11.1.2 Lab – Implement eBGP for IPv4 (Answers)
Topology
Addressing Table
| Device | Interface | IPv4 Address |
|---|---|---|
| R1 | G0/0/0 | 10.1.2.1/24 |
| S0/1/0 | 10.1.3.1/25 | |
| S0/1/1 | 10.1.3.129/25 | |
| Loopback0 | 192.168.1.1/27 | |
| Loopback1 | 192.168.1.65/26 | |
| R2 | G0/0/0 | 10.1.2.2/24 |
| G0/0/1 | 10.2.3.2/24 | |
| Loopback0 | 192.168.2.1/27 | |
| Loopback1 | 192.168.2.65/26 | |
| R3 | G0/0/0 | 10.2.3.3/24 |
| S0/1/0 | 10.1.3.3/25 | |
| S0/1/1 | 10.1.3.130/25 | |
| Loopback0 | 192.168.3.1/27 | |
| Loopback1 | 192.168.3.65/26 |
Objectives
- Part 1: Build the Network and Configure Basic Device Settings and Interface Addressing
- Part 2: Configure and Verify eBGP for IPv4 on all Routers
- Part 3: Configure and Verify Route Summarization and Atomic Aggregate
- Part 4: Configure and Verify Route Summarization with Atomic Aggregate and AS-Set
- Part 5: Configure and Verify the Advertising of a Default Route
Background / Scenario
In this lab you will configure eBGP for IPv4.
Note: This lab is an exercise in developing, deploying, and verifying various path manipulation tools for BGP, and does not reflect networking best practices.
Note: The routers used with CCNP hands-on labs are Cisco 4221 with Cisco IOS XE Release 16.9.4 (universalk9 image). Other routers and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the commands available and the output produced might vary from what is shown in the labs.
Note: Ensure that the routers and switches have been erased and have no startup configurations. If you are unsure contact your instructor.
Instructor Note: Refer to the Instructor Lab Manual for the procedures to initialize and reload devices.
Required Resources
- 3 Routers (Cisco 4221 with Cisco IOS XE Release 16.9.4 universal image or comparable)
- 1 PC (Windows with a terminal emulation program, such as Tera Term)
- Console cables to configure the Cisco IOS devices via the console ports
- Ethernet and serial cables as shown in the topology
Instructions
Part 1: Build the Network and Configure Basic Device Settings and Interface Addressing
In Part 1, you will set up the network topology and configure basic settings and interface addressing on routers.
Step 1: Cable the network as shown in the topology.
Attach the devices as shown in the topology diagram, and cable as necessary.
Step 2: Configure basic settings for each router.
a. Console into each router, enter global configuration mode, and apply the basic settings and interface addressing. A command list for each router is provided below.
Router R1
hostname R1 no ip domain lookup line con 0 logging sync exec-time 0 0 exit interface Loopback0 ip address 192.168.1.1 255.255.255.224 no shut exit interface Loopback1 ip address 192.168.1.65 255.255.255.192 no shut exit interface GigabitEthernet0/0/0 ip address 10.1.2.1 255.255.255.0 no shut exit interface Serial0/1/0 ip address 10.1.3.1 255.255.255.128 no shut exit interface Serial0/1/1 ip address 10.1.3.129 255.255.255.128 no shut exit
Router R2
hostname R2 no ip domain lookup line con 0 logging sync exec-time 0 0 exit interface Loopback0 ip address 192.168.2.1 255.255.255.224 no shut exit interface Loopback1 ip address 192.168.2.65 255.255.255.192 no shut exit interface GigabitEthernet0/0/0 ip address 10.1.2.2 255.255.255.0 no shut exit interface GigabitEthernet0/0/1 ip address 10.2.3.2 255.255.255.0 no shut exit
Router R3
hostname R3 no ip domain lookup line con 0 logging sync exec-time 0 0 exit interface Loopback0 ip address 192.168.3.1 255.255.255.224 no shut exit interface Loopback1 ip address 192.168.3.65 255.255.255.192 no shut exit interface GigabitEthernet0/0/0 ip address 10.2.3.3 255.255.255.0 negotiation auto no shut exit interface Serial0/1/0 ip address 10.1.3.3 255.255.255.128 no shut exit interface Serial0/1/1 ip address 10.1.3.130 255.255.255.128 no shut exit
b. Save the running configuration to startup-config.
