Hi all,
I am doing some projections for growth in our companies FR cloud. I was
checking the limitations of routers when I came across this article. The
link is below.
http://www.cisco.com/warp/public/125/26.html
The following is directly from the article. The last part is what I have a
question about. Does anybody know where the limitations per router platform
come from? The article seems to be missing some information about why the
2500 can only support 60 DLCIs, the 4000 can support 120, etc.
Thanks in advance,
Dave
DLCI Limitations
Subinterfaces count toward the practical upper limit of 230 Interface
Descriptor Blocks (IDBs). In other words, Cisco IOS currently doesn't
support more then 230 interfaces on the router (real or virtual) unless you
have an ISP Geeks Image which has 1024 IDBs. How many DLCIs can one
configure per physical interface? How many DLCIs can one configure in a
specific router? These two questions are frequently asked. Disappointingly,
the answer is, "it depends."
DLCI address space: Approximately 1000 DLCIs can be configured on a single
physical link, given a 10-bit address. Because certain DLCIs are reserved
(vendor-implementation-dependent), the maximum is about 1000. The range for
"cisco" LMI is 16-1007. The stated range for ANSI/ITU is 16-992. These are
the DLCIs carrying user-data.
LMI status update: The LMI protocol requires that all permanent virtual
circuit (PVC) status reports fit into a single packet and generally limits
the number of DLCIs to less than 800, depending on the maximum transmission
unit (MTU) size.
MTU= 4000 bytes Max DLCIs app= (MTU bytes - 20 bytes)/ (5 bytes/DLCI)
(4000-20)/5 = 796
Default MTU on serial interfaces is 1500 bytes, yielding a maximum of 296
DLCIs per interface. Please note that these numbers vary slightly, depending
on the LMI type. The maximum DLCIs per router (not interface) platform
guideline, based on extrapolation from empirical data established on a Cisco
7000 router platform, are listed below:
* Cisco 2500: 1 X T1/E1 link @ 60 DLCIs per interface = 60 total
* Cisco 4000: 1 X T1/E1 link @ 120 DLCIs per interface = 120 total
* Cisco 4500: 3 X T1/E1 links @ 120 DLCIs per interface = 360 total
* Cisco 4700: 4 X T1/E1 links @ 120 DLCIs per interface = 480 total
* Cisco 7000: 4 X T1/E1/T3/E3 links @ 120 DLCIs per interface = 480
total
* Cisco 7200: 5 X T1/E1/T3/E3 links @ 120 DLCIs per interface = 600
total
* Cisco 7500: 6 X T1/E1/T3/E3 links @ 120 DLCIs per interface = 720
total
Note: These numbers are guidelines only, and assume that all traffic is
fast-switched.
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