What is an Offset-list? How to configure EIGRP Offset-list?

 What is an Offset-list?

An Offset-list is an EIGRP and RIPv2 feature that allows you to add a fixed value (an “offset”) to the composite metric of a route that matches an access control list (ACL). You can apply this to inbound (IN) and outbound (OUT) routing updates.

By increasing the metric of a route, you make it less preferable. EIGRP will always choose the path with the lower composite metric. It’s a simple yet powerful way to de-prioritize a path without using a route filter to deny it entirely. Offset value to add to the matric (1-2147483647)

Why would you use one?

Common use cases include:

• ·        Basic load balance: making a primary link preferred while keeping a backup link active by making its metric slightly worse.
• ·        Policy-based routing: influencing outbound traffic to take a specific path based on network policy.
• ·        Cost engineering: simulating a longer path for a route learned from one neighbor compared to another.

 let's see the configuration:-

Topology:-

• Configure the topology as per the diagram
• Configure the IP addresses as per the topology 
• Configure EIGRP 100 to advertise directly connected interfaces
• Configure access-list standard permit 30.1.1.1 network
• Make sure router 1 will use the serial 5/0 to reach the 30.1.1.1/8 network
• Ensure no load balancing is used with the offset list; add 5000 metrics to the serial 5/3 route.

R1(config)#interface serial 5/0
R1(config-if)#ip address 1.1.1.1 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface serial 5/3
R1(config-if)#ip address 4.4.4.2 255.0.0.0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface FastEthernet 0/0
R1(config-if)#ip address 10.1.1.1 255.0.0.0
R1(config-if)#no keepalive
R1(config-if)#no shutdown
R1(config-if)#exit

R2(config)#interface serial 5/0
R2(config-if)#ip address 1.1.1.2 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface serial 5/1
R2(config-if)#ip address 2.2.2.1 255.0.0.0
R2(config-if)#no shutdown
R2(config-if)#exit
R2(config)#interface FastEthernet 0/0
R2(config-if)#ip address 20.1.1.1 255.0.0.0
R2(config-if)#no keepalive
R2(config-if)#no shutdown
R2(config-if)#exit

R3(config)#interface serial 5/2
R3(config-if)#ip address 3.3.3.1 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface serial 5/1
R3(config-if)#ip address 2.2.2.2 255.0.0.0
R3(config-if)#no shutdown
R3(config-if)#exit
R3(config)#interface FastEthernet 0/0
R3(config-if)#ip address 30.1.1.1 255.0.0.0
R3(config-if)#no keepalive
R3(config-if)#no shutdown
R3(config-if)#exit

R4(config)#interface serial 5/2
R4(config-if)#ip address 3.3.3.2 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface serial 5/3
R4(config-if)#ip address 4.4.4.1 255.0.0.0
R4(config-if)#no shutdown
R4(config-if)#exit
R4(config)#interface FastEthernet 0/0
R4(config-if)#ip address 40.1.1.1 255.0.0.0
R4(config-if)#no keepalive
R4(config-if)#no shutdown
R4(config-if)#exit

R1(config)#router eigrp 100

R1(config-router)#network 10.0.0.0

R1(config-router)#network 1.0.0.0

R1(config-router)#network 4.0.0.0

R1(config-router)#no auto-summary

R1(config-router)#exit

R2(config)#router eigrp 100

R2(config-router)#network 20.0.0.0

R2(config-router)#network 1.0.0.0

R2(config-router)#network 2.0.0.0

R2(config-router)#no auto-summary

R2(config-router)#exit

R3(config)#router eigrp 100

R3(config-router)#network 30.0.0.0

R3(config-router)#network 3.0.0.0

R3(config-router)#network 2.0.0.0

R3(config-router)#no auto-summary

R3(config-router)#exit

R4(config)#router eigrp 100

R4(config-router)#network 40.0.0.0

R4(config-router)#network 3.0.0.0

R4(config-router)#network 4.0.0.0

R4(config-router)#no auto-summary

R4(config-router)#exit

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    1.0.0.0/8 is directly connected, Serial5/0