Part 2: Configure and Verify eBGP for IPv4 on all Routers
Step 1: Implement BGP and neighbor relationships on R1.
a. Enter BGP configuration mode from global configuration mode, specifying AS 1000.
R1(config)# router bgp 1000
b. Configure the BGP router-id for R1.
R1(config-router)# bgp router-id 1.1.1.1
c. Based on the topology diagram, configure all the designated neighbors for R1.
R1(config-router)# neighbor 10.1.2.2 remote-as 500 R1(config-router)# neighbor 10.1.3.3 remote-as 300 R1(config-router)# neighbor 10.1.3.130 remote-as 300
d. Configure R1 to advertise the IPv4 prefixes local to ASN 1000.
R1(config-router)# network 192.168.1.0 mask 255.255.255.224 R1(config-router)# network 192.168.1.64 mask 255.255.255.192
Step 2: Implement BGP and neighbor relationships on R2.
a. Enter BGP configuration mode from global configuration mode, specifying AS 500.
R2(config)# router bgp 500
b. Configure the BGP router-id for R2.
R2(config-router)# bgp router-id 2.2.2.2
c. Based on the topology diagram, configure all the designated neighbors for R2.
R2(config-router)# neighbor 10.1.2.1 remote-as 1000 R2(config-router)# neighbor 10.2.3.3 remote-as 300
d. Configure R2 to advertise the IPv4 prefixes local to ASN 500.
R2(config-router)# network 192.168.2.0 mask 255.255.255.224 R2(config-router)# network 192.168.2.64 mask 255.255.255.192
Step 3: Implement BGP and neighbor relationships on R3.
a. Enter BGP configuration mode from global configuration mode, specifying AS 300.
R3(config)# router bgp 300
b. Configure the BGP router-id for R3.
R3(config-router)# bgp router-id 3.3.3.3
c. Unlike the configuration on R1 and R2, disable the default IPv4 unicast behavior.
R3(config-router)# no bgp default ipv4-unicast
The default behavior in IOS is bgp default ipv4-unicast. Routers R1 and R2 were configured using this default behavior. The bgp default ipv4-unicast command enables the automatic exchange of IPv4 address family prefixes. When this command is disabled using no bgp default ipv4-unicast, bgp neighbors must be activated within IPv4 address family (AF) configuration mode. BGP network commands must also be configured within IPv4 AF mode.
d. Based on the topology diagram, configure all the designated neighbors for R3.
R3(config-router)# neighbor 10.2.3.2 remote-as 500 R3(config-router)# neighbor 10.1.3.1 remote-as 1000 R3(config-router)# neighbor 10.1.3.129 remote-as 1000
Step 4: Verifying BGP neighbor relationships.
a. Examine the routing tables on each router. Notice that R1 and R2 are receiving BGP prefixes from each other but not receiving BGP prefixes from R3. And R3 is not receiving any prefixes from R1 or R2. This is because R3 was configured using no bgp default ipv4-unicast and the interfaces must be activated within IPv4 address configuration mode.
R1# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.1.2.2, 00:28:40
B 192.168.2.64/26 [20/0] via 10.1.2.2, 00:28:40
R2# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.1.0/27 [20/0] via 10.1.2.1, 00:29:41
B 192.168.1.64/26 [20/0] via 10.1.2.1, 00:29:41
R3# show ip route bgp | begin Gateway
Gateway of last resort is not set
b. This can be further verified by examining the BGP neighbor adjacencies on R2. Notice the BGP state between R2 and R1 is established, while the BGP state between R2 and R3 is idle.
R2# show ip bgp neighbors
BGP neighbor is 10.1.2.1, remote AS 1000, external link
BGP version 4, remote router ID 1.1.1.1
BGP state = Established, up for 00:35:34
Last read 00:00:28, last write 00:00:35, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
<output omitted>
BGP neighbor is 10.2.3.3, remote AS 300, external link
BGP version 4, remote router ID 0.0.0.0
BGP state = Idle, down for never
Neighbor sessions:
0 active, is not multisession capable (disabled)
<output omitted>
c. The interfaces on R3 need to be activated in IPv4 AF configuration mode. The neighbor activate command in IPv4 AF configuration mode is required to enable the exchange of BGP information between neighbors. This will enable R3 to form an established neighbor adjacency with both R1 and R2. Additionally, because bgp default ipv4-unicast is disabled, network commands must be configured in IPv4 AF configuration mode.
