Table Of Contents
Table Of Contents

Step 6. BGP Scenario and using loopbacks

Goals

Step 6 introduces a more complex, three-AS hierarchical topology (BGP_Topology_4.ned) and demonstrates a complete BGP-based transit scenario. This step focuses on establishing a full-mesh I-BGP configuration within the transit Autonomous System (AS 64600) and using BGP to provide end-to-end connectivity between external ASes.

The network consists of:

  • AS 64500 (RA): A multi-router AS running OSPF internally. RA4 is the border router.

  • AS 64600 (RB): The transit AS. It contains four routers (RB1, RB2, RB3, RB4) running OSPF. RB1 and RB4 are border routers.

  • AS 64700 (RC): Another multi-router AS running OSPF. RC1 is the border router.

Key features demonstrated:

  • E-BGP Peering: RA4 peers with RB1, and RB4 peers with RC1.

  • Full-Mesh I-BGP: Within AS 64600, all routers (RB1, RB2, RB3, RB4) run BGP and are configured as I-BGP peers.

  • Multi-hop I-BGP: Note that some I-BGP sessions (e.g., RB1 to RB4) are not directly connected. They rely on the IGP (OSPF) to provide reachability to the peering addresses.

  • Reachability: The network is configured to allow hosts in AS 64500 (host0) to reach hosts in AS 64700 (host1).

Configuration

This step uses the BGP_Topology_4.ned network, which includes three distinct ASes with several routers and hosts.

../../../_images/BGP_Topology_4.png

The configuration in omnetpp.ini:

[Config Step6]
description = "BGP Scenario and using loopbacks"
network = BGP_Topology_4
*.routingTableRecorder.logfile = "step6.rt"

*.pcapRecorder.pcapFile = "step6.pcap"

# adding default routes in RA4 and RC1 and ask OSPF to distribute it within the AS
*.configurator.config = xml("<config> \
                                <interface hosts='RA4' names='eth0' address='192.168.x.x' netmask='255.x.x.x'/> \
                                <interface hosts='RB1' names='eth2' address='192.168.x.x' netmask='255.x.x.x'/> \
                                <interface hosts='RB4' names='eth2' address='192.168.x.x' netmask='255.x.x.x'/> \
                                <interface hosts='RC1' names='eth2' address='192.168.x.x' netmask='255.x.x.x'/> \
                                \
                                <interface among='host0 RA*' address='10.x.x.x' netmask='255.x.x.x'/> \
                                <interface hosts='RA*' address='10.x.x.x' netmask='255.x.x.x'/> \
                                \
                                <interface hosts='RB*' address='20.x.x.x' netmask='255.x.x.x'/> \
                                \
                                <interface among='host1 RC*' address='30.x.x.x' netmask='255.x.x.x'/> \
                                <interface hosts='RC*' address='30.x.x.x' netmask='255.x.x.x'/> \
                                \
                                <route hosts='host*' destination='*' netmask='0.0.0.0' interface='eth0' /> \
                                \
                                <route hosts='RA4' destination='*' netmask='0.0.0.0' interface='eth0' /> \
                                <route hosts='RC1' destination='*' netmask='0.0.0.0' interface='eth2' /> \
                             </config>")

# OSPF configuration
*.R*.hasOspf = true
*.R*.ospf.ospfConfig = xmldoc("OSPFConfig.xml")

*.RA4.ipv4.routingTable.routerId = "10.0.0.5"
*.RC1.ipv4.routingTable.routerId = "30.0.0.5"
*.RB1.ipv4.routingTable.routerId = "20.0.0.18"
*.RB4.ipv4.routingTable.routerId = "20.0.0.14"

# BGP configuration
*.RA4.hasBgp = true
*.RB1.hasBgp = true
*.RB4.hasBgp = true
*.RC1.hasBgp = true
*.R*.bgp.bgpConfig = xmldoc("BGPConfig_Redist.xml")
*.R*.bgp.redistributeOspf = "O IA"

# BGP routes are distributed internally within the AS
#*.RB1.bgp.redistributeInternal = true
#*.RB4.bgp.redistributeInternal = true

# enable BGP on RB2 and RB3
*.RB2.hasBgp = true
*.RB3.hasBgp = true

The BGP configuration (BGPConfig_Redist.xml) defines the ASes and the full-mesh sessions within AS 64600. It also uses nextHopSelf="true" to ensure I-BGP peers can reach external next hops.

Results

In the simulation, we observe the convergence of the entire network:

  1. IGP Convergence: OSPF converges within each AS, providing internal reachability. In AS 64600, this allows multi-hop BGP sessions (like RB1-RB4) to establish.

  2. BGP Session Establishment:

    • EBGP sessions are established: RA4 <-> RB1 and RB4 <-> RC1.

    • A full mesh of IBGP sessions is established among RB1, RB2, RB3, and RB4.

  3. Route Propagation:

    • RB1 learns RA’s networks (10.x.x.x) and redistributes them into the RB mesh.

    • RB4 learns RC’s networks (30.x.x.x) and redistributes them into the RB mesh.

    • Due to the full mesh, every router in AS 64600 learns the path to both external ASes.

  4. End-to-End Reachability: step6.rt shows that all routers eventually possess routes to all subnets across all three ASes. Host0 can successfully communicate with Host1 through the transit AS 64600.

This step validates the use of a BGP full mesh within a transit AS as a robust solution for internet-scale routing.

Sources: BGP_Topology_4.ned, omnetpp.ini, OSPFConfig.xml BGPConfig_Redist.xml

Discussion

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Step 5b. Enabling BGP on RB3
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Step 7. BGP with RIP redistribution