Lab 1: Basic BGP
Lab Diagram
AS3
.1
24.234.21.0/24
Se0/1/0
R1
24.234.0.1
.2
Se0/0/1
.2
.2
Se0/1/0
24.234.12.0/24
.2
24.234.0.2
Se0/0/0.203
24.234.23.0/24
.3
Se0/0/0.302
24.234.0.3
R3
Se0/0/1
R2
AS12
.2
Se0/0/0.203
24.234.24.0/24
Se0/0/0.203
.4
24.234.0.4
R4
AS4
Lab Addressing
Router
R1
R2
R3
R4
Interface
Loopback0
Serial0/0/1
Serial0/1/0
Loopback0
Serial0/0/1
Serial0/1/0
Serial0/0/0.203
Serial0/0/0.204
Loopback0
Serial0/0/0.302
Loopback0
Serial0/0/0.402
L2 (DLCI, VLAN)
Loopback
HDLC
HDLC
Loopback
HDLC
HDLC
DLCI 203
DLCI 204
Loopback
DLCI 302
Loopback
DLCI 402
IP Address
24.234.0.1/32
24.234.12.1/24
24.234.21.1/24
24.234.0.2/32
24.234.12.2/24
24.234.21.2/24
24.234.23.2/24
24.234.24.2/24
24.234.0.3/32
24.234.23.3/24
24.234.0.4/32
24.234.24.4/24
Lab Walkthrough

Configure BGP between R2 and R3 using directly connected interface.
This is a straightforward task. All we have to do is to start BGP router process and define
neighbors.
R2(config)#router bgp 12
R2(config-router)#neighbor 24.234.23.3 remote-as 3
R3(config)#router bgp 3
R3(config-router)#neighbor 24.234.23.2 remote-as 12
R2#show ip bgp summary
BGP router identifier 24.234.0.2, local AS number 12
BGP table version is 1, main routing table version 1
Neighbor
24.234.23.3
V
4
AS MsgRcvd MsgSent
3
21
21
TblVer
1
InQ OutQ Up/Down State/PfxRcd
0
0 00:00:05
0
To verify configuration, issue “show ip bgp summary” command. Don’t be alarmed if you
see no prefixes received, as we are not supposed to advertise any at this moment.
R3#show ip bgp summary
BGP router identifier 24.234.0.3, local AS number 3
BGP table version is 1, main routing table version 1
Neighbor
24.234.23.2

V
4
AS MsgRcvd MsgSent
12
21
21
TblVer
1
InQ OutQ Up/Down State/PfxRcd
0
0 00:00:52
0
Configure BGP between R1 and R3 and ensure that session is up independent of
either of the interconnecting links being up. You are not allowed to configure any
static routes.
Generally speaking, it is not possible to establish multiple BGP sessions between two
routers. What this task is hinting at is to use loopback interfaces for session. However, we
don’t have routes to loopbacks and we can’t use static routes. Some form of IGP is required.
You can use any, as none is specifically required. We will use OSPF. Again, very simple
configuration – the goal is only to get Loopback0 into the routing table of the neighbor.
R1(config)#router ospf 1
R1(config-router)#network 24.234.0.1 0.0.0.0 area 0
R1(config-router)#network 24.234.12.0 0.0.0.255 area 0
R2(config)#router ospf 1
R2(config-router)#network 24.234.0.2 0.0.0.0 area 0
R2(config-router)#network 24.234.12.0 0.0.0.255 area 0
Make sure OSPF works properly before you continue.
R1#show ip route ospf
24.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
O
24.234.0.2/32 [110/65] via 24.234.12.2, 00:11:15, Serial0/0/1
R2#show ip route ospf
24.0.0.0/8 is variably subnetted, 7 subnets, 2 masks
O
24.234.0.1/32 [110/65] via 24.234.12.1, 00:10:53, Serial0/0/1
In order to use Loopback0 interface as an endpoint of BGP TCP session, use “neighbor
source-interface” command, as shown below.