R3(config-router)# address-family ipv4 R3(config-router-af)# neighbor 10.1.3.1 activate R3(config-router-af)# neighbor 10.1.3.129 activate R3(config-router-af)# neighbor 10.2.3.2 activate R3(config-router-af)# network 192.168.3.0 mask 255.255.255.224 R3(config-router-af)# network 192.168.3.64 mask 255.255.255.192
d. Verify that all BGP speakers are receiving prefixes from their neighbors. The prefixes from R3 are highlighted in the routing tables of R1 and R2.
Note: The prefixes in the lab are for example purposes only. Most service providers do not accept prefixes larger than /24 for IPv4 (/25 through /32).
R1# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.1.2.2, 00:51:09
B 192.168.2.64/26 [20/0] via 10.1.2.2, 00:51:09
192.168.3.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.3.0/27 [20/0] via 10.1.3.3, 00:01:43
B 192.168.3.64/26 [20/0] via 10.1.3.3, 00:01:43
R2# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.1.0/27 [20/0] via 10.1.2.1, 00:51:17
B 192.168.1.64/26 [20/0] via 10.1.2.1, 00:51:17
192.168.3.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.3.0/27 [20/0] via 10.2.3.3, 00:01:51
B 192.168.3.64/26 [20/0] via 10.2.3.3, 00:01:51
R3# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.1.0/27 [20/0] via 10.1.3.1, 00:02:11
B 192.168.1.64/26 [20/0] via 10.1.3.1, 00:02:11
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.2.3.2, 00:02:11
B 192.168.2.64/26 [20/0] via 10.2.3.2, 00:02:11
e. Verify that the BGP state between R2 and R3 has now been established.
R2# show ip bgp neighbors | begin BGP neighbor is 10.2.3.3
BGP neighbor is 10.2.3.3, remote AS 300, external link
BGP version 4, remote router ID 3.3.3.3
BGP state = Established, up for 00:12:16
Last read 00:00:37, last write 00:00:52, hold time is 180, keepalive interval is 60 seconds
Neighbor sessions:
1 active, is not multisession capable (disabled)
<output omitted>
Step 5: Examining the running-configs.
Examine the running-configs on all three routers. Because router R3 was configured using no bgp default ipv4-unicast command, notice that the network commands were automatically entered under the IPv4 AF. This is the same configuration mode where the neighbors were activated to exchange BGP information.
R1# show running-config | section bgp router bgp 1000 bgp router-id 1.1.1.1 bgp log-neighbor-changes network 192.168.1.0 mask 255.255.255.224 network 192.168.1.64 mask 255.255.255.192 neighbor 10.1.2.2 remote-as 500 neighbor 10.1.3.3 remote-as 300 neighbor 10.1.3.130 remote-as 300 R2# show running-config | section bgp router bgp 500 bgp router-id 2.2.2.2 bgp log-neighbor-changes network 192.168.2.0 mask 255.255.255.224 network 192.168.2.64 mask 255.255.255.192 neighbor 10.1.2.1 remote-as 1000 neighbor 10.2.3.3 remote-as 300 R3# show running-config | section bgp router bgp 300 bgp log-neighbor-changes no bgp default ipv4-unicast neighbor 10.1.3.1 remote-as 1000 neighbor 10.1.3.129 remote-as 1000 neighbor 10.2.3.2 remote-as 500 ! address-family ipv4 network 192.168.3.0 mask 255.255.255.224 network 192.168.3.64 mask 255.255.255.192 neighbor 10.1.3.1 activate neighbor 10.1.3.129 activate neighbor 10.2.3.2 activate exit-address-family
Step 6: Verifying BGP operations.
a. To verify the BGP operation on R2, issue the show ip bgp command.
R2# show ip bgp
BGP table version is 11, local router ID is 2.2.2.2
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,
t secondary path, L long-lived-stale,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 192.168.1.0/27 10.2.3.3 0 300 1000 i
*> 10.1.2.1 0 0 1000 i
* 192.168.1.64/26 10.2.3.3 0 300 1000 i
*> 10.1.2.1 0 0 1000 i
*> 192.168.2.0/27 0.0.0.0 0 32768 i
*> 192.168.2.64/26 0.0.0.0 0 32768 i
*> 192.168.3.0/27 10.2.3.3 0 0 300 i
* 10.1.2.1 0 1000 300 i
*> 192.168.3.64/26 10.2.3.3 0 0 300 i
* 10.1.2.1 0 1000 300 i
What does the * at the beginning of an entry indicate?
The entry is valid, reachable.
What does the angle bracket (>) in an entry indicate?
