为什么L2SC用MST而router用SPF - 未名空间MITBBS历史存档

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为什么L2SC用MST而router用SPF

为什么L2SC用MST而router用SPF# EmergingNetworking - 热门网络技术

c*g2007-10-27 07:10

1 楼

突然想到这个问题：
Ethernet switch用Minimum spanning tree 算法来确定如何forward frame,
而IP router用shorted path first, 为什么ethernet switch 不也用spf呢？

b*e2007-10-27 07:10

2 楼

This is a good question, the answer is that there is a basic difference between MAC address and IP address, MAC address does not have built-in hierarchy, i.e, it can not be summarized, in a flat network, spanning tree is the only solution. As Ethernet is gaining more and more traction in SP space, L2 transport relying on spanning tree is no-longer sufficient for large L2 networks, because of inefficiency of spanning tree protocol.
There is an IETF group working on L2 routing, but there won't be

【在 c*****g 的大作中提到】

: 突然想到这个问题：
: Ethernet switch用Minimum spanning tree 算法来确定如何forward frame,
: 而IP router用shorted path first, 为什么ethernet switch 不也用spf呢？

c*g2007-10-27 07:10

3 楼

thanks, but i don't quite understand. I thought an Ethernet switch still
needs to keep something like a route table (called forwarding table?)
for the hosts in its broadcast domain, right? It is the same as ip route
tables except addresses cannot be summarized, isn't it?

is
some

【在 b******e 的大作中提到】

: This is a good question, the answer is that there is a basic difference between MAC address and IP address, MAC address does not have built-in hierarchy, i.e, it can not be summarized, in a flat network, spanning tree is the only solution. As Ethernet is gaining more and more traction in SP space, L2 transport relying on spanning tree is no-longer sufficient for large L2 networks, because of inefficiency of spanning tree protocol.
: There is an IETF group working on L2 routing, but there won't be

j*a2007-10-27 07:10

4 楼

ethernet is a mess and cannot be summerized. so l2 it has to be small or u
have performance issue due to broadcast/multicast.
just think of "routing table" of l2 as a table that has only /32 route. i
see no issue of running spf over l2. but it doesnt justify in any case and
just isn't worth it.

【在 c*****g 的大作中提到】

: thanks, but i don't quite understand. I thought an Ethernet switch still
: needs to keep something like a route table (called forwarding table?)
: for the hosts in its broadcast domain, right? It is the same as ip route
: tables except addresses cannot be summarized, isn't it?
:
: is
: some

b*e2007-10-27 07:10

5 楼

I think we will be wasting time to discuss something that is not there yet,
it is baseless, right?
So can you rephrase your question again? assuming we are talking about how
today's routers and switches work.

【在 c*****g 的大作中提到】

c*g2007-10-27 07:10

6 楼

我说的是现在的情况。假设一个局域网里有几个ethernet switch,
十几个end host, 每个ethernet switch 应该有一个forward table,
记录到每个MAC地址的下一个port应该是什么。这个forward table
是怎么生成的？是通过spanning tree protocol吗？

,

【在 b******e 的大作中提到】

: I think we will be wasting time to discuss something that is not there yet,
: it is baseless, right?
: So can you rephrase your question again? assuming we are talking about how
: today's routers and switches work.

j*a2007-10-27 07:10

7 楼

src mac of incoming frame

【在 c*****g 的大作中提到】

: 我说的是现在的情况。假设一个局域网里有几个ethernet switch,
: 十几个end host, 每个ethernet switch 应该有一个forward table,
: 记录到每个MAC地址的下一个port应该是什么。这个forward table
: 是怎么生成的？是通过spanning tree protocol吗？
:
: ,

c*g2007-10-27 07:10

8 楼

you are right! thanks! So ethernet switch doesn't maintain
a permanent forwarding table at all?

【在 j*a 的大作中提到】

: src mac of incoming frame

c*g2007-10-27 07:10

9 楼

不好意思，还是没明白，l2 switching是不改变ethernet frame的
src&dest mac的，对吧？那么知道src mac又有什么用？你是说incoming
frame的incoming port吗？

【在 j*a 的大作中提到】

: src mac of incoming frame

c*g2007-10-27 07:10

10 楼

又在网上查了一下，我觉得l2用stp主要是为了防止loop，而在l3
loop的问题可以通过ttl来解决。
forwarding table应该就是通过stp来得到的吧

【在 c*****g 的大作中提到】

: 不好意思，还是没明白，l2 switching是不改变ethernet frame的
: src&dest mac的，对吧？那么知道src mac又有什么用？你是说incoming
: frame的incoming port吗？

b*e2007-10-27 07:10

11 楼

I faint!!!! I thought you were thinking ...

【在 c*****g 的大作中提到】

: 又在网上查了一下，我觉得l2用stp主要是为了防止loop，而在l3
: loop的问题可以通过ttl来解决。
: forwarding table应该就是通过stp来得到的吧

z*r2007-10-27 07:10

12 楼

恩？香蕉和苹果比？
stp是为了生成一个loop free l2 network, routing protocol是为了生成一个look up
table plus loop free l3 network。
如何forward frame at leyer 2 取决于cam table和stp的共同结果。
layer2不是说就不可以用routing protocol，目前bgp autodiscovery基本上就是这个
思路吧。

【在 c*****g 的大作中提到】

: 突然想到这个问题：
: Ethernet switch用Minimum spanning tree 算法来确定如何forward frame,
: 而IP router用shorted path first, 为什么ethernet switch 不也用spf呢？

L*t2007-10-27 07:10

13 楼

STP has nothing to do with L2 forwarding table.
STP is just used to shut down some ports to avoid loops.
Conceptually, L2 forwarding table is simply a list of (src-mac, ingress-port
) pair, stuffed by source mac learning.
At layer 3, TTL is the last straw to avoid loop. You can think the routing
protocol can guarantee loop-free forwarding.

【在 c*****g 的大作中提到】

: 又在网上查了一下，我觉得l2用stp主要是为了防止loop，而在l3
: loop的问题可以通过ttl来解决。
: forwarding table应该就是通过stp来得到的吧

c*g2007-10-27 07:10

14 楼

thank you all guys. It is clear to me now. :)

port

【在 L******t 的大作中提到】

: STP has nothing to do with L2 forwarding table.
: STP is just used to shut down some ports to avoid loops.
: Conceptually, L2 forwarding table is simply a list of (src-mac, ingress-port
: ) pair, stuffed by source mac learning.
: At layer 3, TTL is the last straw to avoid loop. You can think the routing
: protocol can guarantee loop-free forwarding.