chapter 5: Link-state protocol’s behavior in a Clos network

[zhaohuabing]

All of the leaves also compute the area summary routes to announce to the other backbone area routers. S11, which saw the change, sends the new updates to its neighbors in the backbone area, the super-spines. This update propagates through the backbone area until all routers in the backbone area have received it. This includes leaves in other pods. We’re assuming that the spines are not summarizing in this case (see the “Route Summarization in Clos Networks” on page 105 for details on summarization).

1. All of the leaves also compute the area summary routes to announce to the other backbone area routers.
   Since leaves are not in backbone area, and spines are ABR, I think it should be"All of the spines also compute the area summary routes to announce to the other backbone area routers(The super spines). "

2. "This includes leaves in other pods." Should it be "This includes spines in other pods."? Because leaves are not in backbone area.

３. If my first two assumptions were correct, then they will contradict this sentence "We’re assuming that the spines are not summarizing in this case (see the “Route Summarization in Clos Networks” on page 105 for details on summarization)."

I think I may misunderstand most of this paragraph, could you please help me with that? Thanks.

[zhaohuabing]

After thinking it over, I think the current statements are correct. I add my understanding as bold words, could you please verify them?

All of the leaves also compute the area summary routes to announce to the other backbone area routers(the spine routers). S11, which saw the change, sends the new updates to its neighbors in the backbone area, the super-spines. This update propagates through the backbone area until all routers in the backbone area have received it. (And super spines propagate the update to other pods, so leaves in other pods receive the update)This includes leaves in other pods. We’re assuming that the spines are not summarizing in this case (see the “Route Summarization in Clos Networks” on page 105 for details on summarization).

[zhaohuabing]

I googled some introduction of OSPF route summary. It says that only ABR and ASBR can summary the routes. I think the leaves in Clos topo are not ABR or ASBR, so why can they advertise area summary routes?

[ddutt]

You're not entirely correct, but not wrong either :). There is a typo. This is a typo that got left behind when I changed the use of "tor, leaf and spine" to "leaf, spine, super-spine". The correct statement should be:

• At the start of the section, change second sentence to: "The spines are the area border routers. The spines and super-spines form the level 2 (or backbone) routers."
• Bullet 5 should read: "All of the leaves also compute the area summary routes. S11, which saw the change, sends the new updates to its neighbors in the backbone area, the super-spines. This update propagates through the backbone area until all routers in the backbone area have received it. We’re assuming that the spines are not summarizing in this case (see the “Route Summarization in Clos Networks” on page 105 for details on summarization).

[zhaohuabing]

Thanks for the explanation. So "Only ABR and ASBR can summary the routes " is not correct. And the leaves do summary the routes of its locally connected networks before advertising them, right?

[ddutt]

Leaves advertise the prefixes associated with the links they have. They do not summarize. But they do recompute the inter-area routes from the area summary advertisements they learn from the backbone routers. Is that clearer?

[zhaohuabing]

"All of the leaves recompute the inter-area routes from the area summary advertisements they learn from the backbone routers."

That's very clear, thanks!