An angle bracket (>) indicates that the route has been selected as the best route.
What is the address of the preferred next hop router to reach the 192.168.1.0/27 network? Explain.
10.1.2.1. It is preferred because it has shorter AS path.
How can you verify that 10.1.2.1 is the next hop router used to reach 192.168.1.0/27?
Display the routing table on R2.
B 192.168.1.0/27 [20/0] via 10.1.2.1, 17:31:46
What does a next hop of 0.0.0.0 indicate?
This advertised prefix is a directly connected network.
b. Use the show ip bgp ip-prefix command to display all the paths for a specific route and the BGP path attributes for that route.
R2# show ip bgp 192.168.1.0
BGP routing table entry for 192.168.1.0/27, version 14
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 1
300 1000
10.2.3.3 from 10.2.3.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
1000
10.1.2.1 from 10.1.2.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
What is the IPv4 address of the next hop router with the best path?
10.1.2.1
c. Examine the BGP neighbor relationships on R2 using the show ip bgp neighbors command.
R2# show ip bgp neighbors
BGP neighbor is 10.1.2.1, remote AS 1000, external link
BGP version 4, remote router ID 1.1.1.1
BGP state = Established, up for 00:00:51
Last read 00:00:00, last write 00:00:51, 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 5
Keepalives: 2 3
Route Refresh: 0 0
Total: 10 11
<output omitted>
BGP neighbor is 10.2.3.3, remote AS 300, external link
BGP version 4, remote router ID 3.3.3.3
BGP state = Established, up for 16:23:45
Last read 00:00:29, last write 00:00:51, 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: 9 5
Keepalives: 1082 1088
Route Refresh: 0 0
Total: 1096 1096
Do log neighbor state changes (via global configuration)
Default minimum time between advertisement runs is 30 seconds
<output omitted>
How many neighbors does R2 have and what are their router IDs?
Two neighbors: 1.1.1.1 and 3.3.3.3
What is the BGP state of both neighbors?
established
What are the keepalive and hold time value for both neighbors?
Keepalive is 60 seconds, hold time is180 seconds
Part 3: Configure and Verify Route Summarization and Atomic Aggregate
Step 1: Configure route summarization using atomic aggregate.
Summarizing prefixes conserves router resources and accelerates best-path calculation by reducing the size of the table. Summarization can be configured either for prefixes originated by the AS or prefixes received from downstream providers. Summarization also provides the benefits of stability by hiding flapping routes or having to install new prefixes when they are contained within a summary.
Although AS 1000 only has two prefixes 192.168.1.0/27 and 192.168.1.64/26, this customer has been allocated the entire 192.168.1.0/24 prefix. R3 in AS 300 has two prefixes 192.168.3.0/27 and 192.168.3.64/26 but has been allocated the entire 192.168.3.0/24 prefix.
Configure R1 and R3 to advertise a summary or aggregate route using the aggregate-address command. The summary-only option suppresses the specific prefixes that are summarized from also being advertised. Notice that this command is configured in address-family ipv4 configuration mode on R3.
R1(config)# router bgp 1000 R1(config-router)# aggregate-address 192.168.1.0 255.255.255.0 summary-only R3(config)# router bgp 300 R3(config-router)# address-family ipv4 R3(config-router-af)# aggregate-address 192.168.3.0 255.255.255.0 summary-only
Step 2: Verify route summarization using atomic aggregate.
a. Examine the routing tables on each router to verify the route summarization for the two prefixes. Verify that R1 and R3 are each receiving the summary route from the other router. Verify that R2 is receiving aggregate routes from both R1 and R3.
R1# show ip route bgp | begin Gateway
Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 5 subnets, 4 masks
B 192.168.1.0/24 [200/0], 00:27:47, Null0
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.1.2.2, 13:34:31
B 192.168.2.64/26 [20/0] via 10.1.2.2, 13:34:31
B 192.168.3.0/24 [20/0] via 10.1.3.3, 00:26:01
R2# show ip route bgp | begin Gateway
Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.1.2.1, 00:33:53
B 192.168.3.0/24 [20/0] via 10.2.3.3, 00:32:08
R3# show ip route bgp | begin Gateway
Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.1.3.1, 00:36:52
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.2.3.2, 02:10:48
B 192.168.2.64/26 [20/0] via 10.2.3.2, 02:10:48
192.168.3.0/24 is variably subnetted, 5 subnets, 4 masks
B 192.168.3.0/24 [200/0], 00:35:07, Null0
Why do R1 and R3 contain an entry with a next hop address of Null0? What is the result of having this Null0 route in the routing table?