D    2.0.0.0/8 [90/2681856] via 1.1.1.2, 00:26:52, Serial5/0

D    3.0.0.0/8 [90/2681856] via 4.4.4.1, 00:26:52, Serial5/3

C    4.0.0.0/8 is directly connected, Serial5/3

D    20.0.0.0/8 [90/2172416] via 1.1.1.2, 00:26:52, Serial5/0

D    40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:26:52, Serial5/3

C    10.0.0.0/8 is directly connected, FastEthernet0/0

D    30.0.0.0/8 [90/2684416] via 4.4.4.1, 00:24:15, Serial5/3

                [90/2684416] via 1.1.1.2, 00:24:15, Serial5/0

R1#traceroute 30.1.1.1

Type escape sequence to abort.

Tracing the route to 30.1.1.1

  1 4.4.4.1 48 msec

    1.1.1.2 28 msec

    4.4.4.1 28 msec

  2 2.2.2.2 80 msec

    3.3.3.1 80 msec

    2.2.2.2 80 msec

R1#show ip route 30.1.1.1 255.0.0.0

Routing entry for 30.0.0.0/8

  Known via "eigrp 100", distance 90, metric 2684416, type internal

  Redistributing via eigrp 100

  Last update from 4.4.4.1 on Serial5/3, 00:01:28 ago

  Routing Descriptor Blocks:

    4.4.4.1, from 4.4.4.1, 00:01:28 ago, via Serial5/3

      Route metric is 2684416, traffic share count is 1

      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

  * 1.1.1.2, from 1.1.1.2, 00:01:28 ago, via Serial5/0

      Route metric is 2684416, traffic share count is 1

      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

(as you can see, both route metrics are the same. So now we are going to add 5000 metric to the serial 5/3 route.

R1(config)#ip access-list standard Offset

R1(config-std-nacl)#permit 30.0.0.0 0.255.255.255

R1(config-std-nacl)#exit

R1(config)#router eigrp 100

R1(config-router)#offset-list Offset in  5000 serial 5/3

R1(config-router)#end

*Aug 30 16:14:11.471: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 4.4.4.1 (Serial5/3) is resync: route configuration changed

R1#clear ip route *

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    1.0.0.0/8 is directly connected, Serial5/0

D    2.0.0.0/8 [90/2681856] via 1.1.1.2, 00:00:39, Serial5/0

D    3.0.0.0/8 [90/2681856] via 4.4.4.1, 00:00:39, Serial5/3

C    4.0.0.0/8 is directly connected, Serial5/3

D    20.0.0.0/8 [90/2172416] via 1.1.1.2, 00:00:39, Serial5/0

D    40.0.0.0/8 [90/2172416] via 4.4.4.1, 00:00:39, Serial5/3

C    10.0.0.0/8 is directly connected, FastEthernet0/0

D    30.0.0.0/8 [90/2684416] via 1.1.1.2, 00:00:39, Serial5/0

R1#traceroute 30.1.1.1

Type escape sequence to abort.

Tracing the route to 30.1.1.1

  1 1.1.1.2 40 msec 32 msec 32 msec

  2 2.2.2.2 76 msec 56 msec 72 msec

R1#show ip route 30.1.1.1 255.0.0.0

Routing entry for 30.0.0.0/8

  Known via "eigrp 100", distance 90, metric 2684416, type internal

  Redistributing via eigrp 100

  Last update from 1.1.1.2 on Serial5/0, 00:01:57 ago

  Routing Descriptor Blocks:

  * 1.1.1.2, from 1.1.1.2, 00:01:57 ago, via Serial5/0

      Route metric is 2684416, traffic share count is 1

      Total delay is 40100 microseconds, minimum bandwidth is 1544 Kbit

      Reliability 255/255, minimum MTU 1500 bytes

      Loading 1/255, Hops 2

R1#show ip eigrp topology
IP-EIGRP Topology Table for AS(100)/ID(10.1.1.1)
Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - reply Status, s - sia Status
P 1.0.0.0/8, 1 successors, FD is 2169856
        via Connected, Serial5/0
P 2.0.0.0/8, 1 successors, FD is 2681856
        via 1.1.1.2 (2681856/2169856), Serial5/0
P 3.0.0.0/8, 1 successors, FD is 2681856
        via 4.4.4.1 (2681856/2169856), Serial5/3
P 4.0.0.0/8, 1 successors, FD is 2169856
        via Connected, Serial5/3
P 10.0.0.0/8, 1 successors, FD is 28160
        via Connected, FastEthernet0/0
P 20.0.0.0/8, 1 successors, FD is 2172416
        via 1.1.1.2 (2172416/28160), Serial5/0
P 30.0.0.0/8, 1 successors, FD is 2684416
        via 1.1.1.2 (2684416/2172416), Serial5/0
        via 4.4.4.1 (2689416/2177416), Serial5/3
P 40.0.0.0/8, 1 successors, FD is 2172416
        via 4.4.4.1 (2172416/28160), Serial5/3

 ( as you can see, the metric has been increased to 2177416, and previously this metric was 2172416)

I hope you like this lab. Please visit our YouTube channel https://www.youtube.com/@internetworkss

What is IP prefix-list? How to configure IP prefix-list?

 IP Prefix List

An IP prefix list is a network filtering tool used primarily in routing protocols to control which routes are accepted or rejected based on their network prefixes.

Key Characteristics

·        Prefix-based filtering: Matches routes based on their network address and prefix length.

·        Ordering processing: Entries are processed top-down until a match is found.

·        Implicit deny: if no match is found, the prefix is denied by default

 

R1(config)#ip prefix-list NAME seq 10 deny 192.168.1.1/24?

  ge  Minimum prefix length to be matched

  le  Maximum prefix length to be matched

  <cr>

Parameters

·        Network/len: The network address and prefix length to match.

·        Ge: “greater than or equal to” – minimum prefix length to match.

·        Le: “less than or equal to” – maximum prefix length to match.

·        Seq: optional sequence number for ordering entries.

Prefix lists are widely used in routing protocols like BGP, OSPF, and EIGRP for precise control over route advertisements and redistributions. 

Topology:-

Goal:.Configure the topology as per the diagram

1. Configure the IP addresses as per the topology
2. Configure IGP protocol EIGRP 100
3. Advertise the directly connected routes 
4. configure IP Prefix list name INTERNET
5. Deny 192.168.31.0/24 - 192.168.32.0/24
6. rest permit
7. deny 192.168.11.0/24 adjust the SEQ number.
8. make sure router 2 deny192.168.31.1/24 & 192.168.32.1/24 and 192.168.11.1/24 
9. change the SEQ

OSPF DR and BDR Explained: The Hidden Heroes of Network Efficiency.

In the world of networking, OSPF (Open Shortest Path First) is one of the most widely used dynamic routing protocols. But did you know that behind the scenes, two special routers —the **Designated Router (DR)** and **Backup Designated Router (BDR)** — work tirelessly to keep your network running smoothly?

In this blog post, we’ll uncover:

 

• 1.      What do DR and BDR do for our network?
• 2.      How do they get elected, and why is this election important?
• 3.      What are the Common mistakes in OSPF configuration?
• 4.      What are the best practices for optimizing DR/BDR selection?

 

Let's dive in-à

·        Why do we need DR and BDR?

Ø  Imagine a conference room where everyone is shouting updates at the same time--- chaos, right? That’s what happens in an OSPF multi-access network (Ethernet) without a DR and BDR.

 

·        The problem: toooo many LSAs*

Ø  Every OSPF router floods Link-State Advertisements (LSAs) to announce network changes. In a large network, this creates excessive traffic and inefficiency.

 

·        The Solution: DR and BDR to the Rescue**

>Designated Router DR -> becomes the central hub for LSA distribution.

> BDR Backup DR->  standby to take over if the DR fails.

> All the other routers (***DROTHERs***) only talk to the DR and BDR, reducing unnecessary chatter.

 

Ø  Result: A cleaner, more efficient OSPF network**

 

·        How does the DR and BDR election work?

Ø  Not every router gets to be the DR- there’s a strict election process.

Ø  Only routers in ** multi-access networks*** (Ethernet, Frame Relay) elect a DR/BDR.

Ø  Point-to-point links (like PPP) don’t need DR/BDR.

 

 

·        What are the Election Rules?

