){kind=link}
Topology
Objectives
- Configure BGP to exchange routing information with two ISPs.
Background
The International Travel Agency (ITA) relies extensively on the Internet for sales. For this reason, the ITA has decided to create a multihomed ISP connectivity solution and contracted with two ISPs for Internet connectivity with fault tolerance. Because the ITA is connecting to two different service providers, you must configure BGP, which runs between the ITA boundary router and the two ISP routers.
Note: This lab uses Cisco 1941 routers with Cisco IOS Release 15.4 with IP Base. The switches are Cisco WS-C2960-24TT-L with Fast Ethernet interfaces, therefore the router will use routing metrics associated with a 100 Mb/s interface. Depending on the router or switch model and Cisco IOS Software version, the commands available and output produced might vary from what is shown in this lab.
Required Resources
- 3 routers (Cisco IOS Release 15.2 or comparable)
- Serial and Ethernet cables
Step 0: Suggested starting configurations.
a. Apply the following configuration to each router along with the appropriate hostname. The exec-timeout 0 0 command should only be used in a lab environment.
Router(config)# no ip domain-lookup Router(config)# line con 0 Router(config-line)# logging synchronous Router(config-line)# exec-timeout 0 0
Step 1: Configure interface addresses.
a. Using the addressing scheme in the diagram, create the loopback interfaces and apply IPv4 addresses to these and the serial interfaces on ISP1 (R1), ISP2 (R3), and ITA (R2). The ISP loopbacks simulate real networks that can be reached through the ISP. The two loopbacks for the ITA router simulate the connections between the ITA boundary router and their core routers. Set a clock rate on the DCE serial interfaces.
ISP1(config)# interface Lo0 ISP1(config-if)# description ISP1 Internet Network ISP1(config-if)# ip address 10.1.1.1 255.255.255.0 ISP1(config-if)# exit ISP1(config)# interface Serial0/0/0 ISP1(config-if)# description ISP1 -> ITA ISP1(config-if)# ip address 10.0.0.1 255.255.255.252 ISP1(config-if)# clock rate 128000 ISP1(config-if)# no shutdown ISP1(config-if)# end ISP1# ITA(config)# interface Lo0 ITA(config-if)# description Core router network link 1 ITA(config-if)# ip address 192.168.0.1 255.255.255.0 ITA(config)# exit ITA(config-if)# interface Lo1 ITA(config-if)# description Core router network link 2 ITA(config-if)# ip address 192.168.1.1 255.255.255.0 ITA(config-if)# exit ITA(config)# interface Serial0/0/0 ITA(config-if)# description ITA -> ISP1 ITA(config-if)# ip address 10.0.0.2 255.255.255.252 ITA(config-if)# no shutdown ITA(config-if)# exit ITA(config)# interface Serial0/0/1 ITA(config-if)# description ITA -> ISP2 ITA(config-if)# ip address 172.16.0.2 255.255.255.252 ITA(config-if)# clock rate 128000 ITA(config-if)# no shutdown ITA(config-if)# end ITA# ISP2(config)# interface Lo0 ISP2(config-if)# description ISP2 Internet Network ISP2(config-if)# ip address 172.16.1.1 255.255.255.0 ISP2(config)# exit ISP2(config-if)# interface Serial0/0/1 ISP2(config-if)# description ISP2 -> ITA ISP2(config-if)# ip address 172.16.0.1 255.255.255.252 ISP2(config-if)# no shutdown ISP2(config-if)# end ISP2#
b. Use ping to test the connectivity between the directly connected routers. Note that router ISP1 cannot reach router ISP2.
Step 2: Configure BGP on the ISP routers.
On the ISP1 and ISP2 routers, configure BGP to peer with the ITA boundary router and advertise the ISP loopback networks.
ISP1(config)# router bgp 200 ISP1(config-router)# neighbor 10.0.0.2 remote-as 100 ISP1(config-router)# network 10.1.1.0 mask 255.255.255.0 ISP2(config)# router bgp 300 ISP2(config-router)# neighbor 172.16.0.2 remote-as 100 ISP2(config-router)# network 172.16.1.0 mask 255.255.255.0
Step 3: Configure BGP on the ITA boundary router.
a. Configure the ITA router to run BGP with both Internet providers.
