Let's see the behavior of next hop for advertised prefixes over eBGP.
R2(config)#router bgp 65100
R2(config-router)#neighbor 172.16.28.8 remote-as 65089 R2(config-router)#network 10.2.2.2 mask 255.255.255.255 R2(config-router)#end R2#
R8 is receiving prefix 10.2.2.2/32:
On R8 we can use another, fancy way to extract what prefixes are learned from a direct neighbor AS 65100 like so:
show ip bgp regex ^65100_
Now, on to configuring R1 and R2 iBGP peering.
R2 Configuration:
R2(config)#router bgp 65100 R2(config-router)#neighbor 172.16.123.1 remote-as 65100 R2(config-router)#end R2#
R1 Configuration:
R1(config)#router bgp 65100 R1(config-router)#bgp router-id 10.1.1.1 R1(config-router)#neighbor 172.16.123.2 remote-as 65100 R1(config-router)#network 10.1.1.1 mask 255.255.255.255 R1(config-router)#end R1#
First let's take a peek at R8 BGP table.
All looks good. The next hop is 172.16.28.2 which is reachable via direct link.
What about R1 BGP table?
Notice the 'next-hop-value' for these prefixes!
R1#show ip route 172.16.28.8 % Subnet not in table R1# R1# R1#show ip bgp 10.8.0.0 BGP routing table entry for 10.8.0.0/28, version 0 Paths: (1 available, no best path) Not advertised to any peer Refresh Epoch 1 65089 172.16.28.8 (inaccessible) from 172.16.123.2 (10.2.2.2) Origin IGP, metric 0, localpref 100, valid, internal R1
The next hop is preserved in eBGP advertisements (R8's interface IP address that is used for peering with R2). And it is not reachable. That is why, BGP prefixes won't get installed in the routing table.
R1#show ip route bgp
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
+ - replicated route, % - next hop override
Gateway of last resort is not set
10.0.0.0/32 is subnetted, 2 subnets
B 10.2.2.2 [200/0] via 172.16.123.2, 00:12:00
R1#
There are a few ways of dealing with this issue.
Method 1
Use 'next-hop-self' Command on R2 for peer R1. It is the recommended command on the router that connects eBGP with iBGP peers.router bgp 65100
bgp router-id 10.2.2.2
bgp log-neighbor-changes
network 10.2.2.2 mask 255.255.255.255
neighbor 172.16.28.8 remote-as 65089
neighbor 172.16.123.1 remote-as 65100
neighbor 172.16.123.1 next-hop-self
R2(config-router)#
The result on R1 is this:
R1 finds next-hop-ip reachable (direct link).
Removing this command and trying a second method.
R2(config)#router bgp 65100 R2(config-router)#no neighbor 172.16.123.1 next-hop-self R2(config-router)#
After a 15 or so seconds the '>' best path shows up in the BGP table on R1:
And the prefixes will be installed in the routing table on R1:
Remove configuration in BGP process:
no neighbor 172.16.123.1 next-hop-self
Method 2
Change 'next-hop' attribute using route-map.route-map NEXT_HOP permit 10 set ip next-hop 172.16.123.2 ! router bgp 65100 bgp router-id 10.2.2.2 bgp log-neighbor-changes network 10.2.2.2 mask 255.255.255.255 neighbor 172.16.28.8 remote-as 65089 neighbor 172.16.123.1 remote-as 65100 neighbor 172.16.123.1 route-map NEXT_HOP out
Remove the configuration and try the third method.
Method 3
Advertise link (IP) between R2 and R8 using an IGP protocol (for instance EIGRP/OSPF/IS-IS, etc).R1 EIGRP Configuration:
router eigrp 100
network 172.16.123.1 0.0.0.0
eigrp router-id 10.1.1.1
R2 EIGRP Configuration:
router eigrp 100 network 172.16.28.2 0.0.0.0 network 172.16.123.2 0.0.0.0 eigrp router-id 10.2.2.2
That does the trick as well. R1 learns how to reach next-hop 172.16.28.8 via EIGRP.
R1#show ip route 172.16.28.8
Routing entry for 172.16.28.0/24
Known via "eigrp 100", distance 90, metric 307200, type internal
Redistributing via eigrp 100
Last update from 172.16.123.2 on Ethernet0/0.123, 00:04:23 ago
Routing Descriptor Blocks:
* 172.16.123.2, from 172.16.123.2, 00:04:23 ago, via Ethernet0/0.123
Route metric is 307200, traffic share count is 1
Total delay is 2000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 1
R1#
And the last method would be to redistribute connected network 172.16.28.0 into IGP protocol.
