Here is a comparison of the existing implementation against 802.1d standard
pseudo-code.
/* Spec */
Boolean supersedes_port_info(port_no, config) /* (8.6.2.2) */
Int port_no;
Config_bpdu *config;
{
return config->root_id < port_info[port_no].designated_root /*
(8.6.2.2 a) */
|| (config->root_id == port_info[port_no].designated_root
&& (config->root_path_cost <
port_info[port_no].designated_cost /* (8.6.2.2 b) */
(config->root_path_cost ==
port_info[port_no].designated_cost
&& (config->bridge_id <
port_info[port_no].designated_bridge /* (8.6.2.2 c) */
|| (config->bridge_id ==
port_info[port_no].designated_bridge /* (8.6.2.2 d) */
&& (config->bridge_id != bridge_info.bridge_id
/* (8.6.2.2 d1) */
|| config->port_id <=
port_info[port_no].designated_port /* (8.6.2.2 d2) */
))))));
}
/* Implementation */
static int br_supersedes_port_info(struct net_bridge_port *p, struct
br_config_bpdu *bpdu)
{
int t;
t = memcmp(&bpdu->root, &p->designated_root, 8);
if (t < 0)
return 1; /* 8.6.2.2 a */
else if (t > 0)
return 0;
if (bpdu->root_path_cost < p->designated_cost) /* 8.6.2.2 b */
return 1;
else if (bpdu->root_path_cost > p->designated_cost)
return 0;
t = memcmp(&bpdu->bridge_id, &p->designated_bridge, 8); /* 8.6.2.2 c */
if (t < 0)
return 1;
else if (t > 0)
return 0;
if (memcmp(&bpdu->bridge_id, &p->br->bridge_id, 8)) /* 8.6.2.2 d */
return 1;
if (bpdu->port_id <= p->designated_port)
return 1;
return 0;
}
You are questioning the part related to 8.6.2.2 of the spec:
8.6.2.2 Use
Following the receipt of a Configuration BPDU conveying protocol information
that
supersedes that already held, i.e., if
a) Case 1. The Root Identifier denotes a Bridge of higher priority than that
recorded as the Designated Root, or
b) Case 2. The Root Identifier is the same as the Designated Root, and the
Root
Path Cost is lower than that recorded as the Designated Cost for the
Port, or
c) Case 3. The Root Identifier and Root Path Cost are as recorded for the
Port, and
the Bridge Identifier denotes a Bridge of higher priority than that
recorded
as the Designated Bridge for the Port, or
d) Case 4. The Root Identifier and Root Path Cost are as recorded for the
Port,
and the Bridge Identifier is the same as that recorded as the
Designated Bridge
for the Port, and either
1) The Bridge receiving the BPDU is not the Designated Bridge for the
Port, or
2) The Port Identifier denotes a Port of priority not less than that
recorded
as the Designated Port.
Linux follows the specification; there is no reason to deviate for this.
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