Subject: [Isis-wg] Isis-wg] - Regarding IS-IS LSP mtu size

From: "Rajesh Hegde" <[email-masked]>
Date: Tue, 17 Jul 2001 20:22:38 -0700

Hi all,

Following are the doubts regarding the usage of size of LSP mtu

By having default size of 1497 bytes works for most of the data link types. Ex : Ethernet, FrameRelay and Serial Links where the size of the DataLink BlockSize is 1500 bytes.  

1 )  How it should be handled If the datalink type is POS where the default DataLinkBlockSize is     512 bytes ?

2) How the negotiation of the Hello PDUs to be done in case of FDDI and Gigabit ethernet where the DataLinkBlockSize is greater than the default size of 1497 bytes ?
(i.e Do we pad the Hello PDU to the size of DataLink Block Size or increase the LSP MTU size ? ) 

3) Shall the parameters L1LSPBufferSize and L2LSPBufferSize (Level 1 lsp mtu and level 2 lsp-mtu) be made configurable ?

Suppose a router with 3 interfaces 1 ethernet(Link Mtu 1500 bytes), 1 Gigabit ethernet (Link MTU is 9180 bytes), 1 POS interface (link MTU 512 bytes )

In this case how to decide the size of the LSP-mtu ?

Thanks and Regards,
Rajesh

---

From: Les Ginsberg <[email-masked]>
Date: Wed, 18 Jul 2001 20:45:40 -0700

> By having default size of 1497 bytes works for most of the data link
types. Ex : 
> Ethernet, FrameRelay and Serial Links where the size of the DataLink
BlockSize is 1500 > bytes. 
>  
> 1 )  How it should be handled If the datalink type is POS where the
default 
> DataLinkBlockSize is     512 bytes ?
>

The LSP MTU size must be <= to the smallest DataLinkBlockSize(MTU) of any
subnetwork over which LSPs are to be propagated. Therefore, if you have even
one link with an MTU of 512 over which LSPs will be propagated, then LSP MTU
cannot be set larger than 512 in any router in the area/domain.

> 2) How the negotiation of the Hello PDUs to be done in case of FDDI and
Gigabit 
> ethernet where the DataLinkBlockSize is greater than the default size of
1497 bytes ?
> (i.e Do we pad the Hello PDU to the size of DataLink Block Size or
increase the LSP MTU > size ? ) 

ISO 10589 is very clear about this. Padding is to the maximum of:

   o dataLinkBlockSize
   o originatingL1LSPBufferSize (If L1 routing supported)
   o originatingL2LSPBUfferSize  (If L2 routing supported)

See 8.2.3.c and 8.4.2.

Setting of the LSP MTU is based NOT upon the dataLinkBlockSize of an
individual link but upon the minimum MTU of all links over which LSPs are to
be propagated. Since there is no way for the protocol to know this, it is up
to the network administartor to configure it correctly and consistently on
all routers in the area/domain.

> 3) Shall the parameters L1LSPBufferSize and L2LSPBufferSize (Level 1 lsp
mtu and level > 2 lsp-mtu) be made configurable ?

They are configurable. ISO 10589 specifies a range of 512-1492.
 
> Suppose a router with 3 interfaces 1 ethernet(Link Mtu 1500 bytes), 1
Gigabit ethernet > (Link MTU is 9180 bytes), 1 POS interface (link MTU 512
bytes )
>In this case how to decide the size of the LSP-mtu ?

LSP MTU must be 512 in this case.

You may also find the SIF work on this subject of interest:

ftp://ftp.juniper.net/pub/isis/lspsize.PDF

Apart from the change of the TLVs from 12/13 to a single value of 14, the
SIF work as written has been incorporated in the upcoming revision 2 of ISO
10589.

> Rajesh

   Les
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