Ø  OSPF Priority (0-255)

Ø  Default - ***1***

Ø  If OSPF priority is 0, it means the router is ineligible to be elected

Ø  If the priorities are equal, the router with the highest RID wins

Ø  Router ID is chosen from—manually configured RID, the highest loopback IP, highest active physical IP.

 

 

·        The surprising Truth: Elections are non-preemptive.

Ø  Once elected, the DR/BDR OSPF routers stay in power unless they fail.

Ø  A new router with a higher priority won't take over the current DR BDR. Unless the current DR/BDR fails.

 

·        Common DR/BDR configuration mistakes.

Ø  Ignoring priority setting > if all routers have default priority 1, the highest router ID wins, which may not be optimal. FIX – manually set priorities on critical routers.

Ø  Adding a new high-priority router won’t change the DR unless the current one fails. FIX restart OSPF on the interface if needed.

Ø  If router IDs are unstable, for example, if they're based on a physical interface, DR election can be unpredictable. FIX always set a STATIC router ID in OSPF configuration.

 

·        Best practice for DR/BDR optimization.

Ø  Set priority 100 on the routers you want as DR/BDR, and set priority 0 on routers that should never become DR.

Ø  Avoid DR on low power routers- the DR does extra work, do not make a weak router the DR.

Ø  Use the loopback interface for router-ID.

 

 Let's see the configuration=>https://www.youtube.com/@internetworkss

Topology:-

• Configure the topology as per the diagram
• Configure the IP addresses as per the topology 
• Configure OSPF 1 and advertise directly connected interfaces 
• Configure router 1 DR, router 2, and router 3 Drothers 
• Configure Router 4 as backup DR
• VERIFY THE CONFIGURATIONS.

What is OSPFv2 (Open Shortest Path First version 2) subcommand?

 OSPFv2 (Open Shortest Path First version 2 is a link-state routing protocol designed for IPv4 networks. It is defined in RFC 2328 and is widely used in enterprise and service provider networks due to its efficiency and scalability.

Key Features of OSPFv2:

1. Link-State Protocol:

• Maintains a complete topological map of the network (LSDB - Link-State Database).
• Uses the Dijkstra (SPF) algorithm to compute the shortest path to all destinations.

2. Hierarchical Routing (Areas):

• Divides networks into areas to reduce routing overhead.
• Backbone Area (Area 0) is mandatory; all other areas must connect to it.

Types of areas:

• Normal Areas (Standard areas)
• Stub Areas (Block external routes)
• Totally Stubby Areas (Block external and inter-area routes)
• Not-So-Stubby Areas (NSSA) (Allows limited external routes)

Metric (Cost):

• Calculated based on interface bandwidth (Cost = Reference BW / Interface BW, default Reference BW = 100 Mbps).

Packet Types:

• Hello (Discover/maintain neighbors)
• DBD (Database Description) (Exchange LSDB info)
• LSR (Link-State Request) (Request specific LSA)
• LSU (Link-State Update) (Send LSA updates)
• LSAck (Link-State Acknowledgment) (Acknowledge LSUs)

LSA Types (Link-State Advertisements):

• Type 1 (Router LSA) – Intra-area router info.
• Type 2 (Network LSA) – Multi-access network info.
• Type 3 (Summary LSA) – Inter-area routes (ABR-generated).
• Type 4 (ASBR Summary LSA) – Location of ASBR.
• Type 5 (External LSA) – Routes from other AS (redistributed).
• Type 7 (NSSA External LSA) – External routes in NSSA.

Neighbor States:

• Down → Init → 2-Way → ExStart → Exchange → Loading → Full

Authentication:

Supports plain-text, MD5, and SHA authentication for security.

Let's see the configuration of the OSPFv2 subcommand.

Topology:-

• Configure the topology as per the diagram
• Configure the IP addresses as per the topology 
• Configure OSPFv2 with the subcommand 
• advertise directly connected routes and loopbacks
• Make sure all the routes are exchanged 
• Verify with ping and show commands

What is a VLAN Hopping Attack?

 

What is a VLAN Hopping Attack?

Virtual Local Area Networks (VLANs) play a crucial role in segmenting traffic for security and performance in today's interconnected networks. However, attackers can exploit misconfigurations to bypass these segmentation controls through VLAN hopping attacks.

