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Posts Tagged ‘router’

While routers accept and generate broadcasts, they do not forward them. This can be quite a problem when a broadcast needs to get to a device such as a DHCP or TFTP server that’s on one side of a router with other subnets on the other side.

If a PC attempts to locate a DNS server with a broadcast, the broadcast will be stopped by the router and will never get to the DNS server. By configuring the ip helper-address command on the router, UDP broadcasts such as this will be translated into a unicast by the router, making the communication possible. The command should be configured on the interface that will be receiving the broadcasts.

R1(config)#int e0

R1(config-if)#ip helper-address ?

A.B.C.D IP destination address

R1(config-if)#ip helper-address 100.1.1.2

Now, you may be wondering if this command covers all UDP services. Sorry, you’re not getting off that easy! The command does forward eight common UDP service broadcasts, though.

TIME, port 37

TACACS, port 49

DNS, port 53

BOOTP/DHCP Server, port 67

BOOTP/DHCP Client, port 68

TFTP, port 69

NetBIOS name service, port 137

NetBIOS datagram service, port 138

That’s going to cover most scenarios where the ip helper-address command will be useful, but what about those situations where the broadcast you need forwarded is not on this list? You can use the ip forward-protocol command to add any UDP port number to the list.

Additionally, to remove protocols from the default list, use the no ip forward-protocol command. In the following example, we’ll add the Network Time Protocol port to the forwarding list while removing the NetBIOS ports. Remember, you can use IOS Help to get a list of commonly filtered ports!

R1(config)#ip forward-protocol udp ?

Port number

biff Biff (mail notification, comsat, 512)

bootpc Bootstrap Protocol (BOOTP) client (68)

bootps Bootstrap Protocol (BOOTP) server (67)

discard Discard (9)

dnsix DNSIX security protocol auditing (195)

domain Domain Name Service (DNS, 53)

echo Echo (7)

isakmp Internet Security Association and Key Management Protocol (500)

mobile-ip Mobile IP registration (434)

nameserver IEN116 name service (obsolete, 42)

netbios-dgm NetBios datagram service (138)

netbios-ns NetBios name service (137)

netbios-ss NetBios session service (139)

ntp Network Time Protocol (123)

pim-auto-rp PIM Auto-RP (496)

rip Routing Information Protocol (router, in.routed, 520)

snmp Simple Network Management Protocol (161)

snmptrap SNMP Traps (162)

sunrpc Sun Remote Procedure Call (111)

syslog System Logger (514)

tacacs TAC Access Control System (49)

talk Talk (517)

tftp Trivial File Transfer Protocol (69)
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Ever since you picked up your first CCNA book, you’ve heard about multicasting, gotten a fair idea of what it is, and you’ve memorized a couple of reserved multicasting addresses. Now as you prepare to pass the BCMSN exam and become a CCNP, you’ve got to take that knowledge to the next level and gain a true understanding of multicasting. Those of you with an eye on the CCIE will truly have to become multicasting experts!

Having said that, we’re going to briefly review the basics of multicasting first, and then future tutorials will look at the different ways in which multicasting can be configured on Cisco routers and switches.

What Is Multicasting?

A unicast is data that is sent from one host to another, while a broadcast is data sent from a host that is destined for “all” host addresses. By “all”, we can mean all hosts on a subnet, or truly all hosts on a network.

There’s a quite a bit of a middle ground there! A multicast is that middle ground, as a multicast is data that is sent to a logical group of hosts, called a multicast group. Hosts that are not part of the multicast group will not receive the data.

Some other basic multicasting facts:

There’s no limit on how many multicast groups a single host can belong to.

The sender is usually unaware of what host devices belong to the multicast group.

Multicast traffic is unidirectional. If the members of the multicast group need to respond, that reply will generally be a unicast.
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If a Layer Two switch doesn’t have the capabilities to run IGMP Snooping, it will be able to run CGMP – Cisco Group Membership Protocol. CGMP allows the multicast router to work with the Layer Two switch to eliminate unnecessary multicast forwarding.

CGMP will be enabled on both the multicast router and the switch, but the router’s going to do all the work. The router will be sending Join and Leave messages to the switch as needed. PIM must be running on the router interface facing the switch before enabling CGMP, as you can see:

R1(config)#int e0

R1(config-if)#ip cgmp

WARNING: CGMP requires PIM enabled on interface

R1(config-if)#ip pim sparse

R1(config-if)#ip cgmp

When CGMP is first enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch that a multicast router is now connected to it. This particular CGMP Join will contain a Group Destination Address (GDA) of 0000.0000.0000 and the MAC address of the sending interface. The GDA is used to identify the multicast group, so when this is set to all zeroes, the switch knows this is an introductory CGMP Join, letting the switch know that the multicast router is online.
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Earning your CCNP certification and passing the BSCI exam depends on knowing the details of many Cisco technologies, ISIS chief among them. To help you prepare for exam success, here’s a list of ISIS terminology and basic concepts that will help you pass this tough exam. Enjoy!

ISIS Terms:

Domain: section of the network under common administrative control

Area: logical segment of the network composed of contiguous routers and their data links

Intermediate System: A router.

End System: A host device.

The four levels of ISIS routing:

Level 0: ES-IS routing in the same subnet.

Level 1: IS-IS routing in the same area.

Level 2: IS-IS routing in the same domain.

Level 3: Inter-domain routing performed by InterDomain Routing Protocol (IDRP).
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