Lab Objective
Configure eBGP peering between two routers in different autonomous systems, advertise a network from each side, and thoroughly verify the BGP table, AS-path information, and the resulting IP routing table entries.
Lab Purpose
eBGP was introduced briefly earlier in this series as part of network fundamentals; this lab revisits it with the deeper verification workflow expected at the CCNP level — examining not just whether the route works, but the specific BGP attributes that explain why it was selected and how it will be advertised further.
Lab Topology
R1 (AS 65001) ---- Serial0/0/0 ------ Serial0/0/0 ---- R2 (AS 65002)
R1: 10.200.200.1/30, LAN 192.168.200.0/24
R2: 10.200.200.2/30, LAN 192.168.201.0/24Task 1: Configure Basic Addressing
Configure the serial interfaces and each router's LAN.
Task 2: Configure BGP on R1
Enable BGP AS 65001 on R1, establish a neighbor relationship with R2, and advertise R1's LAN network.
Task 3: Configure BGP on R2
Enable BGP AS 65002 on R2, establish a neighbor relationship with R1, and advertise R2's LAN network.
Task 4: Verify the BGP Neighbor Relationship
Confirm the neighbor reaches Established state.
Task 5: Examine the BGP Table and AS-Path
Display the BGP table and identify the AS-path attribute for the learned route.
Task 6: Verify the Route in the IP Routing Table
Confirm the route appears with BGP's default administrative distance for external routes.
Solution and Verification
R1(config)# interface serial0/0/0
R1(config-if)# ip address 10.200.200.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip address 192.168.200.1 255.255.255.0
R1(config-if)# no shutdownR2(config)# interface serial0/0/0
R2(config-if)# ip address 10.200.200.2 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown
R2(config-if)# exit
R2(config)# interface gigabitethernet0/0
R2(config-if)# ip address 192.168.201.1 255.255.255.0
R2(config-if)# no shutdownR1(config)# router bgp 65001
R1(config-router)# neighbor 10.200.200.2 remote-as 65002
R1(config-router)# network 192.168.200.0 mask 255.255.255.0R2(config)# router bgp 65002
R2(config-router)# neighbor 10.200.200.1 remote-as 65001
R2(config-router)# network 192.168.201.0 mask 255.255.255.0R1# show ip bgp summary
Neighbor V AS MsgRcvd MsgSent State/PfxRcd
10.200.200.2 4 65002 15 14 1
-- A number in State/PfxRcd (1, not a state
-- name) confirms Established with one
-- prefix receivedR1# show ip bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.168.200.0/24 0.0.0.0 0 32768 i
*> 192.168.201.0/24 10.200.200.2 0 0 65002 i
-- The AS-path for the remote network
-- explicitly shows "65002" -- as this route
-- propagates further, R1 will prepend its
-- own AS number, building a path history
-- any future recipient can use for loop
-- detectionR1# show ip route bgp
B 192.168.201.0/24 [20/0] via 10.200.200.2, 00:02:15
-- Administrative distance of 20, the
-- standard default for eBGP-learned routes,
-- distinct from the 200 used for iBGPKey Takeaway
The AS-path attribute serves two purposes simultaneously: it is BGP's primary loop-prevention mechanism, since a router rejects any route whose AS-path already contains its own AS number, and it also functions as a metric of sorts during best-path selection, where a shorter AS-path is generally preferred — reading this single attribute in the BGP table explains both why a route was chosen and how far it has traveled through the autonomous system topology.