R1(config)#router bgp 12
R1(config-router)#neighbor 24.234.0.2 remote-as 12
R1(config-router)#neighbor 24.234.0.2 update-source loopback0
R2(config)#router bgp 12
R2(config-router)#neighbor 24.234.0.1 remote-as 12
R2(config-router)#neighbor 24.234.0.1 update-source loopbac0
Similarly to the previous task, verify that the session is up. There is no need to verify on
both ends. If it’s up on R1, it will be up on R2, too.
R1#show ip bgp summary
BGP router identifier 24.234.0.1, local AS number 12
BGP table version is 1, main routing table version 1
Neighbor
24.234.0.2

V
4
AS MsgRcvd MsgSent
12
18
18
TblVer
1
InQ OutQ Up/Down State/PfxRcd
0
0 00:01:45
0
Configure BGP between R2 and R4 and ensure that new links can be added in the
future. You are allowed one host static route on R2 and R4 each.
This task in some aspects similar to the previous one – it requires session between
Loopback0 interfaces. However, here we are configuring external BGP (eBGP) and some
additional steps are required. First of all, in real life it is very uncommon to run any IGP
routing protocol between autonomous systems. In order to get Loopback0 of the neighbor
into the routing table, we will use static host routes on both routers.
R2(config)#ip route 24.234.0.4 255.255.255.255 24.234.24.4 name R4_Loopback0
R4(config)#ip route 24.234.0.2 255.255.255.255 24.234.24.2 name R2_Loopback0
We need to have in mind are eBGP session establishment rules. Due to very basic security
reasons, session will not be established between neighbors that are not directly connected.
Loopback0 interfaces are not directly connected, even though routers are! In order to
establish the session, we need to “loosen” the security check and allow session to be
established. Use “neighbor ebgp-multihop” command to accomplish that.
R2(config)#router bgp 12
R2(config-router)#neighbor 24.234.0.4 remote-as 4
R2(config-router)#neighbor 24.234.0.4 update-source loopback0
R2(config-router)#neighbor 24.234.0.4 ebgp-multihop 2
R4(config)#router bgp 4
R4(config-router)#neighbor 24.234.0.2 remote-as 12
R4(config-router)#neighbor 24.234.0.2 update-source loopback0
R4(config-router)#neighbor 24.234.0.2 ebgp-multihop 2
Hint: It is a good habit to verify after configuration. It takes only few seconds and can save
you valuable time down the road in the lab.
R4#show ip bgp summary
BGP router identifier 24.234.0.4, local AS number 4
BGP table version is 1, main routing table version 1
Neighbor
24.234.0.2

V
4
AS MsgRcvd MsgSent
12
24
24
TblVer
1
InQ OutQ Up/Down State/PfxRcd
0
0 00:03:12
0
Protect all eBGP sessions with password "cisco". Use the most secure method.
Hint: Don’t fall for “mind tricks”. At this moment, BGP supports only one password
protection method – there is no less and more secure one!
R2(config)#router bgp 12
R2(config-router)#neighbor 24.234.0.4 password cisco
R2(config-router)#neighbor 24.234.23.3 password cisco
R2(config-router)#end
R2#clear ip bgp *
R3(config)#router bgp 3
R3(config-router)#neighbor 24.234.23.2 password cisco
R4(config)#router bgp 4
R4(config-router)#neighbor 24.234.0.2 password cisco
R2#show ip bgp summary
BGP router identifier 24.234.0.2, local AS number 12
BGP table version is 1, main routing table version 1
Neighbor
24.234.0.1
24.234.0.4
24.234.23.3

V
4
4
4
AS MsgRcvd MsgSent
12
20
20
4
24
24
3
24
24
TblVer
1
1
1
InQ OutQ Up/Down State/PfxRcd
0
0 00:03:28
0
0
0 00:03:26
0
0
0 00:03:27
0
On each router, advertise Loopback0 interface into the BGP. Ensure full reachability
between Loopback0 interfaces. Links connecting to any external AS are not allowed
into any IGP.