In this blog, we’ll explore:

• What VLAN hopping is
• How VLAN hopping attacks work
• Types of VLAN hopping attacks
• How to prevent VLAN hopping

What is BGP Allowas-in Feature? How to configure BGP Allowas-in? GNS3

 BGP Allowas-in is a configuration option in Border Gateway Protocol (BGP) routing that allows a router to accept routes with its own AS (Autonomous System) number in the AS_PATH attribute.
There's a risk of creating routing loops if not managed properly. It should be used cautiously and with a clear understanding of network topology.

• Purpose: Normally, BGP prevents loops by rejecting routes that contain the local AS number in the AS_PATH. However, in scenarios like multi-homed environments or when dealing with MPLS VPNs, you should allow routes to come back into your AS through different paths.

• Functionality: When you configure allowas-in, you specify how many times your AS number is allowed to appear in the AS_PATH. This number can be set between 1 to 10, although this varies by router vendor and model. For example, setting allowas-in 2 would allow routes with up to two instances of your AS number in the path.

Usage Scenario:

1. Multi-homed Networks: If you have connections to the same ISP in different locations, allowas-in can prevent unnecessary route blackholing by allowing these routes back into your network.
2. MPLS VPNs: In scenarios where customer routes are sent through a service provider network and back, allowas-in ensures these routes are not dropped.

Configuration Example (Cisco IOS-like syntax):

plaintext
router bgp 65100
 neighbor 12.1.1.2 remote-as 65200
 neighbor 12.1.1.2 allowas-in 2

end

To allow the re-advertisement of all prefixes that contain duplicate ASNs, use the neighbor allowas-in command in router configuration mode in Router

Risks:

• There's a risk of creating routing loops if not managed properly. It should be used cautiously and with a clear understanding of network topology.
• It can also lead to suboptimal routing if not configured correctly.

Best Practices:

• Use sparingly and only where necessary.
• Monitor BGP updates closely when employing this feature.
• Combine with route-maps or other BGP policies to control which routes are accepted.

This configuration can significantly affect network reachability and BGP convergence, so it should be deployed with a good understanding of BGP dynamics and network design.
 

let's see the configuration:-

Topology:-

Goal- allow the same AS routes installed in router 1 and router 5 routing table. 

• configure the IP addresses as per the topology
• configure EBGP IBGP protocols
• verify the 10.0.0.0/8 in the router 5 routing table
• verify the 50.0.0.0/8 in the router 1 routing table
• configure BGP allowas-in feature on routers 1 and 5
• ensure route 10.1.1.1 and 50.1.1.1 in router 1 and 5 routing table.
• in the end, ensure the connectivity with ping traceroute commands

R1(config)#interface ethernet 5/0

R1(config-if)#ip address 10.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit

