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General Routing &Amp; Switching | Layer 3 Switching - Part 3
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Aug 302012

When the packet travels down the OSI and reaches the network layer, the MAC address must be resolved from the destination host for the encapsulation process. Traditionally this was done in IPv4 using ARP which uses broadcast packets. As in IPv6 we do not have Broadcasts, this must be done with the help of Multicasts. In IPv6 we do not have a separate protocol to accomplish this; it is built into the IPv6 stack itself. The Layer 3 to Layer 2 resolution in done with ICMP neighbor discovery.

Each host which has an IPv6 address configured listens to a particular multicast group. Thus there could be many hosts which listen to the same group. Finally the neighbor discovery request will itself contain the L3 address to be resolved, and only the host which is configured with the address in question will answer the request.

The neighbor discovery packet is an ICMPv6 Packet which has a type code of 135. This is called a neighbor solicitation packet. This is equivalent to the ARP request. The host which has the address configured will answer with an ICMPv6 packet with a type code of 136. This is called a neighbor advertisement packet. This is equivalent to ARP reply.

When an interface is assigned an IPv6 address, it will join a multicast group of FF02:0:0:0:0:1:FFXX:XXXX. The X is the last 24 bits of the IPv6 address which is assigned to an interface. Let us look into the details, when R1 has the IPv6 address of 123::1/64 assigned. It will then join a multicast group of FF02::1:FF00:1.

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Jul 092012

Using PIM in sparse mode is much more effective, because of the explicit requests. A multicast packet is only flooded from the sender to the first hop router. Only when a client requests data, the multicast packets are flooded out of the broadcast domain. The core control point of PIM-Sparse mode is the Rendezvous Point (RP). The RP tracks all the senders and receivers in the multicast domain.

In the figure below the loopback 0 of R1 is configured as the RP address. OSPF area 0 and PIM in sparse mode is enabled on all interfaces of all routers.

Let us check the scenario when a multicast receiver joins to a group. We assume now there are no senders.

  • Client sends an IGMP membership report out on the LAN.
  • IGMP join will then the converted to PIM join and sent upstream to the RP.
  • A (*,G) entry will be installed on all routers in the IGP path to RP

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Jun 272012

We can configure a Cisco router as a frame-relay switch. The router which I use is a 3700 platform and runs the IOS version 12.4(15)T14. The router will then switch DLCIs between interfaces. For this we have to do the following tasks:

  • Activate frame-relay switching.
  • Configure interfaces as frame-relay DCE.
  • Create frame-relay routes.

First let’s configure the router as to do frame-relay switching

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Jun 042012

R4 does redistribution of the network as a Type-7 LSA with metric-type of 1. In this case the forwarding address will be an interface attached to R4. R1 and R2 which are Border routes will retain the forwarding address when translating Type-7 to Type-5 LSA.

The forwarding address will be:

  • The highest IP address from among the loopback interfaces on which OSPF is activated
  • If there are no loopback interfaces, then the IP address of the physical interface on which OSPF is activated

With this theory, let’s check the current packet flow towards the external network. Each interface in the topology has a cost of 1.
Route calculation for R1: R3 which has a higher RID ( than R1 ( will do the Type-5 translation. R1 will get a Type-5 route and a Type-7 route each having a cost of 20. Then it calculates the cumulative cost to reach the forwarding address which will be 22 for both. Thus R1 will have an E1 route in its RIB.

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Jun 032012

An external route could be injected into OSPF domain either as a Type-5 or Type-7. The Type-7 route will be injected by an ASBR from a NSSA area. The Type-5 route will be injected by an ASBR from any normal area.

A Type-7 LSA will be translated to Type-5 by an NSSA ABR and flooded it to the whole OSPF domain. If there are multiple NSSA ABRs, the one with higher RID will do the translation. The forwarding address and the metric-type will remain the same as the Type-7 route. If the forwarding address is non-zero, then metric / cost to the forwarding address is accounted or else the metric / cost is to reach the advertising router (ASBR).

If there are multiple external paths to an identical destination, the NSSA ABR will use the following rules for path selection.

1.       E1 and N1 routes are preferred over E2 or N2
2.       If both are Type-1 or Type-2 routes, the route with the lower metric / cost will be installed in the RIB
3.       If both are Type-1 or Type-2 routes, and both routes have the same metric / cost E1 will be preferred over N1 and E2 will be preferred over N2
4.       If maximum-paths 1 command is configured, the route advertised from highest RID will be considered

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