peer because it is assumed that routing information to reach that is already available.
Now if you choose to use iBGP to route within your autonomous system, then of course you can do that, but you will need the “next hop self” command/configuration.
I hope this has been helpful!
Laz
Hi Rene,
Aside from this scenario, Is there other scenario that we can use BGP Next Hop Self?
What is the best practice to implementing BGP Next Hop Self?
Best Regards,
Fritz Macahilig
Hello Fritz
The next hop self feature is used primarily for this purpose, to allow iBGP routers to change the next hop IP address in order avoid potential reachability issues.
A slightly different scenario that comes to mind where BGP next hop self is used is when you have two internet routers running eBGP with their ISPs and iBGP between them. If the next hop self command is not implemented, then each iBGP neighbour will have the eBGP ISP as the next hop for some networks, something that you may not want for your network. The principle is the same, the scenario is somewhat different.
Best practice for using next hop self is to use it whenever an internal peer must see the iBGP peer as the next hop rather than the eBGP peer to which it may not have a route. Now you could have an IGP provide the route to that peer, however, because it is outside of the AS, it is not ideal. IGPs should function only within an AS and have only BGP operate on an interAS basis.
I hope this has been helpful!
Laz
Hi,
Advertise Network
The first solution is simple, we can advertise the network in iBGP (or an IGP if you use one) so that R1 is able to reach the next hop. Let’s advertise 192.168.23.0 /24 in BGP:R2(config)#router bgp 12
R2(config-router)#network 192.168.23.0 mask 255.255.255.0
Hi,
Some places you are saying we need an igp in iBGP and the above example shows it uses network command to advertise the network .
I am really struggling to understand .
Please help
Thanks
Hello Sims
iBGP requires that an IGP be installed and configured within the AS. In this example, an IGP should be configured between R1 and R2 so that connectivity between routers in the same AS can be achieved. This allows BGP routers to become neighbors even if they are not directly connected. In this example, an IGP is not necessary because the BGP routers are directly connected. If they weren’t then an IGP is imperative.
The command in the quote you mention in your post is there in order to include the specific network in the exchange of BGP prefixes. This means that R2 will tell BGP what networks should be advertised. In this particular case, this network will be advertised to R1 using iBGP and to R3 using eBGP.
I hope this has been helpful!
Laz
Hi Guys - I’ve been labbing in GNS3 and have come across a situation where ‘next hop self’ would be useful but my question is how can a BGP route be deemed valid if the next hop is not reachable?
R1>sh ip bgp 33.33.33.33
BGP routing table entry for 33.33.33.33/32, version 0
Paths: (1 available, no best path)
Not advertised to any peer
300
3.3.3.3 (inaccessible) from 2.2.2.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, internalHello Gareth
That’s one of the idiosyncrasies of BGP. Within the AS there must be connectivity between the iBGP routers and this is achieved via an IGP or static routing. In this lab, this is already achieved via directly connected networks, so the routers can find each other. The “next hop self” feature exists to solve a different problem, specifically, to make sure that the next hop IP associated with a particular path is one that is in the same AS (and not in another AS and thus unreachable via IGPs), and is indeed reachable.
So IGPs solve reachablility within an AS, while next hop self ensures that the next hop IP is within the AS and thus reachable via IGPs. Both must exist in order for a network to function correctly.
I hope this has been helpful!
Laz
Thanks Laz - makes sense. I’ve believed for a while that the next hop must be reachable for a route to even appear in the BGP table but now realise that this is just a condition for a route to appear in the RIB. I guess the ‘valid’ part of the output:
Origin IGP, metric 0, localpref 100, valid, internal.
But also stating that the next hop is inaccessible threw me a little. 
can you help me? Why vios4 dont change next-hop(ebgp) when it send 3.3.3.3/32 to vios5? On vios5 i see network 3.3.3.3 with next-hop 192.168.1.3
Hello Pavel
When eBGP routers exchange routing information between them, by default, the router changes the next-hop attribute of the BGP route (to its own address) when the router sends out a route.
Now in your topology, since there is no eBGP peering between AS 65002 and AS65003, AS 65001 plays the intermediary role of exchanging these routes with the other two devices. Now if all default configurations are maintained, then vIOS5 should have a next-hop IP address of 192.168.1.1. The only reason you would not have this is if you have accidentally created an iBGP peering between vIOS4 and vIOS5 or if you have configured the BGP Next Hop Unchanged feature.
I suggest you go over your configs again, as there is no other reason for you to see this behaviour.
I hope this has been helpful!
Laz
I have a quick inquiry regrading IBGP with EBGP:
- I have build below network topology in GNS3:
1- I have configured IBGP between R01 and R02 as they are in same AS.
2- I have configured EBGP between R02 and R03 as they are in different AS’s.
so after that i have seen the route received in R01 , R02 , R03 which is as below:
My question why R03 is able to reach loopback0 of R01 which is 1.1.1.1 and its added in the routing table of it , however R01 was not able to reach loopback0 of R03 which is 3.3.3.3 and 3.3.3.3 was not added in routing table of R01 ,it can be seen only under R01#show ip bgp.
Great i have tried it in my lab and it works i have configured neighbor 192.168.12.1 next-hop-self in R02 bgp, and then R01 was able to install 3.3.3.0 in its routing table and next hop became correct 192.168.12.2 instead of 192.168.23.3 , but when i tried to ping 3.3.3.3 from R01 the ping reached to R03 but in echo reply the packet dropped and this is make sense as when the packet reached to R03 the source was 192.168.12.1 and dest 3.3.3.3 then when R03 wanted to reply in echo msg the destination became 192.168.12.1 and R03 didn’t have this subnet in its routing table, so it was dropped so the only way to solve this we need to advertise 192.168.12.0 and 192.168.23.0 in R02.
Hello Ziad
This is a typical situation which results because iBGP doesn’t change the next hop IP address when it advertises a route it learns via eBGP. In this case, destination network 3.3.3.3 is advertised by R03 to R02 using eBGP. In R02, the next hop IP address for this route is 192.168.23.2 which is the IP address of R03. This is the way it should be.
However, when R02 advertises this route to R1 using iBGP, it advertises it using the same next-hop IP address of 192.168.23.2. R01 learns of this route, and you can see it in your BGP table of R01 with the right next-hop IP. However, R01 doesn’t have a route to this next-hop IP. Notice it doesn’t exist in the routing table of R01. So R01 knows of the 3.3.3.3 network, knows the next-hop IP, but doesn’t know how to get to the next-hop IP. A BGP router will only put a BGP route into the routing table if it has a valid next-hop IP.
How do you resolve this? By using the BGP next-hop-self feature. You can read about this in the following lesson, which has a scenario very similar to yours (actually it’s almost identical!).
Now you say you are able to ping 1.1.1.1 on R01 from R03? The ping will indeed reach R01, but it should not make it back to R03 because there is no route to the 192.168.23.0/24 subnet, which is where the pings originate from. Can you confirm that you are indeed able to ping with a response? Or do you simply see the pings reach R1 without returning? Let us know…
I hope this has been helpful!
Laz