This is the same prefix as the aggregate route advertised by this router. The router’s routing table contains more specific routes. The Null0 route is a discard route for loop prevention. Any packets with a destination IPv4 address matching the aggregated prefix with the Null0 egress interface, but do not match a more specific route, will be dropped.
b. Examine the BGP table on router R2 to verify the route summarization. When a prefix has the default classful mask, the subnet mask is not displayed. Both 192.168.1.0 and 192.168.3.0 prefixes have a /24 prefix length which would be the default mask for a Class C address.
R2# show ip bgp
BGP table version is 69, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
<output omitted>
Network Next Hop Metric LocPrf Weight Path
* 192.168.1.0 10.2.3.3 0 300 1000 i
*> 10.1.2.1 0 0 1000 i
*> 192.168.2.0/27 0.0.0.0 0 32768 i
*> 192.168.2.64/26 0.0.0.0 0 32768 i
* 192.168.3.0 10.1.2.1 0 1000 300 i
*> 10.2.3.3 0 0 300 i
c. Examine the BGP table on routers R2 and R3 and verify that each router is receiving the summary route from the other router.
R1# show ip bgp
BGP table version is 69, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
<output omitted>
Network Next Hop Metric LocPrf Weight Path
s> 192.168.1.0/27 0.0.0.0 0 32768 i
*> 192.168.1.0 0.0.0.0 32768 i
s> 192.168.1.64/26 0.0.0.0 0 32768 i
* 192.168.2.0/27 10.1.3.130 0 300 500 i
* 10.1.3.3 0 300 500 i
*> 10.1.2.2 0 0 500 i
* 192.168.2.64/26 10.1.3.130 0 300 500 i
* 10.1.3.3 0 300 500 i
*> 10.1.2.2 0 0 500 i
* 192.168.3.0 10.1.2.2 0 500 300 i
* 10.1.3.130 0 0 300 i
*> 10.1.3.3 0 0 300 i
R3# show ip bgp
BGP table version is 22, local router ID is 3.3.3.3
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,
t secondary path, L long-lived-stale,
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found
Network Next Hop Metric LocPrf Weight Path
* 192.168.1.0 10.2.3.2 0 500 1000 i
*> 10.1.3.1 0 0 1000 i
* 10.1.3.129 0 0 1000 i
* 192.168.2.0/27 10.1.3.1 0 1000 500 i
* 10.1.3.129 0 1000 500 i
*> 10.2.3.2 0 0 500 i
* 192.168.2.64/26 10.1.3.1 0 1000 500 i
* 10.1.3.129 0 1000 500 i
*> 10.2.3.2 0 0 500 i
s> 192.168.3.0/27 0.0.0.0 0 32768 i
*> 192.168.3.0 0.0.0.0 32768 i
s> 192.168.3.64/26 0.0.0.0 0 32768 i
Why do two of the entries have the status code of “s”? Specifically, this is the result of what command or option that was configured on these two routers?
This indicates that the route was suppressed. These entries were suppressed because a summary route (“*” valid) was advertised instead. Prefixes with a next hop of 0.0.0.0 are directly connected networks on this router. The summary-only option in the aggregate-address command suppressed these more specific prefixes from being advertised.
d. Examine the explicit 192.168.1.0 prefix entry in R2’s BGP table. The route’s NLRI information indicates that the route was aggregated in AS 1000 by router with the RID 1.1.1.1.
R2# show ip bgp 192.168.1.0
BGP routing table entry for 192.168.1.0/24, version 45
Paths: (2 available, best #2, table default)
Advertised to update-groups:
1
Refresh Epoch 1
300 1000, (aggregated by 1000 1.1.1.1)
10.2.3.3 from 10.2.3.3 (3.3.3.3)
Origin IGP, localpref 100, valid, external, atomic-aggregate
rx pathid: 0, tx pathid: 0
Refresh Epoch 2
1000, (aggregated by 1000 1.1.1.1)
10.1.2.1 from 10.1.2.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, external, atomic-aggregate, best
rx pathid: 0, tx pathid: 0x0
Part 4: Configure and Verify Route Summarization with Atomic Aggregate and AS-Set
Step 1: Configure route summarization using atomic aggregate and AS-Set.
a. Shut down both serial interfaces on R1. This will create a single path from R1 (AS 1000) to R2 (AS 500) to R3 (AS 300).
