How to troubleshoot GRE

Utilizing non specific steering embodiment (GRE) burrows on Cisco switches can prove to be useful with Cisco switch organization, and arranging GRE passages is generally simple. David Davis has the points of interest in this release of Cisco Routers and Switches.


Initially created by Cisco, non specific steering embodiment (GRE) is currently a standard, characterized in RFC 1701, RFC 1702, and RFC 2784. GRE is a burrowing convention used to transport parcels from one system through an alternate system.



In the event that this sounds like a virtual private system (VPN) to you that is on account of it hypothetically is: Technically, a GRE shaft is a sort of a VPN-however it isn't a protected burrowing strategy. On the other hand, you can scramble GRE with an encryption convention, for example, IP sec to structure a protected VPN. Truth be told, the point-to-point burrowing convention (PPTP) really utilizes GRE to make VPN burrows. For instance, in the event that you arrange Microsoft VPN burrows, of course, you utilize PPTP, which utilizes GRE.



Benefits of using the GRE

You have to encode multicast movement. GRE passages can convey multicast parcels much the same as genuine system interfaces rather than utilizing Ipsec without anyone else's input, which can't encode multicast activity. A few cases of multicast movement are OSPF, EIGRP, and Ripv2. Additionally, various feature, VOIP, and streaming music applications use multicast.


You have a convention that isn't routable, for example, Netbios or non-IP movement over an IP system. For instance, you could utilize GRE to passage IPX or Apple talk through an IP system.



You have to interface two comparable systems associated by an alternate system with distinctive IP addressing.

Troubleshooting the GRE

Since GRE takes one bundle and typifies it in an alternate parcel, you may run into a circumstance where the bundle you're sending is bigger than your interface permits. The answer for this issue is to arrange ip TCP modify mss 1436on the shaft interface. While GRE doesn't give encryption, you can empower a key on each one side of the shaft utilizing the passage key order. This is similar to a straightforward clear-message secret key with no encryption.



Since GRE shafts are stateless, it's workable for one side of the passage to go down while the other side stays up. The answer for this issue is to empower keep alive parcels on each one side of the shaft. By doing this, each one side of the shaft occasionally sends a keep alive to the next side. On the off chance that one side doesn't get a keep alive in the indicated time, the passages go down on each one side.



Requirements for the GRE

GRE passages are useful for a system executive from numerous points of view really. For instance, chances that you are running a directing convention or may be static steering with your ISP are bleak. Be that as it may you don't need ISP to see your inner system. So all things considered with the assistance of Static Routing or IGP (that ISP is running too with you) you will secure essential integration between your extensions with help of ISP. Presently later you can make burrows between all extensions with help of fundamental network you had secured prior and after that you can run steering convention over the passage & can trade courses. Presently actually once GRE shaft is created between your extension work places then ISP can't see what information is going inside the tunnel or in other words your courses & information.


An alternate circumstance where we can presumably utilize GRE shaft is to pass OSPF LSAs through Stub/Totally Stub Area. For instance once Area is designed as stub, of course it won't permit you to make virtual connection over it or to pass LSAs which it should hinder as Stub range yet somehow you need do these things which mean you get the idea. There are numerous different winds in Real world and CCIE lab exam where you can consider utilizing tunnels. There are a lot of people more reasons however these are most basic one.



Setting up the tunnel for the requirements:

The prerequisite to setup passage is exceptionally straightforward. Everything you need is the achieve capacity between the two end purposes of the shaft. Normally you will see individuals utilizing loopback interface as passage source address however actually you can pick any interface as source at your side yet end will dependably be some ip location of other side and can't be interface that really bodes well as well as Local Router don't think about what interfaces we have on the other side.



Yet one thing you ought to remember while picking any interface as source location is that naturally it will pick the essential IP of the interface, so on the off chance that you need to pick optional IP then specify Secondary IP address particularly as source ip address.



Drawbacks for the GRE

There is stand out significant downside of GRE shaft and that is - it adds two additional headers to our unique IP parcel. So those fellows who are concerned with Bandwidth may have a few issues. However in the event that you change the shaft mode to "IPIP" then you can diminish some overhead as "IPIP" passage includes 1 header rather than 2 contrast with "GRE". With the goal that makes it more proficient. You can likewise utilize other strategy to avoid transfer speed like conventional TCP layering methods or cutting edge MQC based procedures. An alternate disadvantage is that GRE shafts are not extremely adaptable result. The default BW for GRE shafts is only 9 Kbps with MTU size 1514 byes, so your directing convention may not take after the way you craving to, so watch out for such things all things considered.


At this moment Tunnel Source and Destinations (Which are Lo0 of R1/R3) are educated by means of ospf and inside the passage we are running EIGRP. Anyway say in the event that I empower EIGRP on Lo0 over R1 & R2 then by method for Basic steering essential, EIGRP will begin publicizing the Lo0 interfaces on both switches and soon switch will upgrade its directing table and will introduce EIGRP courses for Lo0s. Since EIGRP has less AD contrast with OSPF.