There are many ways in which a network can “end up” in BGP. Two most common ones are
using “network” statement and redistribution. General rule of thumb is to use “network”
statement if the task is talking about “advertising” and use redistribution only if asked to do
so. However, if in doubt on the exam day – ask the Proctor for clarification.
Proctor’s tip: Do not redistribute.
R1(config)#router bgp 12
R1(config-router)#network 24.234.0.1 mask 255.255.255.255
R2(config)#router bgp 12
R2(config-router)#network 24.234.0.2 mask 255.255.255.255
R3(config)#router bgp 3
R3(config-router)#network 24.234.0.3 mask 255.255.255.255
R4(config)#router bgp 4
R4(config-router)#network 24.234.0.4 mask 255.255.255.255
Check BGP tables on all routers to ensure that all have proper routing information.
R1#show ip bgp
BGP table version is 10, local router ID is 24.234.0.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
Next Hop
Metric LocPrf Weight Path
*> 24.234.0.1/32
0.0.0.0
0
32768 i
r>i24.234.0.2/32
24.234.0.2
0
100
0 i
* i24.234.0.3/32
24.234.23.3
0
100
0 3 i
* i24.234.0.4/32
24.234.0.4
0
100
0 4 i
R1#show ip route
Codes: 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
Gateway of last resort is not set
C
O
C
C
24.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
24.234.21.0/24 is directly connected, Serial0/1/0
24.234.0.2/32 [110/65] via 24.234.12.2, 00:05:41, Serial0/0/1
24.234.0.1/32 is directly connected, Loopback0
24.234.12.0/24 is directly connected, Serial0/0/1
Notice the problem above! R1 is not installing R3 and R4 networks into the routing table,
even though it’s receiving them in BGP from R2. The reason for that can be found in “nexthop” attribute. By default, routers will not change “next-hop” when advertising networks to
iBGP neighbors. In this case, “next-hop” addresses are not known to R1 and it will not
install those prefixes into the routing table. We will fix this later on, but let’s first ensure
that there are no further surprises!
R2#show ip bgp
BGP table version is 10, local router ID is 24.234.0.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
Next Hop
Metric LocPrf Weight Path
r>i24.234.0.1/32
24.234.0.1
0
100
0 i
*> 24.234.0.2/32
0.0.0.0
0
32768 i
*> 24.234.0.3/32
24.234.23.3
0
0 3 i
r> 24.234.0.4/32
24.234.0.4
0
0 4 i
R2#show ip route
Codes: 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
Gateway of last resort is not set
C
C
C
C
B
O
S
C
24.0.0.0/8 is variably subnetted, 8 subnets, 2 masks
24.234.23.0/24 is directly connected, Serial0/0/0.203
24.234.21.0/24 is directly connected, Serial0/1/0
24.234.24.0/24 is directly connected, Serial0/0/0.204
24.234.0.2/32 is directly connected, Loopback0
24.234.0.3/32 [20/0] via 24.234.23.3, 00:05:07
24.234.0.1/32 [110/65] via 24.234.12.1, 00:05:14, Serial0/0/1
24.234.0.4/32 [1/0] via 24.234.24.4
24.234.12.0/24 is directly connected, Serial0/0/1
R3#show ip bgp
BGP table version is 7, local router ID is 24.234.0.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
Next Hop
Metric LocPrf Weight Path
*> 24.234.0.1/32
24.234.23.2
0 12 i
*> 24.234.0.2/32
24.234.23.2
0
0 12 i
*> 24.234.0.3/32
0.0.0.0
0
32768 i
*> 24.234.0.4/32
24.234.23.2
0 12 4 i
R3#show ip route
Codes: 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
Gateway of last resort is not set
C
B
C
B
B
24.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
24.234.23.0/24 is directly connected, Serial0/0/0.302
24.234.0.2/32 [20/0] via 24.234.23.2, 00:05:17
24.234.0.3/32 is directly connected, Loopback0
24.234.0.1/32 [20/0] via 24.234.23.2, 00:05:47
24.234.0.4/32 [20/0] via 24.234.23.2, 00:00:18
R4#show ip bgp
BGP table version is 11, local router ID is 24.234.0.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
Next Hop
Metric LocPrf Weight Path