R1(config)# interface s0/1/0 R1(config-if)# shutdown R1(config-if)# exit R1(config)# interface s0/1/1 R1(config-if)# shutdown
b. Remove route aggregation previously configured on R1.
R1(config)# router bgp 1000 R1(config-router)# no aggregate-address 192.168.1.0 255.255.255.0 summary-only
c. Verify that R3 is now receiving the non-summarized prefixes 192.168.1.0/27 and 192.168.1.64/26.
R3# show ip route 192.168.1.0 Routing entry for 192.168.1.0/24, 2 known subnets Variably subnetted with 2 masks B 192.168.1.0/27 [20/0] via 10.2.3.2, 00:01:26 B 192.168.1.64/26 [20/0] via 10.2.3.2, 00:01:26
d. On R2, summarize the prefixes 192.168.1.0/27 and 192.168.1.64/26 received from R1 as 192.168.1.0/24.
R2(config)# router bgp 500 R2(config-router)# aggregate-address 192.168.1.0 255.255.255.0 summary-only
Step 2: Verify route summarization using atomic aggregate and AS-Set.
a. Verify that R3 is receiving the aggregated prefix 192.168.1.0/24.
R3# show ip route bgp | begin Gateway
Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.2.3.2, 00:00:51
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.2.3.2, 08:46:37
B 192.168.2.64/26 [20/0] via 10.2.3.2, 08:46:37
192.168.3.0/24 is variably subnetted, 5 subnets, 4 masks
B 192.168.3.0/24 [200/0], 08:46:07, Null0
b. Examine R3’s BGP table. Notice that the AS path only includes the AS that summarized the route, AS 500, router R2.
R3# show ip bgp
<output omitted>
Network Next Hop Metric LocPrf Weight Path
*> 192.168.1.0 10.2.3.2 0 0 500 i
*> 192.168.2.0/27 10.2.3.2 0 0 500 i
*> 192.168.2.64/26 10.2.3.2 0 0 500 i
s> 192.168.3.0/27 0.0.0.0 0 32768 i
*> 192.168.3.0 0.0.0.0 32768 i
s> 192.168.3.64/26 0.0.0.0 0 32768 i
c. On R2, remove the current route aggregation for the 192.168.1.0/24 prefix and configure it again, this time using the as-set option.
R2(config)# router bgp 500 R2(config-router)# no aggregate-address 192.168.1.0 255.255.255.0 summary-only R2(config-router)# aggregate-address 192.168.1.0 255.255.255.0 as-set summary-only
d. Verify that R3 is receiving the aggregated prefix 192.168.1.0/24.
R3# show ip route bgp | begin Gateway
Gateway of last resort is not set
B 192.168.1.0/24 [20/0] via 10.2.3.2, 00:01:35
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.2.3.2, 08:50:02
B 192.168.2.64/26 [20/0] via 10.2.3.2, 08:50:02
192.168.3.0/24 is variably subnetted, 5 subnets, 4 masks
B 192.168.3.0/24 [200/0], 08:49:32, Null0
e. Examine R3’s BGP table again. Notice that the entry for 192.168.1.0 this time includes the entire AS path. The output from the show ip bgp 192.168.1.0 command displays both AS numbers and identifies that R2 (2.2.2.2) aggregated the route.
R3# show ip bgp
<output omitted>
Network Next Hop Metric LocPrf Weight Path
*> 192.168.1.0 10.2.3.2 0 0 500 1000 i
*> 192.168.2.0/27 10.2.3.2 0 0 500 i
*> 192.168.2.64/26 10.2.3.2 0 0 500 i
s> 192.168.3.0/27 0.0.0.0 0 32768 i
*> 192.168.3.0 0.0.0.0 32768 i
s> 192.168.3.64/26 0.0.0.0 0 32768 i
R3# show ip bgp 192.168.1.0 | begin Refresh
Refresh Epoch 7
500 1000, (aggregated by 500 2.2.2.2)
10.2.3.2 from 10.2.3.2 (2.2.2.2)
Origin IGP, metric 0, localpref 100, valid, external, best
rx pathid: 0, tx pathid: 0x0
Part 5: Configure and Verify the Advertising of a Default Route
Step 1: Configure default route advertisement on R2.
Configure R2 to advertise a default router to R1. R2 does not necessarily have to have a default route of its own. Core internet routers that have full internet routing tables and do not require a default route are referred to as being in a default-free zone (DFZ).
