Challenges related to growing metro ethernet rings STP/RSTP issues.

[jjohonness]

How are people handling the limitations of STP/RSTP when their Metro Ethernet Rings grow very large.

[wong34539]

IEEE 802.1D STP is widely used in L2 networks, including for failure recovery and loop avoidance. However, STP performs poorly in large Metro Ethernetnetworks, taking 10s of seconds to provide recovery link failures.

The IEEE 802.1d spanning tree protocol is the only standards based protocol available for providing loopfree connectivity while providing an alternate path for failed links, ports or switches. However, standard STP is prone to slow convergence times of 30 to 50 seconds and does not scale well as the Metro network expands with more spanning tree nodes. Additionally, spanning tree domains must be continuous in order to ensure a loopfree data path. Changes within the spanning tree domain can affect all spanning tree members of that domain. Such ripple effects can cause problems in one city to affect other cities of a Metro network, for example.

[jjohonness]

Yes but what about rapid spaning tree? Are there still the same design constraints?

[jmackenzie]

Keep your access rings small (<6 x 3550-24Ts in one Gig.E ring), and run both ends of each ring back to the same salient NE (we use 6513s). This limits the STP regionally, keeping the CPU and STP activity way down. It also provides VLAN reuse (reduces future VLAN exhaustion issues), and keeps your MTBF >> 99.99% in your fiber access.

Also, running UplinkFast on all trunk ports does a great job of reducing your reconvergence times. Our Wpg MAN has over 50 Network Elements, runs >600 VLANs, and has a worst-case, tested reconvergence time of <16 seconds for the whole network.

- Jim M.

[jliste]

Yes, Keeping GE STP rings small (wrt number of nodes) is a good advice..but how small is small..

If you are running 802.1d, you might run very quickly into STP diameter issues, here your rings cannot have more than 7 nodes (including the collection node)

On the other hand when you use MST/RSTP 802.1W/.1S you can actually configure the number of hos that BPDUs can traverse before being discarded (TTL). That number can go as high as 40 ---- but again this is looking at it from a theoretical perspective, you have to also look at the SLAs you want to offer and the traffic patterns

Below is a sample config for the number of hops I mentioned:

R8-3550-24-2(config)#spanning-tree mst ?

WORD MST instance range, example: 0-3,5,7-9

configuration Enter MST configuration submode

forward-time Set the forward delay for the spanning tree

hello-time Set the hello interval for the spanning tree

max-age Set the max age interval for the spanning tree

max-hops Set the max hops value for the spanning tree

R8-3550-24-2(config)#spanning-tree mst max

R8-3550-24-2(config)#spanning-tree mst max-h

R8-3550-24-2(config)#spanning-tree mst max-hops ?

<1-40> maximum number of hops a BPDU is valid

Jose