Re: Spanning Tree Question - Root Port Selection
Nathan, Check the original paper by Radia Perlman, or better yet have a read of Chapter 3 of her book "Interconnections - Bridges, Routers, switches and Internetworking Protocols" 2nd Edition. Serge :) - Original Message - From: "Miller, Nathan - BSC" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, February 06, 2001 2:08 PM Subject: Spanning Tree Question - Root Port Selection > I have been looking for a while for further documentation of the process by > which a switch selects its root port. Most of the books that I have > searched for this information say something similar to the following quote > from a CCO page: "A bridge's root port is the port through which the root > bridge can be reached with the least aggregate path cost, a value that is > called the root path cost." My problem is that they all seem to stop there. > My question is this. If the root path cost is the same on multiple switch > ports, how does STA determine which is the root port? Does it follow the > same course as it would when selecting a designated port (root bridge, root > path cost, sender ID, sender port). > Many thanks for your thoughts. > Nathan Miller > > _ > FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html > Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Spanning Tree Question - Root Port Selection
>From the CISCO web site. http://www.cisco.com/warp/public/473/5.html#oper Rules of Operation - STP works as follows: When the switches first come up, they start the root switch selection process by each switch transmitting BPDU to its directly connected switch on a per-VLAN basis. As the BPDU goes out through the network, each switch compares the BPDU it sent out to the one it received from its neighbors. From this comparison, the switches come to an agreement as to who the root switch is. The switch with the lowest priority in the network that wins this election process. (Remember, there will be one root switch identified per VLAN.) After that root switch has been identified, the switches follow the rules defined below: STP Rule One: All ports of the root switch must be in forwarding mode (except for some corner cases where self-looped ports are involved). Next, each switch determines their best path to get to the root. They determine this path by comparing the information in all the BPDUs received on all their ports. The port with the smallest information contained in its BPDU is used to get to the root switch; that port is called the root port. After a switch figures out its root port, it proceeds to Rule Two. STP Rule Two: Once a switch determines its root port, that port must be set to forwarding mode. In addition, for each LAN segment, the switches communicate with each other to determine which switch on that LAN segment is best to use for moving data from that segment to the root bridge. This switch is called the designated switch. STP Rule Three: In a given LAN segment, the designated switch's port that connects to that LAN segment must be placed in forwarding mode. STP Rule Four: All other ports in all the switches (VLAN-specific) must be placed in blocking mode. This is only for ports that are connected to other bridges or switches. Ports connected to workstations or PCs are not affected by STP; they remain forwarded. FOR a Look at path cost you might take a look at http://www.cisco.com/univercd/cc/td/doc/product/lan/cat3ks/3000/aicfgcsl.htm #11023 -Original Message- From: Tom Pruneau [mailto:[EMAIL PROTECTED]] When switch send spanning tree updates those updates are sent in BPDUs (bridge protocol data units). The BPDU will have a source mac address associated with the originating switch/VLAN number. If a switch recieves multiple BPDUs that indicate the same root cost it will pick the one which came from the switch with the lowest (I'm pretty sure it's lowest and not highest but I may be wrong) MAC address. There is also a port priority which I believe (I'm not sure) can be configured to aid in the selection of the root port _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Spanning Tree Question - Root Port Selection
When switch send spanning tree updates those updates are sent in BPDUs (bridge protocol data units). The BPDU will have a source mac address associated with the originating switch/VLAN number. If a switch recieves multiple BPDUs that indicate the same root cost it will pick the one which came from the switch with the lowest (I'm pretty sure it's lowest and not highest but I may be wrong) MAC address. There is also a port priority which I believe (I'm not sure) can be configured to aid in the selection of the root port At 09:01 AM 02/06/2001 -0600, Jim Dixon wrote: >Hi Nathan, > >Have you read Radia Perlman's Interconnections. There are two. >The second edition I believe is the latest. >She wrote spanning tree. This book does cover it in detail. >ISBN# 0201634481 > >At the time I looked Amazon had a used one in GOOD condition for 15 bucks. >(FYI) > > >-Original Message- >From: Miller, Nathan - BSC [mailto:[EMAIL PROTECTED]] >Sent: Tuesday, February 06, 2001 8:09 AM >To: [EMAIL PROTECTED] >Subject: Spanning Tree Question - Root Port Selection > > >I have been looking for a while for further documentation of the process by >which a switch selects its root port. Most of the books that I have >searched for this information say something similar to the following quote >from a CCO page: "A bridge's root port is the port through which the root >bridge can be reached with the least aggregate path cost, a value that is >called the root path cost." My problem is that they all seem to stop there. >My question is this. If the root path cost is the same on multiple switch >ports, how does STA determine which is the root port? Does it