ITA(config)# router bgp 100 ITA(config-router)# neighbor 10.0.0.1 remote-as 200 ITA(config-router)# neighbor 172.16.0.1 remote-as 300 ITA(config-router)# network 192.168.0.0 ITA(config-router)# network 192.168.1.0
You should see BGP neighbor peering messages on the console similar to the following.
*Sep 8 16:00:21.587: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Up
b. To verify the configuration, check the ITA routing table with the show ip route command.
ITA# show ip route
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
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks
C 10.0.0.0/30 is directly connected, Serial0/0/0
L 10.0.0.2/32 is directly connected, Serial0/0/0
B 10.1.1.0/24 [20/0] via 10.0.0.1, 00:01:10
172.16.0.0/16 is variably subnetted, 3 subnets, 3 masks
C 172.16.0.0/30 is directly connected, Serial0/0/1
L 172.16.0.2/32 is directly connected, Serial0/0/1
B 172.16.1.0/24 [20/0] via 172.16.0.1, 00:00:53
192.168.0.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.0.0/24 is directly connected, Loopback0
L 192.168.0.1/32 is directly connected, Loopback0
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, Loopback1
L 192.168.1.1/32 is directly connected, Loopback1
ITA#
ITA has BGP routes to the loopback networks at each ISP router.
c. Run the following Tcl script on all routers to verify connectivity If these pings are not successful, troubleshoot. Use exit to exit the Tcl script.
Note: The WAN subnets connecting ITA (R2) to the ISPs (R1 and R3) are not advertised in BGP, so the ISPs will not be able to ping each other’s serial interface address.
ITA# tclsh
foreach address {
10.0.0.1
10.0.0.2
10.1.1.1
172.16.0.1
172.16.0.2
172.16.1.1
192.168.0.1
192.168.1.1
} {
ping $address }
Step 4: Verify BGP on the routers.
a. To verify the BGP operation on ITA, issue the show ip bgp command.
ITA# show ip bgp
BGP table version is 5, local router ID is 192.168.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.1.1.0/24 10.0.0.1 0 0 200 i
*> 172.16.1.0/24 172.16.0.1 0 0 300 i
*> 192.168.0.0 0.0.0.0 0 32768 i
*> 192.168.1.0 0.0.0.0 0 32768 i
ITA#
What is the local router ID?
_________________________________________________________
Which table version is displayed?
_________________________________________________________
An asterisk (*) next to a route indicates that it is valid. An angle bracket (>) indicates that the route has been selected as the best route.
b. To verify the operation of ISP1, issue the show ip bgp command.
ISP1# show ip bgp
BGP table version is 5, local router ID is 10.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 10.1.1.0/24 0.0.0.0 0 32768 i
*> 172.16.1.0/24 10.0.0.2 0 100 300 i
*> 192.168.0.0 10.0.0.2 0 0 100 i
*> 192.168.1.0 10.0.0.2 0 0 100 i
ISP1#
Which table version is displayed and is it the same as the BGP table version for ITA?
________________________________________________________________
From ISP1, what is the path to network 172.16.1.0/24?
________________________________________________________________
c. On the ISP1 router, issue the shutdown command on Loopback0. Then on ITA, issue the show ip bgp command again.
ISP1(config)# interface loopback 0
ISP1(config-if)# shutdown
ISP1(config-if)#
ITA# show ip bgp
BGP table version is 6, local router ID is 192.168.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter,
x best-external, a additional-path, c RIB-compressed,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
*> 172.16.1.0/24 172.16.0.1 0 0 300 i
*> 192.168.0.0 0.0.0.0 0 32768 i
*> 192.168.1.0 0.0.0.0 0 32768 i
ITA#
Which table version is displayed? Why?
_______________________________________________________________
What happened to the route for network 10.1.1.0/24?
