[c-nsp] ASR9000, bridging, RPVSTP, TCN and MAC flush

Gert Doering gert at greenie.muc.de
Sun Dec 1 13:42:18 EST 2019


as a preface, I know the standard answer is "do not do that", but due to 
local constraints, I can't avoid it...

If I have a bridge-group on an ASR9001 with (rapid per-vlan) spanning-tree
active, everything works "as expected".  

Except for TCN-induced mac-flushes, that is

 - somewhere else in this STP topology, a link fails
 - adjacent switches notice and send STP TCNs
 - the ASR9001 would now need to send out packets for a given host
   to a different interface (I've constructed the outage purposely that
   way to see if it works) - normally, STP TCNs force a MAC table flush
 - this does not happen - it sticks to its MAC tables unless a packet
   *from* that host is seen, and then it updates everything fine.

Reading documentation on STP modes on the ASR9001 is a bit sparse - and
all mentioning of TCN lead to "MST", which is a no-go.  In general, and
even more in this particular environment...

So - since all the knobs are well-hidden - is there a knob I have not
yet found to make the ASR9001 flush MAC tables on STP TCN, or is this
just "not supported"?

IOS XR version 6.5.3SP1 (32 bit, as ASR9001).

This is the config:

interface TenGigE0/0/0/0.3028 l2transport
 encapsulation dot1q 3028
 rewrite ingress tag pop 1 symmetric
 mtu 1518

interface TenGigE0/0/1/0.3028 l2transport
 encapsulation dot1q 3028
 rewrite ingress tag pop 1 symmetric
 mtu 1518

 bridge group vlandb
  bridge-domain v3028
   interface TenGigE0/0/0/0.3028
   interface TenGigE0/0/1/0.3028
   routed interface BVI3028
panning-tree pvrst vlandb
 vlan 3028
  priority 8192
 interface TenGigE0/0/0/0
 interface TenGigE0/0/1/0

"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert at greenie.muc.de
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