*> 24.234.0.1/32
24.234.0.2
0 12 i
r> 24.234.0.2/32
24.234.0.2
0
0 12 i
*> 24.234.0.3/32
24.234.0.2
0 12 3 i
*> 24.234.0.4/32
0.0.0.0
0
32768 i
R4#show ip route
Codes: 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
Gateway of last resort is not set
C
S
B
B
C
24.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
24.234.24.0/24 is directly connected, Serial0/0/0.402
24.234.0.2/32 [1/0] via 24.234.24.2
24.234.0.3/32 [20/0] via 24.234.0.2, 00:00:18
24.234.0.1/32 [20/0] via 24.234.0.2, 00:00:18
24.234.0.4/32 is directly connected, Loopback0
In order to fix the problem between R1 and R2, we will configure R2 to make itself “nexthop” for prefixes that it’s advertising to R1. Note that this applies only to eBGP-learned
prefixes.
R2(config)#router bgp 12
R2(config-router)#neighbor 24.234.0.1 next-hop-self
R2(config-router)#end
R2#clear ip bgp 24.234.0.1
R1#show ip route
Codes: 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
Gateway of last resort is not set
C
O
B
C
24.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
24.234.21.0/24 is directly connected, Serial0/1/0
24.234.0.2/32 [110/65] via 24.234.12.2, 00:07:32, Serial0/0/1
24.234.0.3/32 [200/0] via 24.234.0.2, 00:00:18
24.234.0.1/32 is directly connected, Loopback0
B
C
24.234.0.4/32 [200/0] via 24.234.0.2, 00:00:18
24.234.12.0/24 is directly connected, Serial0/0/1
At the end, let’s verify connectivity.
R1#ping 24.234.0.2 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.2, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R1#ping 24.234.0.3 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.3, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/59/60 ms
R1#ping 24.234.0.4 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.4, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/60/68 ms
R3#ping 24.234.0.4 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.4, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.3
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 112/113/116 ms

R2 will be connected to other internal neighbors in AS12. Configure R2 such that
minimal configuration will be required when these neighbors are added.
When there are multiple neighbors that share common parameters, their configuration can
be greatly simplified using “peer-groups” or in more recent IOS versions, templates. As
there are no limitations as to what method to use, we will use peer-groups.
Hint: You will need to remove currently configured neighbor if you want to add it to “peergroup”. If you have read the entire lab before configuration, you may have spotted the
requirement and adjusted your initial configuration accordingly. Well done if you have done
it!
R2#show running-config | include 24.234.0.1
neighbor 24.234.0.1 remote-as 12
neighbor 24.234.0.1 update-source Loopback0
neighbor 24.234.0.1 next-hop-self
R2(config)#router bgp 12
R2(config-router)#neighbor AS12 peer-group
R2(config-router)#neighbor AS12 remote-as 12
R2(config-router)#neighbor AS12 update-source loopback0
R2(config-router)#neighbor AS12 next-hop-self
R2(config-router)#no neighbor 24.234.0.1
R2(config-router)#neighbor 24.234.0.1 peer-group AS12
Make sure that everything still works as expected.
R1#show ip route bgp
24.0.0.0/8 is variably subnetted, 6 subnets, 2 masks
B
24.234.0.3/32 [200/0] via 24.234.0.2, 00:00:12
B
24.234.0.4/32 [200/0] via 24.234.0.2, 00:00:12
R1#ping 24.234.0.3 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.3, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/59/60 ms
R1#ping 24.234.0.4 source loopback0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 24.234.0.4, timeout is 2 seconds:
Packet sent with a source address of 24.234.0.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 56/58/60 ms