What is ERGR Stack

Answered Question
Jan 20th, 2008
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Hai all,


Probly this may be a very simple quiestion ..in one interview they asked me ...What is EIGRP Stack ? ..i am not sure what xactly it is .... i try to find some docs but cudint find any ..


Can some one help me regarding the same


Lijesh

Correct Answer by mohammedmahmoud about 9 years 5 months ago

Hi,


The protocol number (Assigned by IANA) defines the protocol used in the data portion of the IP datagram , it is simply a numeric identification of the upper layer protocol that an IP packet should be sent to. Stored in the IP header of an IP packet, the most common IP protocol numbers are TCP (6), UDP (17) and ICMP (1), thus in your case when the router sees that the IP protocol number is 88 it understands that this is not a TCP or UDP packet, and that it is an EIGRP packet.


The link provided by Pavlo is IANAs reference for all these numbers.


BR,

Mohammed Mahmoud.

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pcameron Sun, 01/20/2008 - 03:38
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EIGRP stands for Enhanced Interior Gateway Routing Protocol , which is a Cisco proprietary routing protocol for passing of IP routing information between routers.


In normal terms, a 'stack' is a description of how the data processing applications sit in the ISO 7 layer data communications model.


For example, a PC would have a TCP/IP stack that sits on top of the ethernet MAC layer, it may also have a Novell IPX stack stack sitting on top of the ethernet MAC layer as well.


For a router , it may be running EIGRP, RIP, OSPF, RIP and BGP concurrently, so each of these applications would have their own processing stack sitting above the transport/IP layer - information for EIGRP and OSPF is multicast, so the IP layer needs to differentiate the different multicast addresses and pass the traffic to the relevent routing protocol.


Here is a good diagram showing the concept of software layering and protocol stacks.


http://www.pcmag.com/encyclopedia_term/0,2542,t=protocol+stack&i=49882,00.asp


So ... getting back to your original question - EIGRP can handle IP, IPX and appletalk, so the EIGRP sits on top of these underlying networking protocols. The layers of software are stacked on top of each other.


Have a read of this guide (ignore the stuff on FDDI, DECNET and other obsolete protocols!) , it will give you a better understanding of all these terms -


http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/index.htm



mohammedmahmoud Sun, 01/20/2008 - 04:34
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Hi,


Let me address this question from a different prospective, the designers of RIP and of BGP decided to utilize existing transport protocols to carry their routing traffic (BGP uses TCP 179 and RIP uses UDP 520). The developers of OSPF and of EIGRP decided that there were advantages in not using an existing transport layer protocol and just encapsulating their content directly in IP (EIGRP uses IP protocol 88 and OSPF uses IP protocol 89 - they encapsulate their messages directly into IP packets). Part of the advantage of this is that it simplifies the formatting of the packet and simplifies the processing of the packet when it is received (rather than passing the data from layer 3 IP to layer 4 UDP or TCP which will look at the data and determine that it needs to be sent to the routing protocol, the data goes directly from layer 3 IP to the routing protocol), simply EIGRP/OSPF data or payload is just encapsulated inside the IP Packet, with no higher OSI layer utilized. There are a number of other protocols where IP packets going through the network do not have TCP or UDP transport layer. For example protocol 47 is for GRE packets which do not use either UDP or TCP. And protocol 50 is for ESP (part of IPSec) which does not use UDP or TCP.


[edit] After reading both replies together (my reply, and Paul's reply) i think that they are complementary to each other, i hope that we have illustrated the logic of routing protocols stack.


BR,

Mohammed Mahmoud.

CSCO10758684 Sun, 01/20/2008 - 23:57
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Hai Mohmd,


Thanks a lot for your detaild xplanation ..bothe replys were xclnt and valuable ones....


one more dout you ve mentioned that Eigrp and Ospf uses ip protocol 88 and 89 ..is this actualy the port or ...??? smal confusion ..wud be nice if you can xplain ..


Lijesh.N.C

Correct Answer
mohammedmahmoud Mon, 01/21/2008 - 03:18
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Hi,


The protocol number (Assigned by IANA) defines the protocol used in the data portion of the IP datagram , it is simply a numeric identification of the upper layer protocol that an IP packet should be sent to. Stored in the IP header of an IP packet, the most common IP protocol numbers are TCP (6), UDP (17) and ICMP (1), thus in your case when the router sees that the IP protocol number is 88 it understands that this is not a TCP or UDP packet, and that it is an EIGRP packet.


The link provided by Pavlo is IANAs reference for all these numbers.


BR,

Mohammed Mahmoud.

CSCO10758684 Mon, 01/21/2008 - 06:35
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Hai Mohmd,


Now its clear and got in depth details ...sorry to ask you this quiestion ...how we can define "EIGRP STACK" in few sentance ...it shud contain all detail

sorry for the trble ....i think m trbling a lot


Thanks alot

Lijesh.N.C


mohammedmahmoud Mon, 01/21/2008 - 06:58
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Hi Lijesh,


No trouble at all, i am glade that i was helpful, and please never hesitate to ask further questions, IMHO the simple answer could be; EIGRP stack consists of an IP packet that uses ip protocol number 88.



BR,

Mohammed Mahmoud.

CSCO10758684 Mon, 01/21/2008 - 07:40
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Hai Mohmd,


Whah .....Greaat ..no words ..... ...


Thanks a lot


Lijesh.N.C

CSCO10758684 Sun, 01/20/2008 - 23:55
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Hai PAUL,


Thanks a lot for your detaild explanation ..really this a a valuable information for me ....


once again thanks a lot


Lijesh.N.C

saimbt Mon, 01/21/2008 - 04:56
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Hi,

One more quick information.


1) Port numbers are used by APPLICATIONS.

2) Protocol numbers are used by PROTOCOLS.



amolwaghmare Mon, 01/21/2008 - 06:12
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Very well said by both of them.Especially mohammedmahmoud


mohammedmahmoud how did u got such gr8 knowledge because even if u read RFC you won't get that.


Can you both suggest any book?

mohammedmahmoud Mon, 01/21/2008 - 06:37
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Hi Amol,


Thank you very much, i can at least recommend 2 books beside Cisco's documents and the RFCs:


TCP/IP Illustrated (Richard Stevens).

Routing TCP/IP (Jeff Doyle).


BR,

Mohammed Mahmoud.

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