Intermediate system to system TLVs

IS-IS, initially intended for Open System Interconnection (OSI) directing, uses TLV parameters to convey data in Link State Packets (LSPs). The TLVs make IS-IS extendable. IS-IS can hence convey various types of data in the LSPs. As characterized by ISO 10589, Seems to be IS backings just the Connectionless Network Protocol (CLNP). Then again, IS-IS was reached out for IP directing in RFC 1195 leavingcisco.com with the enlistment of TLV 128 which contains a set of 12-octet fields to convey IP data.


In the IS-IS Protocol Data Unit (PDU), there is a settled and a variable piece of the header. The settled piece of the header contains fields that are constantly showed, and the variable piece of the header contains the TLV which allows the adaptable encoding of parameters inside connection state records. These fields are recognized by one octet of sort (T), one octet of length (L) and "L" octets of worth (V). The Type field shows the kind of things in the Value field. The Length field demonstrates the length of the Value field. The Value field is the information parcel of the bundle. Not all switch usage help all TLVs, however they are obliged to overlook and retransmit the overlooked sorts.



As clarified by RFC 1195 leavingcisco.com, TLV 128 amplifies IS-IS to convey IP, notwithstanding Connectionless Network Service (CLNS), steering data in the same bundle. DEC has likewise executed an augmentation to IS-IS with TLV 42. This augmentation permits the IS-IS to hold data about Decent Phase IV systems. Later on, another TLV may be actualized permitting CLNS to convey Ipv6 steering data.


A few directing conventions use TLVs to convey an assortment of qualities. Cisco Discovery Protocol (CDP), Label Discovery Protocol (LDP), and Border Gateway Protocol (BGP) are cases of conventions that utilization TLVs. BGP utilizes TLVs to convey properties, for example, Network Layer reach ability Information (NLRI), Multiple Exit Discriminator (MED), and neighborhood inclination.



Sub TLVs and the traffic engineering

Sub-TLVs utilize the same ideas as TLVs. The distinction is that TLVs exist inside IS-IS bundles, while TLV are utilized to add additional data to specific TLVs. Each one sub-TLV comprises of three fields. An one-octet Type field, and one-octet Length field, and zero or more octets of Value. The Type field shows the kind of things in the Value field. The Length field demonstrates the length of the Value field in octets. Each one sub-TLV can conceivably hold different things. The quantity of things in a sub-TLV could be processed from the length of the entire sub-TLV, when the length of everything is known. Obscure sub-TLVs are to be overlooked and skipped on receipt.



Working of a GRE Keep Alive

The GRE keep alive peculiarity empowers the keep alive interface charge for shafts, and permits you to arrange keep alives for point-to-point GRE burrows. You can design keep alives with the keep alive summon, and alternatively with its new expansion.


GRE shafts give a strategy to epitomize self-assertive bundles inside a vehicle convention. They likewise offer a structural engineering intended to give the administrations needed to actualize any standard point-to-point embodiment plan. Here are a portion of the points of interest of GRE shafts:


GRE passages give multi-convention neighborhood arranges over a solitary convention spine. GRE passages give workarounds to systems that contain conventions with constrained jump tallies. GRE shafts unite broken sub-systems. GRE shafts permit VPNs crosswise over WANs. In any case, in the current execution of GRE shafts, an arranged shaft does not can cut down the line convention of either passage endpoint, if the far end is inaccessible. Subsequently, the movement sent from the shaft is dark holed, and it can't take after option ways in light of the fact that the passage dependably stays up.


This circumstance is valid for shafts that depend on static courses or on directing conventions that total courses to discover a course to the shaft goal. It is additionally valid in circumstances where the information in the control plane takes after an alternate way from the information in the information plane.



Keep Alive packeting

When you empower keep alives on the passage endpoint of Router A, the switch at each interim develops the inward IP header. At the end of the header, the switch additionally adds a GRE header with a Protocol Type (PT) of 0, and no other payload. The switch then sends that parcel through the shaft, which brings about its embodiment with the external IP header, and a GRE header with the PT of IP. The passage keep alive counter augmentations by one. In the event that there is an approach to achieve the far end burrow endpoint, and the shaft line convention is not down because of different reasons, the bundle touches base on Router B. It is then matched against Tunnel 0, is encapsulated, and sent to the objective IP, which is the passage source, Router A. Upon landing on Router A, the bundle is again encapsulated, and the PT is checked. In the event that the aftereffect of the PT check is 0, it means that this is a keep alive parcel. In such a case, the shaft keep alive counter is reset to 0, and the bundle is tossed.


In the event that Router B is inaccessible, Router A keeps on developing and sends the keep alive bundles alongside ordinary activity. On the off chance that the line convention is down, the keep alives don't return to Router A. Subsequently, the keep alive counter keeps on increaing. The shaft line convention stays up just the length of the passage keep alive counter stays zero, or short of what a designed quality. In the event that that condition is not genuine, whenever you endeavor to send a keep alive to Router B, the line convention is cut down, when the keep alive counter achieves the arranged keep alive esteem. In the up/down express, the passage does not forward or transform any movement separated from the keep alive bundles. For this to work for keep alive parcels just, the shaft must be forward-and-get agreeable. So the shaft lookup calculation must be effective in all cases, and must dispose of just the information parcels if the line convention is down. At the point when a keep alive parcel is gotten, it infers that the shaft endpoint is again reachable. The passage keep alive counter is then reset to 0, and the line convention returns up.


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