R2(config)# router bgp 500 R2(config-router)# neighbor 10.1.2.1 default-originate
Step 2: Verify default route advertisement on R1.
a. Examine R1’s routing table to verify that it has received a default route.
R1# show ip route bgp | begin Gateway
Gateway of last resort is 10.1.2.2 to network 0.0.0.0
B* 0.0.0.0/0 [20/0] via 10.1.2.2, 00:00:37
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
B 192.168.2.0/27 [20/0] via 10.1.2.2, 21:24:43
B 192.168.2.64/26 [20/0] via 10.1.2.2, 21:24:43
B 192.168.3.0/24 [20/0] via 10.1.2.2, 12:41:58
b. Examine R1’s BGP table to verify that it has received a default route.
R1# show ip bgp
<output omitted>
Network Next Hop Metric LocPrf Weight Path
*> 0.0.0.0 10.1.2.2 0 500 i
*> 192.168.1.0/27 0.0.0.0 0 32768 i
*> 192.168.1.64/26 0.0.0.0 0 32768 i
*> 192.168.2.0/27 10.1.2.2 0 0 500 i
*> 192.168.2.64/26 10.1.2.2 0 0 500 i
*> 192.168.3.0 10.1.2.2 0 500 300 i
Router Interface Summary Table
| Router Model | Ethernet Interface #1 | Ethernet Interface #2 | Serial Interface #1 | Serial Interface #2 |
|---|---|---|---|---|
| 1800 | Fast Ethernet 0/0 (F0/0) | Fast Ethernet 0/1 (F0/1) | Serial 0/0/0 (S0/0/0) | Serial 0/0/1 (S0/0/1) |
| 1900 | Gigabit Ethernet 0/0 (G0/0) | Gigabit Ethernet 0/1 (G0/1) | Serial 0/0/0 (S0/0/0) | Serial 0/0/1 (S0/0/1) |
| 2801 | Fast Ethernet 0/0 (F0/0) | Fast Ethernet 0/1 (F0/1) | Serial 0/1/0 (S0/1/0) | Serial 0/1/1 (S0/1/1) |
| 2811 | Fast Ethernet 0/0 (F0/0) | Fast Ethernet 0/1 (F0/1) | Serial 0/0/0 (S0/0/0) | Serial 0/0/1 (S0/0/1) |
| 2900 | Gigabit Ethernet 0/0 (G0/0) | Gigabit Ethernet 0/1 (G0/1) | Serial 0/0/0 (S0/0/0) | Serial 0/0/1 (S0/0/1) |
| 4221 | Gigabit Ethernet 0/0/0 (G0/0/0) | Gigabit Ethernet 0/0/1 (G0/0/1) | Serial 0/1/0 (S0/1/0) | Serial 0/1/1 (S0/1/1) |
| 4300 | Gigabit Ethernet 0/0/0 (G0/0/0) | Gigabit Ethernet 0/0/1 (G0/0/1) | Serial 0/1/0 (S0/1/0) | Serial 0/1/1 (S0/1/1) |
Note: To find out how the router is configured, look at the interfaces to identify the type of router and how many interfaces the router has. There is no way to effectively list all the combinations of configurations for each router class. This table includes identifiers for the possible combinations of Ethernet and Serial interfaces in the device. The table does not include any other type of interface, even though a specific router may contain one. An example of this might be an ISDN BRI interface. The string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands to represent the interface.
