Junos
Junos

ISIS Padding TLV

Sunday

After reading the book: "OSPF_and_IS-IS_Choosing_an_IGP_for_Large-Scale_Networks" I understood very well why with ISIS the fragmentation is a bit harder.

 

Expanation: In OSPF we have few LSA packets (LSA1,2,3,4,5) and there are LSA packets with lower size than others...but in the case of ISIS we only have a LSP packets to send update one for level1 and another for Level2; that's why they needs to contain much more informatin and the size is higher than for example LSA3 in the case of OSPF.

Additionall the LSP is not an IP packet, it cannot take advantage of IP’s fragmentation mechanism. Therefore, IS-IS must have its own fragmentation mechanism called padding TLV."

 

With that said I would like to know if is required to enable padding TLV or if is enabled by default. At the same time some articles say that is not recommended to enable in large networks because adds overhead.

Do you know which commands could I use to verify if is enabled and which type of TLV padding (loos, strict...) is enabled?

I didn't found any traceoption command to debug "padding TLV"

Many thanks, 

1 REPLY 1
Junos

Re: ISIS Padding TLV

Sunday

Hello,

 


@alfredo.bosca wrote:

IS-IS must have its own fragmentation mechanism called padding TLV."

 

 


 

ISIS Padding TLV and ISIS LSP fragments are not related.

ISIS Padding TLV is present only in IIIH packets (in ISIS Hello) and IIH are never fragmented.

https://www.iana.org/assignments/isis-tlv-codepoints/isis-tlv-codepoints.xhtml

 

 


@alfredo.bosca wrote:

 

With that said I would like to know if is required to enable padding TLV or if is enabled by default.

 


 

The behaviour depends on the vendor. CSCO and JNPR behaviour is explained in the Hannes Gredler book titled "The Complete IS-IS Routing protocol" https://www.amazon.com/Complete-Routing-Protocol/dp/1852338229 

 

Additionally, check out these links

https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-...

https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/hello-pad...

 


@alfredo.bosca wrote:

 

I didn't found any traceoption command to debug "padding TLV"

 


 

If You record all ISIS packets (either in traceoptions file or with monitor traffic interface or tcpdump from shell), You will see the Padding TLV present in IIH packets.

HTH

Thx

Alex

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