This article explains how to understand unrecognized attributes in BGP routes.

When executing "show route extensive" command, sometimes the received bgp route contains unrecognized Attributes. 

Refer below example:

user@mx# run show route 172.0.0.0 extensive
inet.0: 11 destinations, 11 routes (11 active, 0 holddown, 0 hidden)
172.0.0.0/24 (1 entry, 1 announced)
    *BGP  Preference: 170/-11
        Next hop type: Indirect, Next hop index: 0
        Address: 0xc68bc30
        Next-hop reference count: 1
        Source: 192.168.1.2
        Protocol next hop: 192.168.1.2
        Indirect next hop: 0x2 no-forward INH Session ID: 0x0
        State: <Active Int Ext>
        Local AS: 65001 Peer AS: 65001
        Age: 13     Metric2: 0
        Validation State: unverified
        Task: BGP_65001.192.168.1.2
        Announcement bits (1): 6-Resolve tree 4
        AS path: I
        Unrecognized Attributes: 32 bytes     
        Attr flags e0 code 23: 00 00 fa fa
        Attr flags e0 code ff: 00 00 07 db 00 00 00 01 00 01 00 0a ff 08 00 00 00 00 0c ca 6e 17
        Accepted
        ...

This route includes two JUNOS unsupported path attributes, refer below bgp update message content:

Border Gateway Protocol - UPDATE Message
  Marker: ffffffffffffffffffffffffffffffff
  Length: 80
  Type: UPDATE Message (2)
  Withdrawn Routes Length: 0
  Total Path Attribute Length: 53
  Path attributes
    Path Attribute - Unknown (35)
      Flags: 0xe0, Optional, Transitive, Partial
        1... .... = Optional: Set
        .1.. .... = Transitive: Set
        ..1. .... = Partial: Set
        ...0 .... = Extended-Length: Not set
        .... 0000 = Unused: 0x0
      Type Code: Unknown (35)
      Length: 4
      Unknown Path attributes
    Path Attribute - Unknown (255)
      Flags: 0xe0, Optional, Transitive, Partial
        1... .... = Optional: Set
        .1.. .... = Transitive: Set
        ..1. .... = Partial: Set
        ...0 .... = Extended-Length: Not set
        .... 0000 = Unused: 0x0
      Type Code: Unknown (255)
      Length: 22
      Unknown Path attributes
    Path Attribute - ORIGIN: IGP
    Path Attribute - AS_PATH: empty
    Path Attribute - NEXT_HOP: 192.168.1.2 
    Path Attribute - LOCAL_PREF: 10
  Network Layer Reachability Information (NLRI)

Two Path attributes flags are both "Optional, Transitive, Partial".

First unsupported path attribute type code is 0x23(35), the length 4 means this attribute content is 4 bytes.

This attribute total length is "flag+type code+length+content=1+1+1+4=7 bytes".

Second unsupported path attribute type code is 0xff(255). This attribute total length is "flag+type code+length+content=1+1+1+22=25 bytes"

Two Path attributes total length is "7+25=32 bytes".

BGP code instruction refer below link:

https://www.iana.org/assignments/bgp-parameters/bgp-parameters.xhtml

If customer wants to remove JUNOS unsupported path attribute from bgp route, refer below KB:

https://kb.juniper.net/KB26966 [juniper.net]