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Aug 082013

The dynamic tunnels are point to multipoint, virtually treating the underlying IPv4 network as NBMA. This is because the destination IPv4 address is dynamically discovered from the destination IPv6 address. Thus if a protocol uses the destination of multicast address, they will get mapped to an IPv4 address which cannot be routed, there by dropped.

Because of this reason we have to trick the protocol to use unicast IPv6 destination addresses. The implementation of this depends on the routing protocol itself. In this blog I will be demonstrating using OSPFv3 over different tunnel types.


In the above topology R2 which will be the hub is configured for following tunneling methods:

  • Automatic 6to4 tunneling towards R1.
  • ISATAP tunneling towards R3, where it acts as a client.
  • ISATAP tunneling towards R4 where both R2 and R4 are servers.

Caveats: The most important point to take into account is the IPv6 address itself. The transport IPv6 destination address will be automatically discovered with help of the IPv6 destination address. So we must make take care that the IPv6 destination could be properly mapped to the IPv4 destination.

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Jul 302013

The automatic tunneling mechanisms like Automatic 6to4 tunnels and ISATAP (Intra Site Automatic Tunnel Addressing Protocol) tunnels are point to multipoint tunnels. The destination IPv4 address is encoded in the IPv6 address itself. These tunneling mechanisms treat the underlying IPv4 network as an NBMA network.

As the destination IPv4 address is found out from the IPv6 address itself, care must be taken when deploying dynamic routing protocols over these tunnels. Care must be taken that the intermediate routers on the path does know the route to the encoded IPv4 address in the IPv6 packet.


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Apr 072013

An IPv6 router does not need a global IP address on its interface like an IPv4 router because it only uses the link local address (LLA) for the convergence of the routing domain. In IPv6 an interface always has a LLA as soon as the interface is enabled for IPv6. The LLA as the name states is not routed across the local link (Broadcast Domain). Routing protocols either of link state nature or distance vector nature exchange updates with their corresponding LLA as source.


The core routers R1 and R2 only have a single global address assigned so that they could be accessed from outside their broadcast domain. This loopback address must also be advertised into the IGP which in our case is OSPF. The global address is also needed on the core routes so that they could send ICMP control messages like packet too big, network unreachable, TTL exceeded etc. to the requester.

The non-core routers R4 and R3 do have also global address assigned from their respective broadcast domains to the appropriate interfaces, so that the prefix could be advertised via OSPF for the network reachability.

Let’s now check the importance for having a global address on the core routers. First with a traceroute from H1 to H4

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Mar 302013

In short Hot Standby Routing Protocol and Gateway Load Balancing Protocol do first hop redundancy, the former is open standard, and the latter is Cisco proprietary. HSRP works in an active standby fashion, whereas in GLBP all routers are forwarding packets.

In HSRP the active router answers the ARP requests with the MAC address assigned to the standby group, thus only the active router forward packets. The standby router only will take over the forwarding process if it does not hear from the active router (3 hello messages by default) or by changing the priority/preemption. Thus a router could be taken out of service or taken into service by simply altering the priorities.

In GLBP the Active Virtual Gateway (AVG) is responsible for the ARP requests. It will reply with ARP replies with mac address of Active Virtual Forwarders (AVF) by default in a round robin fashion. Thus all routers will forward packets equally. Naturally all routers generate hello packets with the virtual MAC as source, so that the switches can learn the virtual mac address, which is the same with HSRP.

In GLBP as all routers are forwarding traffic, we have to alter the Weight value to make a router not to forward traffic. For this a tracking object must be defined. If the tracked object is UP, the router will be in service. Below is the example of such tracking. The loopback interface is tracked for its status. If the loopback 100 is shutdown, then the weight will be decremented by 100. When the weight value is below the lower threshold, the router will not send hello packets with Virtual MAC address as source, causing the L2 switches to remove the MAC from their forwarding table.

track 100 interface Loopback100 line-protocol
interface FastEthernet0/0
 glbp 1 ip
 glbp 1 weighting track 100 decrement 100
 glbp 1 weighting 100 lower 1 upper 100


if we want to take a GLBP router out of service, it must also not be the GLBP AVG.

Dec 302012

Each OSPF routers have a unique identification number which represents the router to the OSPF domain. If the ID is configured using the router-id command, it will be preferred over any highest loopback IPv4 address.

When looking out at the detailed output of a route in the routing table, the advertising routers RID will be displayed. Thus looking at the RIB we can determine to which router the network is attached.

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