Device Configs – Final
Router R1
R1# show running-config Building configuration... Current configuration : 1576 bytes ! ! Last configuration change at 21:51:24 UTC Thu Dec 19 2019 ! version 16.9 service timestamps debug datetime msec service timestamps log datetime msec platform qfp utilization monitor load 80 no platform punt-keepalive disable-kernel-core ! hostname R1 ! boot-start-marker boot-end-marker ! no aaa new-model ! no ip domain lookup ! login on-success log ! subscriber templating ! multilink bundle-name authenticated ! license udi pid ISR4221/K9 sn FGL23313183 no license smart enable diagnostic bootup level minimal ! spanning-tree extend system-id ! redundancy mode none ! interface Loopback0 ip address 192.168.1.1 255.255.255.224 ! interface Loopback1 ip address 192.168.1.65 255.255.255.192 ! interface GigabitEthernet0/0/0 ip address 10.1.2.1 255.255.255.0 negotiation auto ! interface GigabitEthernet0/0/1 no ip address negotiation auto ! interface Serial0/1/0 ip address 10.1.3.1 255.255.255.128 shutdown ! interface Serial0/1/1 ip address 10.1.3.129 255.255.255.128 shutdown ! router bgp 1000 bgp router-id 1.1.1.1 bgp log-neighbor-changes network 192.168.1.0 mask 255.255.255.224 network 192.168.1.64 mask 255.255.255.192 neighbor 10.1.2.2 remote-as 500 neighbor 10.1.3.3 remote-as 300 neighbor 10.1.3.130 remote-as 300 ! ip forward-protocol nd no ip http server ip http secure-server ! control-plane ! ! line con 0 exec-timeout 0 0 logging synchronous transport input none stopbits 1 line aux 0 stopbits 1 line vty 0 4 login ! end
Router R2
R2# show running-config Building configuration... Current configuration : 1519 bytes ! ! Last configuration change at 15:45:14 UTC Sun Dec 22 2019 ! version 16.9 service timestamps debug datetime msec service timestamps log datetime msec platform qfp utilization monitor load 80 no platform punt-keepalive disable-kernel-core ! hostname R2 ! boot-start-marker boot-end-marker ! no aaa new-model ! no ip domain lookup ! login on-success log ! subscriber templating ! multilink bundle-name authenticated ! license udi pid ISR4221/K9 sn FGL23313182 no license smart enable diagnostic bootup level minimal ! spanning-tree extend system-id ! redundancy mode none ! interface Loopback0 ip address 192.168.2.1 255.255.255.224 ! interface Loopback1 ip address 192.168.2.65 255.255.255.192 ! interface GigabitEthernet0/0/0 ip address 10.1.2.2 255.255.255.0 negotiation auto ! interface GigabitEthernet0/0/1 ip address 10.2.3.2 255.255.255.0 negotiation auto ! router bgp 500 bgp router-id 2.2.2.2 bgp log-neighbor-changes network 192.168.2.0 mask 255.255.255.224 network 192.168.2.64 mask 255.255.255.192 aggregate-address 192.168.1.0 255.255.255.0 as-set summary-only neighbor 10.1.2.1 remote-as 1000 neighbor 10.1.2.1 default-originate neighbor 10.2.3.3 remote-as 300 ! ip forward-protocol nd no ip http server ip http secure-server ! control-plane ! line con 0 exec-timeout 0 0 logging synchronous transport input none stopbits 1 line aux 0 stopbits 1 line vty 0 4 login ! end
Router R3
R3# show running-config Building configuration... Current configuration : 1781 bytes ! ! Last configuration change at 18:11:15 UTC Sat Dec 21 2019 ! version 16.9 service timestamps debug datetime msec service timestamps log datetime msec platform qfp utilization monitor load 80 no platform punt-keepalive disable-kernel-core ! hostname R3 ! boot-start-marker boot-end-marker ! no aaa new-model ! no ip domain lookup ! login on-success log ! subscriber templating ! multilink bundle-name authenticated ! license udi pid ISR4221/K9 sn FGL23313186 no license smart enable diagnostic bootup level minimal ! spanning-tree extend system-id ! redundancy mode none ! interface Loopback0 ip address 192.168.3.1 255.255.255.224 ! interface Loopback1 ip address 192.168.3.65 255.255.255.192 ! interface GigabitEthernet0/0/0 ip address 10.2.3.3 255.255.255.0 negotiation auto ! interface GigabitEthernet0/0/1 no ip address negotiation auto ! interface Serial0/1/0 ip address 10.1.3.3 255.255.255.128 ! interface Serial0/1/1 ip address 10.1.3.130 255.255.255.128 ! router bgp 300 bgp router-id 3.3.3.3 bgp log-neighbor-changes no bgp default ipv4-unicast neighbor 10.1.3.1 remote-as 1000 neighbor 10.1.3.129 remote-as 1000 neighbor 10.2.3.2 remote-as 500 ! address-family ipv4 network 192.168.3.0 mask 255.255.255.224 network 192.168.3.64 mask 255.255.255.192 aggregate-address 192.168.3.0 255.255.255.0 summary-only neighbor 10.1.3.1 activate neighbor 10.1.3.129 activate neighbor 10.2.3.2 activate exit-address-family ! ip forward-protocol nd no ip http server ip http secure-server ! control-plane ! line con 0 exec-timeout 0 0 logging synchronous transport input none stopbits 1 line aux 0 stopbits 1 line vty 0 4 login ! end
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