interface don't need to up when use bridge

bridge.log

  williamherry | odd things, interface(em2) not up, but bridge on it work fine, can some one know the reason, what's interface 'UP' means?                                     │ aaket504
          tje2 | williamherry ..  I don't believe the interface has to be "up" to *bridge* (read: NOT route), but to route packets (such as via MASQ/NAT, etc) the interface   │ AAYUSH
               | would have to be "UP".                                                                                                                                        │ abbaZaba
       melacon | Mohsen_Hassani:Oh ok. What operating system are you running on your PC?                                                                                       │ abchirk_
          tje2 | williamherry ..  Familiar at all w/ Cisco devices and the UP/UP vs. DOWN/DOWN states of those ifaces?                                                         │ aboudreault
Mohsen_Hassani | melacon, The same as my Laptop, Debian Sid.                                                                                                                   │ abyss
         horse | morning chaps.  Will a simple "yum update" update both packages and my kernel? (rhel 5 machine)                                                               │ AC`97
       melacon | Mohsen_Hassani: Can you ssh to any other machine?                                                                                                             │ aca20031
     kanupatar |  May I know "why we have the name RAM disk" ?                                                                                                                 │ aca_work_
Mohsen_Hassani | melacon, I don't have access to any other machines;                                                                                                           │ AccessGranted
           gry | horse: no, just packages ; "yum update --force" will also update kernel packages.                                                                             │ aceofspades19
         horse | thanks gry                                                                                                                                                    │ acrocity 
         horse | what about yum update all?                                                                                                                                    │ acrocity_
  williamherry | tje2: so, you mean, they don't need to up if they just use to bridge                                                                                          │ AcTiVaTe
          tje2 | williamherry ..  Yes, that's what I mean to say.  :)   Basically, since there's a physical (presumably electric) link, it can function as a bridge.   But if  │ ad_
               | you want to actually examine packet/frame headers (for routing/firewall decisions, etc), then the interface must be up.                                       │ adaam
          tje2 | So, basically, if you stick to only layer 2 functionality, it should be fine.... The "UP", I would guess, refers to layer-3 functionality.                    │ adaptr
  williamherry | tje2: I see, how about the bridge will route packets, interface don't need to up too?                                                                         │ ader10
 AccessGranted | nirvank0:7.2 # sudo cat /dev/urandom | /dev/snd/controlC1                                                                                                     │ Adiemus
 AccessGranted | bash: /dev/snd/controlC1: Permission denied                                                                                                                   │ admboom
 AccessGranted | Why                                                                                                                                                           │ AdmV0rl02
 AccessGranted | I have some problems with sound on Linux Arch.                                                                                                                │ adnauseam
          tje2 | williamherry .. "bridging" != "routing".   I think you're using the term "routing" incorrectly.   Agreed, packets come in one interface and leave via         │ AdrienG
               | another....sounds enough like routing, but there are subtle nuances, such as if the layer 3 (IP-layer) header data is examined in determining what to do with │ afgdsgh
               | the packet/frame (drop it, forward it out iface X, etc).                                                                                                      │ ageis
  williamherry | tje2: got it, thanks                                                                                                                                          │ Ahnberg
          tje2 | williamherry ..  Unless you plan to go at least CCNA-level network admin (i.e., junior admin/engineer), it's probably not worth your time to really learn the │ akiress
               | distinction.... But then again, most of us don't do this because we NEED to, we do because we WANT to.   :)