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Article by Moshiachy Thisyday
To earn your CCNA or CCNP certification, you've got to understand the basics of trunking. This is not just a CCNA topic - it's essential to have a complicated understanding of trunking and etherchannels to move the BCMSN exam and earn your CCNP as well. Before we deal with those superior topics, though, it's worthwhile to master the basics!
A trunk allows inter-VLAN visitors to move between immediately connected switches. By default, a trunk port is a member of all VLANs, so visitors for any and all VLANs can travel throughout this trunk. That includes broadcast site visitors!
The default mode of a swap port does differ between fashions, so always check your documentation. On Cisco 2950 switches, every single port is in dynamic fascinating mode by default, that means that every port is actively attempting to trunk. On these switches, the only motion wanted from us is to physically join them with a crossover cable. In just a few seconds, the port light turns inexperienced and the trunk is up and running. The command show interface trunk will confirm trunking.
How does the receiving change know what VLAN the body belongs to? The frames are tagged by the transmitting change with a VLAN ID, reflecting the number of the VLAN whose member ports should receive this frame. When the body arrives at the remote change, that change will look at this ID and then ahead the frame appropriately.
There are main trunking protocols it's essential to understand and evaluate successfully, these being ISL and IEEE 802.1Q. Let's check out the main points of ISL first.
ISL is a Cisco-proprietary trunking protocol, making it unsuitable for a multivendor environment. That's one disadvantage, but there are others. ISL will place both a header and trailer onto the body, encapsulating it. This increases the overhead on the trunk line.
You know that the default VLAN is also referred to as the "native VLAN", and another disadvantage to ISL is that ISL does not use the concept of the native VLAN. Which means that every single body transmitted across the trunk will likely be encapsulated.
The 26-byte header that's added to the frame by ISL accommodates the VLAN ID; the 4-byte trailer comprises a Cyclical Redundancy Test (CRC) value. The CRC is a frame validity scheme that checks the frame's integrity.
In flip, this encapsulation leads to one other potential issue. ISL encapsulation adds 30 bytes complete to the dimensions of the frame, potentially making them too massive for the switch to handle. (The maximum size for an Ethernet frame is 1518 bytes.)
IEEE 802.1q differs considerably from ISL. In distinction to ISL, dot1q doesn't encapsulate frames. A four-byte header is added to the body, resulting in much less overhead than ISL. If the body is destined for hosts residing in the native VLAN, that header is not added. Because the header is barely four bytes in dimension, and is not even placed on each frame, using dot1q lessens the possibility of outsized frames. When the remote port receives an untagged body, the swap knows that these untagged frames are destined for the native VLAN.
Knowing the main points is the distinction between passing and failing your CCNA and CCNP exams. Maintain learning, get some fingers-on practice, and you're in your approach to Cisco certification success!
BGP is without doubt one of the most complicated matters you'll examine when pursuing your CCNP, if not essentially the most complex. I do know from personal expertise that when I was incomes my CCNP, BGP is the topic that gave me the most trouble at first. One thing I preserve reminding today's CCNP candidates about, though, is that no Cisco technology is unimaginable to grasp for those who just break it down and understand the fundamentals before you start making an attempt to know the more advanced configurations.
BGP attributes are one such topic. You have bought well-identified necessary, well-known discretionary, transitive, and non-transitive. Then you definately've bought each particular person BGP attribute to recollect, and the order wherein BGP considers attributes, and what attributes even are... and a lot more! As with every other Cisco matter, we have to stroll before we can run. Let's check out what attributes are and what they do in BGP.
BGP attributes are much like what metrics are to OSPF, RIP, IGRP, and EIGRP. You won't see them listed in a routing table, but attributes are what BGP considers when selecting the perfect path to a destination when multiple legitimate (loop-free) paths exist.
When BGP has to decide between such paths, there's an order by which BGP considers the trail attributes. For success on the CCNP exams, you'll want to know this order. BGP looks at path attributes on this order:
Highest weight (Cisco-proprietary BGP worth)
Highest local preference (LOCAL_PREF)
Favor locally originated route.
Shortest AS_PATH is preferred.
Choose route with lowest origin code. Inner paths are most popular over exterior paths, and external paths are preferred over paths with an origin of "incomplete".Lowest multi-exit discriminator (MED)
Exterior BGP routes most popular over Inside BGP routes.
If no external route, choose path with lowest IGP value to the following-hop router for iBGP.
Choose most recent route.
Select lowest BGP RID (Router ID).
If you do not know what these values are, or how they're configured, do not panic! The subsequent several parts of this BGP tutorial will explain it all. So spend some time studying this order, and partly II of this free BGP tutorial, we'll look at every of these values in detail. Preserve studying
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