subject: Cisco CCNA / CCNP / BCMSN Exam Evaluate: Trunking And Trunking Protocols [print this page] Cisco CCNA / CCNP / BCMSN Exam Evaluate: Trunking And Trunking Protocols
To earn your CCNA or CCNP certification, you've got to perceive the basics of trunking. This isn't only a CCNA subject - you have to have a complicated understanding of trunking and etherchannels to go the BCMSN exam and earn your CCNP as well. Earlier than we address these superior subjects, though, you have to master the fundamentals!
A trunk allows inter-VLAN site visitors to circulate between instantly linked switches. By default, a trunk port is a member of all VLANs, so site visitors for any and all VLANs can journey across 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, each single port is in dynamic fascinating mode by default, meaning that each port is actively attempting to trunk. On these switches, the one motion needed from us is to physically connect them with a crossover cable. In just some seconds, the port mild turns green and the trunk is up and running. The command show interface trunk will verify trunking.
How does the receiving swap know what VLAN the frame belongs to? The frames are tagged by the transmitting switch with a VLAN ID, reflecting the number of the VLAN whose member ports should obtain this frame. When the frame arrives on the remote swap, that swap will examine this ID and then ahead the frame appropriately.
There are main trunking protocols you could understand and examine successfully, those 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 is one disadvantage, but there are others. ISL will place both a header and trailer onto the frame, encapsulating it. This increases the overhead on the trunk line.
You recognize that the default VLAN is also called the "native VLAN", and one other disadvantage to ISL is that ISL doesn't use the idea of the native VLAN. This means that each single frame transmitted throughout the trunk can be encapsulated.
The 26-byte header that is 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 results in one other potential issue. ISL encapsulation provides 30 bytes total to the size of the frame, probably making them too massive for the swap to handle. (The utmost dimension for an Ethernet frame is 1518 bytes.)
IEEE 802.1q differs substantially from ISL. In distinction to ISL, dot1q does not encapsulate frames. A 4-byte header is added to the frame, leading to much less overhead than ISL. If the body is destined for hosts residing within the native VLAN, that header isn't added. Because the header is simply four bytes in measurement, and is not even placed on each body, utilizing dot1q lessens the possibility of outsized frames. When the distant port receives an untagged frame, the change is aware of that these untagged frames are destined for the native VLAN.
Knowing the details is the distinction between passing and failing your CCNA and CCNP exams. Keep finding out, get some palms-on follow, and you're in your option to Cisco certification success!
BGP is likely one of the most complex matters you'll examine when pursuing your CCNP, if not the most complex. I know from personal experience that once I was incomes my CCNP, BGP is the topic that gave me the most trouble at first. One factor I hold reminding right now's CCNP candidates about, though, is that no Cisco expertise is impossible to know in the event you just break it down and perceive the fundamentals earlier than you begin attempting to know the extra advanced configurations.
BGP attributes are one such topic. You've acquired nicely-known necessary, effectively-identified discretionary, transitive, and non-transitive. Then you've acquired each individual BGP attribute to recollect, and the order through which BGP considers attributes, and what attributes even are... and a lot more! As with every different Cisco topic, we now have to stroll earlier than we will 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 desk, but attributes are what BGP considers when selecting the best path to a vacation spot when a number of valid (loop-free) paths exist.
When BGP has to resolve between such paths, there may be an order through which BGP considers the path attributes. For success on the CCNP exams, that you must know this order. BGP looks at path attributes in this order:
Highest weight (Cisco-proprietary BGP value)
Highest local choice (LOCAL_PREF)
Desire domestically originated route.
Shortest AS_PATH is preferred.
Choose route with lowest origin code. Inner paths are most popular over external paths, and exterior paths are preferred over paths with an origin of "incomplete".
Lowest multi-exit discriminator (MED)
Exterior BGP routes preferred over Internal BGP routes.
If no exterior route, choose path with lowest IGP value to the next-hop router for iBGP.
Choose most recent route.
Choose lowest BGP RID (Router ID).
If you don't know what these values are, or how they're configured, do not panic! The following a number of elements of this BGP tutorial will explain it all. So spend a while studying this order, and in part II of this free BGP tutorial, we'll look at each of those values in detail. Keep finding out