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d. Bring ISP1 router Loopback0 back up by issuing the no shutdown command.
ISP1(config)# interface loopback 0 ISP1(config-if)# no shutdown ISP1(config-if)#
e. On ITA, issue the show ip bgp neighbors command. The following is a partial sample output of the command showing neighbor 172.16.0.1.
ITA# show ip bgp neighbors
BGP neighbor is 10.0.0.1, remote AS 200, external link
BGP version 4, remote router ID 10.1.1.1
BGP state = Established, up for 00:20:47
Last read 00:00:49, last write 00:00:41, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
Neighbor capabilities:
Route refresh: advertised and received(new)
Four-octets ASN Capability: advertised and received
Address family IPv4 Unicast: advertised and received
Enhanced Refresh Capability: advertised and received
Multisession Capability:
Stateful switchover support enabled: NO for session 1
Message statistics:
InQ depth is 0
OutQ depth is 0
Sent Rcvd
Opens: 1 1
Notifications: 0 0
Updates: 5 1
Keepalives: 15 17
Route Refresh: 0 0
Total: 21 19
Default minimum time between advertisement runs is 30 seconds
Based on the output of this command, what is the BGP state between this router and ISP2?
__________________________________________________________________
How long has this connection been up?
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Step 5: Configure route filters.
a. Check the ISP2 routing table using the show ip route command. ISP2 should have a route that belongs to ISP1, network 10.1.1.0.
ISP2# show ip route
If ITA advertises a route belonging to ISP1, ISP2 installs that route in its table. ISP2 might then attempt to route transit traffic through the ITA. This would make ITA a transit router. A traceroute to ISP1’s Lo0 interface illustrates this issue.
ISP2# traceroute 10.1.1.1 Type escape sequence to abort. Tracing the route to 10.1.1.1 VRF info: (vrf in name/id, vrf out name/id) 1 172.16.0.2 8 msec 4 msec 8 msec 2 * * * 3 * * * 4 * * * ISP2#
The traceroute 10.1.1.1 fails because ISP1 does not have a route to the source IPv4 address of the traceroute, 172.16.0.1. It is common in BGP networks not to advertise the links between providers in BGP. A traceroute using the source IPv4 address of ISP2’ Lo0 interface is successful, showing that ITA is a transit router for this network.
ISP2# traceroute 10.1.1.1 source loopback0 Type escape sequence to abort. Tracing the route to 10.1.1.1 VRF info: (vrf in name/id, vrf out name/id) 1 172.16.0.2 8 msec 4 msec 8 msec 2 10.0.0.1 12 msec * 12 msec ISP2#
b. Configure the ITA router so that it advertises only ITA networks 192.168.0.0 and 192.168.1.0 to both providers. On the ITA router, configure the following access list.
ITA(config)# access-list 1 permit 192.168.0.0 0.0.1.255
c. Apply this access list as a route filter using the distribute-list keyword with the BGP neighbor statement.
ITA(config)# router bgp 100 ITA(config-router)# neighbor 10.0.0.1 distribute-list 1 out ITA(config-router)# neighbor 172.16.0.1 distribute-list 1 out
d. Check the routing table for ISP2 again. The route to 10.1.1.0, ISP1, should still be in the table.
ISP2# show ip route
e. Return to ITA and issue the clear ip bgp * command. Wait until the routers reach the established state, which might take several seconds, and then recheck the ISP2 routing table. The route to ISP1, network 10.1.1.0, should no longer be in the routing table for ISP2, and the route to ISP2, network 172.16.1.0, should not be in the routing table for ISP1.