follow the >same course as it would when selecting a designated port (root bridge, root >path cost, sender ID, sender port). >Many thanks for your thoughts. >Nathan Miller > >_ >FAQ, list archives, and subscription info: >http://www.groupstudy.com/list/cisco.html >Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] > >_ >FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html >Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] > > Tom Pruneau Trainer Network Operations GENUITY 3 Van de Graff Drive Burlington Ma. 01803 24 Hr. Network Operations Center 800-436-8489 If you need to get a hold of me my hours are 8AM-4PM ET Mon-Fri --- This email is composed of 82% post consumer recycled data bits --- "Once in a while you get shown the light in the strangest of places if you look at it right" _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Spanning Tree Question - Root Port Selection
Hi Nathan, Have you read Radia Perlman's Interconnections. There are two. The second edition I believe is the latest. She wrote spanning tree. This book does cover it in detail. ISBN# 0201634481 At the time I looked Amazon had a used one in GOOD condition for 15 bucks. (FYI) -Original Message- From: Miller, Nathan - BSC [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 06, 2001 8:09 AM To: [EMAIL PROTECTED] Subject: Spanning Tree Question - Root Port Selection I have been looking for a while for further documentation of the process by which a switch selects its root port. Most of the books that I have searched for this information say something similar to the following quote from a CCO page: "A bridge's root port is the port through which the root bridge can be reached with the least aggregate path cost, a value that is called the root path cost." My problem is that they all seem to stop there. My question is this. If the root path cost is the same on multiple switch ports, how does STA determine which is the root port? Does it follow the same course as it would when selecting a designated port (root bridge, root path cost, sender ID, sender port). Many thanks for your thoughts. Nathan Miller _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Spanning Tree Question - Root Port Selection
Yes. The same decision tree is used. A great explanation is in "Cisco LAN Switching" by Clark and Hamilton. Cisco Press > -Original Message- > From: Miller, Nathan - BSC [mailto:[EMAIL PROTECTED]] > Sent: Tuesday, February 06, 2001 8:09 AM > To: [EMAIL PROTECTED] > Subject: Spanning Tree Question - Root Port Selection > > > I have been looking for a while for further documentation of > the process by > which a switch selects its root port. Most of the books that I have > searched for this information say something similar to the > following quote > from a CCO page: "A bridge's root port is the port through > which the root > bridge can be reached with the least aggregate path cost, a > value that is > called the root path cost." My problem is that they all seem > to stop there. > My question is this. If the root path cost is the same on > multiple switch > ports, how does STA determine which is the root port? Does > it follow the > same course as it would when selecting a designated port > (root bridge, root > path cost, sender ID, sender port). > Many thanks for your thoughts. > Nathan Miller > > _ > FAQ, list archives, and subscription info: > http://www.groupstudy.com/list/cisco.html > Report misconduct > and Nondisclosure violations to [EMAIL PROTECTED] > _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
RE: Spanning Tree Question - Root Port Selection
I think one of the factors is the port priority. Three factors that influence Root bridge port selection are: port cost, path to the root, and port priority. I think the only one you can change is port priority. This can be set for different values depending on the type of switch you are running. -Original Message- From: Miller, Nathan - BSC [mailto:[EMAIL PROTECTED]] Sent: Tuesday, February 06, 2001 9:09 AM To: [EMAIL PROTECTED] Subject: Spanning Tree Question - Root Port Selection I have been looking for a while for further documentation of the process by which a switch selects its root port. Most of the books that I have searched for this information say something similar to the following quote from a CCO page: "A bridge's root port is the port through which the root bridge can be reached with the least aggregate path cost, a value that is called the root path cost." My problem is that they all seem to stop there. My question is this. If the root path cost is the same on multiple switch ports, how does STA determine which is the root port? Does it follow the same course as it would when selecting a designated port (root bridge, root path cost, sender ID, sender port). Many thanks for your thoughts. Nathan Miller _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED] _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
Spanning Tree Question - Root Port Selection
I have been looking for a while for further documentation of the process by which a switch selects its root port. Most of the books that I have searched for this information say something similar to the following quote from a CCO page: "A bridge's root port is the port through which the root bridge can be reached with the least aggregate path cost, a value that is called the root path cost." My problem is that they all seem to stop there. My question is this. If the root path cost is the same on multiple switch ports, how does STA determine which is the root port? Does it follow the same course as it would when selecting a designated port (root bridge, root path cost, sender ID, sender port). Many thanks for your thoughts. Nathan Miller _ FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
