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.
With multicast routing, the destination is a multicast IP group address. It's the multicast router's job to decide which paths will lead back to the source (upstream) and which paths are downstream from the source. Reverse Path Forwarding refers to the router's behavior of sending multicast packets away from the source rather than toward a specific destination.
The RPF Check is run against any incoming multicast packet. The multicast router examines the interface that the packet arrived on. If the packet comes in on an upstream interface - that is, an interface found on the reverse path that leads back to the source - the packet passes the check and will be forwarded. If the packet comes in on any other interface, the packet is dropped.
The RPF Check serves to verify the integrity of your multicasting network, and also serves as a reminder that the basic operation of multicasting is a lot different than unicasting!
Showing posts with label reverse. Show all posts
Showing posts with label reverse. Show all posts
Thursday, December 25, 2008
Wednesday, December 24, 2008
Cisco CCNA / CCNP Home Lab Setup: How To Configure Reverse Telnet
Occasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", and why is it so important to a Cisco CCNA / CCNP home lab setup?
Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.
In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn't a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.
Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.
An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:
ip host FRS 2007 100.1.1.1
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
ip host R4 2004 100.1.1.1
ip host R5 2005 100.1.1.1
ip host SW1 2006 100.1.1.1
interface Loopback0
ip address 100.1.1.1 255.255.255.255
no ip directed-broadcast
This configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type "R1" at the console line, for example, you'll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!
Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.
In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn't a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.
Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.
An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:
ip host FRS 2007 100.1.1.1
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
ip host R4 2004 100.1.1.1
ip host R5 2005 100.1.1.1
ip host SW1 2006 100.1.1.1
interface Loopback0
ip address 100.1.1.1 255.255.255.255
no ip directed-broadcast
This configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type "R1" at the console line, for example, you'll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!
Cisco CCNA / CCNP Home Lab Setup: How To Configure Reverse Telnet
Occasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", and why is it so important to a Cisco CCNA / CCNP home lab setup?
Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.
In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn't a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.
Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.
An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:
ip host FRS 2007 100.1.1.1
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
ip host R4 2004 100.1.1.1
ip host R5 2005 100.1.1.1
ip host SW1 2006 100.1.1.1
interface Loopback0
ip address 100.1.1.1 255.255.255.255
no ip directed-broadcast
This configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type "R1" at the console line, for example, you'll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!
Where a telnet session is started by a remote user who wants to remotely control a router or switch, a reverse telnet session is started when the host device itself imitates the telnet session.
In a CCNA / CCNP home lab, reverse telnet is configured and used on the access server. The access server isn't a white box server like most of us are used to; an access server is a Cisco router that allows you to connect to multiple routers and switches with one session without having to move a rollover cable from device to device.
Your access server will use an octal cable to connect to the other routers and switches in your home lab. The octal cable has one large serial connector that will connect to the access server, and eight rj-45 connectors that will connect to your other home lab devices. Your access server then needs an IP Host table in order to perform reverse telnet.
An IP Host table is easy to put together (and you better know how to write one to pass the CCNA!). The IP Host table is used for local name resolution, taking the place of a DNS server. A typical access server IP Host table looks like this:
ip host FRS 2007 100.1.1.1
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
ip host R4 2004 100.1.1.1
ip host R5 2005 100.1.1.1
ip host SW1 2006 100.1.1.1
interface Loopback0
ip address 100.1.1.1 255.255.255.255
no ip directed-broadcast
This configuration will allow you to use your access server to connect to five routers, a frame relay switch, and a switch without ever moving a cable. When you type "R1" at the console line, for example, you'll be connected to R1 via reverse telnet. If you have a smaller lab, an access server is still a real timesaver and an excellent investment. And by getting a static IP address to put on your access server, you can even connect to your home lab from remote locations!
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