ITA# clear ip bgp * ITA# *Sep 8 16:47:25.179: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Down User reset *Sep 8 16:47:25.179: %BGP_SESSION-5-ADJCHANGE: neighbor 10.0.0.1 IPv4 Unicast topology base removed from session User reset *Sep 8 16:47:25.179: %BGP-5-ADJCHANGE: neighbor 172.16.0.1 Down User reset *Sep 8 16:47:25.179: %BGP_SESSION-5-ADJCHANGE: neighbor 172.16.0.1 IPv4 Unicast topology base removed from session User reset *Sep 8 16:47:25.815: %BGP-5-ADJCHANGE: neighbor 10.0.0.1 Up *Sep 8 16:47:25.819: %BGP-5-ADJCHANGE ITA#: neighbor 172.16.0.1 Up ITA#
Note: The clear ip bgp * command is disruptive because it completely resets all BGP adjacencies. This is acceptable in a lab environment but could be problematic in a production network. Instead, if only a change of inbound/outbound routing policies is to be performed, it is sufficient to issue the clear ip bgp * in or clear ip bgp * out commands. These commands perform only a new BGP database synchronization without the disruptive effects of a complete BGP adjacency reset. All current Cisco IOS versions support the route refresh capability that replaces the inbound soft reconfiguration feature that previously had to be configured on a per-neighbor basis.
ISP2# show ip route
Step 6: Configure primary and backup routes using floating static routes.
With bidirectional communication established with each ISP via BGP, configure the primary and backup routes. This can be done with floating static routes or BGP.
a. Issue the show ip route command on the ITA router.
ITA# show ip route
Notice that there is no gateway of last resort defined. This is a problem because ITA is the border router for the corporate network.
b. Configure static routes to reflect the policy that ISP1 is the primary provider and that ISP2 acts as the backup by specifying a lower distance metric for the route to ISP1 (210) as compared to the backup route to ISP2 (distance metric 220).
ITA(config)# ip route 0.0.0.0 0.0.0.0 10.0.0.1 210 ITA(config)# ip route 0.0.0.0 0.0.0.0 172.16.0.1 220
c. Verify that a default route is defined using the show ip route command.
ITA# show ip route
d. Test this default route by creating an unadvertised loopback on the router for ISP1.
ISP1# config t ISP1(config)# interface loopback 100 ISP1(config-if)# ip address 192.168.100.1 255.255.255.0
e. Issue the show ip route command to ensure that the newly added 192.168.100.0 /24 network does not appear in the routing table.
ITA# show ip route
f. In extended ping mode, ping the ISP1 loopback 1 interface 192.168.100.1 with the source originating from the ITA loopback 1 interface 192.168.1.1.
ITA# ping Protocol [ip]: Target IP address: 192.168.100.1 Repeat count [5]: Datagram size [100]: Timeout in seconds [2]: Extended commands [n]: y Source address or interface: 192.168.1.1 Type of service [0]: Set DF bit in IP header? [no]: Validate reply data? [no]: Data pattern [0xABCD]: Loose, Strict, Record, Timestamp, Verbose[none]: Sweep range of sizes [n]: Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.100.1, timeout is 2 seconds: Packet sent with a source address of 192.168.1.1 !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 12/14/16 ms ITA#
Note: You can bypass extended ping prompted mode and ping while specifying a source address using one of these abbreviated commands:
ITA# ping 192.168.100.1 source 192.168.1.1
or
ITA# ping 192.168.100.1 source Lo1
Note: Testing the default route by creating an unadvertised network on ISP1 and pinging it works only because the default route also points toward ISP1. If the preferred default route pointed toward ISP2, the ping to that unadvertised network on ISP1 would not succeed. If the link to ISP1 failed, the default route to ISP2 would become active, but the pings would be successful only if ISP1 and ISP2 have another working interconnection and appropriate BGP peering between them, which is currently not the case.
Step 7: Using BGP to propagate a default route.
a. ISP router will be used to inject a default route via BGP. First, remove the current default routes on ITA.
ITA(config)# no ip route 0.0.0.0 0.0.0.0 10.0.0.1 210 ITA(config)# no ip route 0.0.0.0 0.0.0.0 172.16.0.1 220
b. Next, configure the ISP1 router to send a default route to its neighbor, the ITA router. This command does not require the presence of 0.0.0.0 in the local ISP1 router.
ISP1(config)# router bgp 200 ISP1(config-router)# neighbor 10.0.0.2 default-originate ISP1(config-router)#
c. Verify that the default route was received by ITA using BGP.
ITA# show ip route