R1(config)#interface serial 4/0

R1(config-if)#ip address 12.1.1.1 255.0.0.0

R1(config-if)#no shutdown

R1(config-if)#exit

R2(config)#interface ethernet 5/0

R2(config-if)#ip address 20.1.1.1 255.0.0.0

R2(config-if)#no shutdown

R2(config-if)#exit

R2(config)#interface serial 4/0

R2(config-if)#ip address 12.1.1.2 255.0.0.0

R2(config-if)#no shutdown

R2(config-if)#exit

R2(config)#interface serial 4/1

R2(config-if)#ip address 23.1.1.1 255.0.0.0

R2(config-if)#no shutdown

R2(config-if)#exit

R3(config)#interface ethernet 5/0

R3(config-if)#ip address 30.1.1.1 255.0.0.0

R3(config-if)#no shutdown

R3(config-if)#exit

R3(config)#interface serial 4/0

R3(config-if)#ip address 34.1.1.1 255.0.0.0

R3(config-if)#no shutdown

R3(config-if)#exit

R3(config)#interface serial 4/1

R3(config-if)#ip address 23.1.1.2 255.0.0.0

R3(config-if)#no shutdown

R3(config-if)#exit

R4(config)#interface ethernet 5/0

R4(config-if)#ip address 40.1.1.1 255.0.0.0

R4(config-if)#no shutdown

R4(config-if)#exit

R4(config)#interface serial 4/0

R4(config-if)#ip address 34.1.1.2 255.0.0.0

R4(config-if)#no shutdown

R4(config-if)#exit

R4(config)#interface serial 4/1

R4(config-if)#ip address 45.1.1.1 255.0.0.0

R4(config-if)#no shutdown

R4(config-if)#exit

R5(config)#interface ethernet 5/0

R5(config-if)#ip address 50.1.1.1 255.0.0.0

R5(config-if)#no shutdown

R5(config-if)#exit

R5(config)#interface serial 4/1

R5(config-if)#ip address 45.1.1.2 255.0.0.0

R5(config-if)#no shutdown

R5(config-if)#exit

R1(config)#router bgp 65100

R1(config-router)#neighbor 12.1.1.2 remote-as 65200

R1(config-router)#network 10.0.0.0 mask 255.0.0.0

R1(config-router)#network 12.0.0.0 mask 255.0.0.0

R1(config-router)#no sync

R1(config-router)#exit

*Feb 18 12:04:32.531: %BGP-5-ADJCHANGE: neighbor 12.1.1.2 Up

R2(config)#router bgp 65200

R2(config-router)#neighbor 12.1.1.1 remote-as 65100

R2(config-router)#neighbor 23.1.1.2 remote-as 65200

R2(config-router)#network 20.0.0.0 mask 255.0.0.0

R2(config-router)#network 12.0.0.0 mask 255.0.0.0

R2(config-router)#network 23.0.0.0 mask 255.0.0.0

R2(config-router)#no sync

R2(config-router)#exit

*Feb 18 12:04:32.687: %BGP-5-ADJCHANGE: neighbor 12.1.1.1 Up

*Feb 18 12:05:30.427: %BGP-5-ADJCHANGE: neighbor 23.1.1.2 Up

R3(config)#router bgp 65200

R3(config-router)#neighbor 34.1.1.2 remote-as 65300

R3(config-router)#neighbor 23.1.1.1 remote-as 65200

R3(config-router)#network 30.0.0.0 mask 255.0.0.0

R3(config-router)#network 34.0.0.0 mask 255.0.0.0

R3(config-router)#network 23.0.0.0 mask 255.0.0.0

R3(config-router)#no sync

R3(config-router)#exit

*Feb 18 12:05:30.535: %BGP-5-ADJCHANGE: neighbor 23.1.1.1 Up

*Feb 18 12:09:19.379: %BGP-5-ADJCHANGE: neighbor 34.1.1.2 Up

R4(config)#router bgp 65300

R4(config-router)#neighbor 34.1.1.1 remote-as 65200

R4(config-router)#neighbor 45.1.1.2 remote-as 65100

R4(config-router)#network 40.0.0.0 mask 255.0.0.0

R4(config-router)#network 34.0.0.0 mask 255.0.0.0

R4(config-router)#network 45.0.0.0 mask 255.0.0.0

R4(config-router)#no sync

R4(config-router)#exit

*Feb 18 12:09:18.955: %BGP-5-ADJCHANGE: neighbor 34.1.1.1 Up

*Feb 18 12:09:56.799: %BGP-5-ADJCHANGE: neighbor 45.1.1.2 Up

R5(config)#router bgp 65100

R5(config-router)#neighbor 45.1.1.1 remote-as 65300

R5(config-router)#network 50.0.0.0 mask 255.0.0.0

R5(config-router)#network 45.0.0.0 mask 255.0.0.0

R5(config-router)#no sync

R5(config-router)#exit

*Feb 18 12:09:56.643: %BGP-5-ADJCHANGE: neighbor 45.1.1.1 Up

R1#show ip bgp

BGP table version is 9, local router ID is 12.1.1.1

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

*> 10.0.0.0         0.0.0.0                  0         32768 i

*  12.0.0.0         12.1.1.2                 0             0 65200 i

*>                  0.0.0.0                  0         32768 i

*> 20.0.0.0         12.1.1.2                 0             0 65200 i

*> 23.0.0.0         12.1.1.2                 0             0 65200 i

*> 30.0.0.0         12.1.1.2                               0 65200 i

*> 34.0.0.0         12.1.1.2                               0 65200 i

*> 40.0.0.0         12.1.1.2                               0 65200 65300 i

*> 45.0.0.0         12.1.1.2                               0 65200 65300 I

(as you can see router 1 does not have a 50.1.1.1/8 network in its routing table because of the loop prevention feature)

