Configure A Next Hop Ip Address For Static Routes

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One potential issue with iBGP is that it doesn’t change the next hop IP address. Sometimes this can cause reachability issues. Let’s look at an example:

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Above we have R1 and R2 in AS 12 running iBGP. R3 is in AS 3 & we use eBGP between R2 và R3. Once we advertise network 3.3.3.0 /24 on R3 in BGP then R2 will learn this prefix và stores it in its BGP table, the next hop IP adress will be 192.168.23.3.

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Once R1 learns about prefix 3.3.3.0 /24 then the next hop IP address will remain 192.168.23.3. When R1 doesn’t know how to lớn reach this IP address then it will fail to lớn install 3.3.3.0 /24 in its routing table.

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Let’s take a look at the configuration, I’ll show you two methods how we can deal with this issue.

Configuration

Here’s the BGP configuraton that we will use:

R1(config)#router bgp 12R1(config-router)#neighbor 192.168.12.2 remote-as 12R2(config)#router bgp 12R2(config-router)#neighbor 192.168.12.1 remote-as 12R2(config-router)#neighbor 192.168.23.3 remote-as 3R3(config)#router bgp 3R3(config-router)#neighbor 192.168.23.2 remote-as 12R3(config-router)#network 3.3.3.0 mask 255.255.255.0The configuration is pretty straight forward. We use iBGP between R1/R2 & eBGP between R2/R3. On R3 we advertised 3.3.3.0 /24 in BGP. Let’s take a look at the BGP tables:

R2#show ip bgpBGP table version is 2, local router ID is 192.168.23.2Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*> 3.3.3.0/24 192.168.23.3 0 0 3 iR2 has installed 3.3.3.0 /24 in its BGP table & it is a valid route, the next hop is 192.168.23.3. Let’s kiểm tra R1:

R1#show ip bgpBGP table version is 1, local router ID is 192.168.12.1Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path* i3.3.3.0/24 192.168.23.3 0 100 0 3 iR1 learns the prefix but it’s unable lớn install it in the routing table:

R1#show ip route bgpThe problem here is that the next hop IP address is 192.168.23.3. Does R1 have any clue how to reach this address?

R1#show ip routeCodes: 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 routeGateway of last resort is not setC 192.168.12.0/24 is directly connected, FastEthernet0/0R1 doesn’t know so it’s impossible khổng lồ install 3.3.3.0 /24 in the routing table. How can we fix this? I’ll show you two different methods.

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 khổng lồ reach the next hop. Let’s advertise 192.168.23.0 /24 in BGP:

R2(config)#router bgp 12R2(config-router)#network 192.168.23.0 mask 255.255.255.0Now take a look at R1:

R1#show ip bgpBGP table version is 3, local router ID is 192.168.12.1Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S StaleOrigin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path*>i3.3.3.0/24 192.168.23.3 0 100 0 3 i*>i192.168.23.0 192.168.12.2 0 100 0 iR1 learns about 192.168.23.0 /24 so now it knows how lớn reach the next hop for 3.3.3.0 /24. It can now install this network in the routing table:

R1#show ip route bgp 3.0.0.0/24 is subnetted, 1 subnetsB 3.3.3.0 <200/0> via 192.168.23.3, 00:16:20B 192.168.23.0/24 <200/0> via 192.168.12.2, 00:16:25

hostname R1!interface fastEthernet0/0 ip address 192.168.12.1 255.255.255.0!router bgp 12 neighbor 192.168.12.2 remote-as 12!end

hostname R2!interface fastEthernet1/0 ip address 192.168.12.2 255.255.255.0!interface fastEthernet0/1 ip address 192.168.23.1 255.255.255.0!router bgp 12 neighbor 192.168.12.1 remote-as 12 neighbor 192.168.23.3 remote-as 3 neighbor 192.168.12.1 next-hop-self network 192.168.23.0 mask 255.255.255.0!end

hostname R3!interface fastEthernet0/0 ip address 192.168.23.3 255.255.255.0!router bgp 3 neighbor 192.168.23.2 remote-as 12 network 3.3.3.0 mask 255.255.255.0!end
This will work but there is another solution that is easier. Let’s clean up before we continue:

R2(config)#router bgp 12R2(config-router)#no network 192.168.23.0 mask 255.255.255.0Now we can try something else…