> For the complete documentation index, see [llms.txt](https://jncia-workbook.gitbook.io/workbook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jncia-workbook.gitbook.io/workbook/junos-switching/2-spanning-tree.md).
# 2) Spanning-Tree
### Pre-Work
Load the config called **lab2.cfg** using the **load\_config\_on\_nodes.py** script.
```
python3 load_config_on_nodes.py --lab_dir switched-lan --config_filename lab2.cfg
```
### Lab
The basic L2 switching that you completed in the previous lab is already pre-configured.
Our goal is to safely add trunking to SW1. Because this will form a physical loop, enable RSTP on all three switches as follows:
* SW1 should be the root bridge with priority 4096
* All ports on SW1 should automatically participate in RSTP, even if more trunk ports are added in the future
* SW2 and SW3 `ge-0/0/2` and `ge-0/0/3` should be edge ports
* There should be one RSTP topology no matter how many VLANs there are
Once RSTP is configured, configure the SW1-SW2 and SW1-SW3 links as trunks. Ensure you define the VLANs on SW1 as well. (This is already preconfigured on SW2 and SW3).
### Answer
Expand to reveal
Configure RSTP:
```
#SW1
set protocols rstp interface all
set protocols rstp bridge-priority 4096
#SW2 and SW3
set protocol rstp interface ge-0/0/0
set protocol rstp interface ge-0/0/1
set protocol rstp interface ge-0/0/2 edge
set protocol rstp interface ge-0/0/3 edge
```
\
Configure trunk ports:
```
#SW1
set vlans PROD vlan-id 100
set vlans DEV vlan-id 200
set interfaces ge-0/0/2.0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/2.0 family ethernet-switching vlan members [ PROD DEV ]
set interfaces ge-0/0/3.0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/3.0 family ethernet-switching vlan members [ PROD DEV ]
#SW2 and SW3
set interfaces ge-0/0/0.0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/0.0 family ethernet-switching vlan members [ PROD DEV ]
```
### Explanation
Expand to reveal
It seems that by default, spanning-tree is not enabled on Junos switches. It's possible that this depends on the platform you are using, I am not certain. However, I did verify that without RSTP explicitly enabled, I was able to form a data plane loop in my lab. This does seem strange to me, as typically STP is enabled by default on switches to protect against accidental loops.
\
The "default" mode for spanning-tree on Junos would be RSTP. If you want to use STP you must use the `force-version` command under RSTP. This forces RSTP verison 0 which is compatible with 802.1D (classic STP). In RSTP "version 0," the switch sends 802.1D BPDUs instead of 802.1w BPDUs. Version 0 normally happens automatically when the switch receives an 802.1D BPDU from a connected switch.
```
set protocols rstp force-version stp
```
\
We enable RSTP on a port-by-port basis. We can either use `interface all`, or specify individual interfaces. (`Interface all` is done on SW1 in this lab to fulfill the requirement that all interfaces automatically have spanning-tree enabled). To enable "portfast" functionality (in Cisco terminology), we use RSTP edge ports.
\
Bridge priority is configured with the `bridge-priority` command. Note that we can also use a shorthand such as `4k`, `8k`, etc. instead of remembering the exact power of 2.
```
set protocols rstp bridge-priority 4k
```
* The switch will really use 4096 when we configure `4k`.
\
SW2 and SW3 should see that SW1 is the root bridge:
```
admin@SW2> show spanning-tree bridge
STP bridge parameters
Routing instance name : GLOBAL
Context ID : 0
Enabled protocol : RSTP
Root ID : 4096.2c:6b:f5:78:ad:d0
Root cost : 20000
Root port : ge-0/0/0
Hello time : 2 seconds
Maximum age : 20 seconds
Forward delay : 15 seconds
Message age : 1
Number of topology changes : 3
Time since last topology change : 7 seconds
Local parameters
Bridge ID : 32768.2c:6b:f5:cf:e5:d0
Extended system ID : 0
```
\
One port on either SW2 or SW3 should be blocking:
```
admin@SW2> show spanning-tree interface
Spanning tree interface parameters for instance 0
Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ge-0/0/1 128:490 128:490 32768.2c6bf5162bd0 20000 BLK ALT
ge-0/0/2 128:491 128:491 32768.2c6bf5cfe5d0 20000 FWD DESG
ge-0/0/3 128:492 128:492 32768.2c6bf5cfe5d0 20000 FWD DESG
ge-0/0/0 128:493 128:491 4096.2c6bf578add0 20000 FWD ROOT
```
On SW2 and SW3, `ge-0/0/2` and `ge-0/0/3` should be edge ports:
```
admin@SW2> show spanning-tree interface ge-0/0/2 detail
Spanning tree interface parameters for instance 0
Interface name : ge-0/0/2
Port identifier : 128.491
Designated port ID : 128.491
Port cost : 20000
Port state : Forwarding
Designated bridge ID : 32768.2c:6b:f5:cf:e5:d0
Port role : Designated
Link type : Pt-Pt/EDGE <-------------
Boundary port : NA
```
\
Note that this is not "per-vlan" spanning-tree. There is only one single instance. To do "per-vlan" spanning-tree you would use `set protocols vstp` instead of `set protocols rstp`. Typically this is only done when you have to interoperate with Cisco equipment that is running per-vlan spanning-tree.
### Further Reading