R5#show ip bgp

BGP table version is 9, local router ID is 50.1.1.1

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

*> 12.0.0.0         45.1.1.1                               0 65300 65200 i

*> 20.0.0.0         45.1.1.1                               0 65300 65200 i

*> 23.0.0.0         45.1.1.1                               0 65300 65200 i

*> 30.0.0.0         45.1.1.1                               0 65300 65200 i

*> 34.0.0.0         45.1.1.1                 0             0 65300 i

*> 40.0.0.0         45.1.1.1                 0             0 65300 i

*  45.0.0.0         45.1.1.1                 0             0 65300 i

*>                  0.0.0.0                  0         32768 i

*> 50.0.0.0         0.0.0.0                  0         32768 i

(as you can see router 5 does not have a 10.1.1.1/8 network in its routing table because of the loop prevention feature both routers are in the same AS 65100)

R1(config)#router bgp 65100

R1(config-router)#neighbor 12.1.1.2 allowas-in

R1(config-router)#exit

R1(config)#end

R5(config)#router bgp 65100

R5(config-router)#neighbor 45.1.1.1 allowas-in

R5(config-router)#exit

R5(config)#end

R1#show ip bgp

BGP table version is 10, local router ID is 12.1.1.1

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

*> 10.0.0.0         0.0.0.0                  0         32768 i

*  12.0.0.0         12.1.1.2                 0             0 65200 i

*>                  0.0.0.0                  0         32768 i

*> 20.0.0.0         12.1.1.2                 0             0 65200 i

*> 23.0.0.0         12.1.1.2                 0             0 65200 i

*> 30.0.0.0         12.1.1.2                               0 65200 i

*> 34.0.0.0         12.1.1.2                               0 65200 i

*> 40.0.0.0         12.1.1.2                               0 65200 65300 i

*> 45.0.0.0         12.1.1.2                               0 65200 65300 i

*> 50.0.0.0         12.1.1.2                               0 65200 65300 65100 i

R5#show ip bgp

BGP table version is 10, local router ID is 50.1.1.1

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

*> 10.0.0.0         45.1.1.1                               0 65300 65200 65100 i

*> 12.0.0.0         45.1.1.1                               0 65300 65200 i

*> 20.0.0.0         45.1.1.1                               0 65300 65200 i

*> 23.0.0.0         45.1.1.1                               0 65300 65200 i

*> 30.0.0.0         45.1.1.1                               0 65300 65200 i

*> 34.0.0.0         45.1.1.1                 0             0 65300 i

*> 40.0.0.0         45.1.1.1                 0             0 65300 i

*  45.0.0.0         45.1.1.1                 0             0 65300 i

*>                  0.0.0.0                  0         32768 i

*  50.0.0.0         45.1.1.1                               0 65300 65100 i

*>                  0.0.0.0                  0         32768 I

(as you can see after we allows-in feature both the routers installed same AS routes in to their routing table)

R1#ping 50.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 50.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 132/154/180 ms

R1#traceroute 50.1.1.1

Type escape sequence to abort.

Tracing the route to 50.1.1.1

  1 12.1.1.2 20 msec 32 msec 32 msec

  2 23.1.1.2 [AS 65200] 72 msec 64 msec 64 msec

  3 34.1.1.2 [AS 65200] 96 msec 92 msec 104 msec

  4 45.1.1.2 [AS 65300] 148 msec 116 msec 140 msec

R5#ping 10.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 120/148/180 ms

R5#traceroute 10.1.1.1

Type escape sequence to abort.

Tracing the route to 10.1.1.1

  1 45.1.1.1 32 msec 32 msec 32 msec

  2 34.1.1.1 [AS 65300] 64 msec 64 msec 64 msec

  3 23.1.1.1 [AS 65200] 104 msec 96 msec 96 msec

  4 12.1.1.1 [AS 65200] 128 msec 152 msec 132 msec

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