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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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BCMSN exam success and earning your CCNP certification requires you to add to your knowledge of VLAN configuration. When you studied for your CCNA exam, you learned how to place ports into a VLAN and what the purpose of VLANs was, but you may not be aware that there are two types of VLAN membership. To pass the BCMSN exam, you must know the details of both types.

In this tutorial, we’ll take a look at the VLAN type you are most familiar with, the “static VLAN”. As you know, VLANs are a great way to create smaller broadcast domains in your network. Host devices connected to a port belonging to one VLAN will receive broadcasts and multicasts only if they were originated by another host in that same VLAN. The drawback is that without the help of a Layer 3 switch or a router, inter-VLAN communication cannot occur.

The actual configuration of a static VLAN is simple enough. In this example, by placing switch ports 0/1 and 0/2 into VLAN 12, the only broadcasts and multicasts hosts connected to those ports will receive are the ones transmitted by ports in VLAN 12.

SW1(config)#int fast 0/1

SW1(config-if)#switchport mode access

SW1(config-if)#switchport access vlan 12

% Access VLAN does not exist. Creating vlan 12

SW1(config-if)#int fast 0/2

SW1(config-if)#switchport mode access

SW1(config-if)#switchport access vlan 12

One of the many things I love about Cisco switches and routers is that if you have forgotten to do something, the Cisco device is generally going to remind you or in this case actually do it for you. I placed port 0/1 into a VLAN that did not yet exist, so the switch created it for me! Read the rest of this entry »

Multicasting is a vital topic on your BCMSN, CCNP, and CCIE exams, and it can also be very confusing when you first start studying it. Multicasting uses concepts that are unlike anything you’ve run into in your routing protocol studies, and that can throw you at first. I speak from experience that multicasting is like any other Cisco technology – learn the basics, master the fundamentals, and then build your skills on that foundation.

One such fundamental is the RPF Check, or Reverse Path Forwarding Check.

A fundamental difference between unicasting and multicasting is that a unicast is routed by sending it toward the destination, while a multicast is routed by sending it away from its source.

“toward the destination” and “away from its source” sound like the same thing, but they’re not. A unicast is going to follow a single path from source to destination. The only factor the routers care about is the destination IP address – the source IP address isn’t a factor.
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In your BCMSN / CCNP exam study, it’s easy to overlook some of the details of Spanning Tree Protocol (STP). After all, you learned all of that in your CCNA studies, right? Not necessarily! While some of the BCMSN material will be a review for you, there are some details regarding familiar topics that you need to learn. That includes the timers for STP – Hello Time, MaxAge, and Forward Delay.

You may remember these timers from your CCNA studies as well, and you should also remember that these timers should not be changed lightly. What you might not have known is that if you decide to change any and all of these timers, that change must be configured on the root bridge! The root bridge will inform the nonroot switches of the change via BPDUs.

Hello Time is the interval between BPDUs, two seconds by default.

Forward Delay is the length of both the listening and learning STP stages, with a default value of 15 seconds.

Maximum Age, referred to by the switch as MaxAge, is the amount of time a switch will retain a BPDU’s contents before discarding it. The default is 20 seconds.

The value of these timers can be changed with the spanning-tree vlan command shown below. Verify the changes with the show spanning-tree command.

SW1(config)#spanning-tree vlan 1 ?

forward-time Set the forward delay for the spanning tree

hello-time Set the hello interval for the spanning tree

max-age Set the max age interval for the spanning tree

priority Set the bridge priority for the spanning tree

root Configure switch as root

SW1(config)#spanning-tree vlan 1 hello-time 5

SW1(config)#spanning-tree vlan 1 max-age 30

SW1(config)#spanning-tree vlan 1 forward-time 20

SW1(config)#^Z

SW1#show spanning-tree vlan 1

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32769
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